LTC3738 [Linear]
3-Phase Buck Controller for Intel VRM9/VRM10 with Active Voltage Positioning; 3相降压控制器,用于Intel VRM9 / VRM10与有源电压定位型号: | LTC3738 |
厂家: | Linear |
描述: | 3-Phase Buck Controller for Intel VRM9/VRM10 with Active Voltage Positioning |
文件: | 总32页 (文件大小:316K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
LTC3738
3-Phase Buck Controller
for Intel VRM9/VRM10 with
Active Voltage Positioning
U
FEATURES
DESCRIPTIO
TheLTC®3738isa3-phasesynchronousstep-downswitch-
ing regulator controller that drives all N-channel external
power MOSFET stages in a phase-lockable, fixed fre-
quency architecture. The 3-phase technique effectively
triples the fundamental frequency, improving transient
response while operating each controller at an optimal
frequency for efficiency and ease of thermal design. Light
load efficiency is optimized by using a choice of output
Stage Shedding or Pulse Skip mode technology.
■
3-Phase Controller with Onboard MOSFET Drivers
■
Programmable Active Voltage Positioning (AVP)
with True Current Sensing
■
Programmable External Thermal Detection or
Internal Thermal Sensing
■
Precise Output Current Matching Optimizes
Thermal Performance and Solution Size
■
Supports Starting into Precharged VOUTS
■
Differential Amplifier Accurately Senses VOUT
PWM, Pulse Skip and Stage SheddingTM Operation
■
The LTC3738 also allows users to program load slope via
a resistor for AVP control. Both external and internal
thermal sensing are available from the on-chip thermal
detector and comparator.
■
Synchronizable
■
210kHz to 530kHz Per Phase, Fixed Frequency
■
Output Power Good Indicator with Adaptive Blanking
■
Adjustable Soft-Start Current Ramping
■
■
■
A differential amplifier provides sensing of both the high
and low sides of the output voltage.
Short-Circuit Shutdown Timer with Defeat Option
OPTI-LOOP® Compensation Minimizes COUT
38-Lead (5mm × 7mm) QFN Package
U
Soft-start and a defeatable, timed short-circuit shutdown
protect the MOSFETs and the load. A foldback current
circuit also provides protection for the external MOSFETs
under short-circuit or overload conditions. An all- “1” VID
detector turns off the regulator after a 1µs timeout.
APPLICATIO S
■
High Performance Notebook Computers
Servers, Desktop Computers and Workstations
■
, LTC and LT are registered trademarks of Linear Technology Corporation.
OPTI-LOOP is a registered trademark of Linear Technology Corporation.
Stage Shedding is a trademark of Linear Technology Corporation.
U.S. Patent Numbers: 5481178, 5994885, 5929620, 6177787, 6144194, 6580258, 6462525,
6593724, 6674274, 6100678 pending on AVP technique.
U
TYPICAL APPLICATIO
VRM10 Step-Down Controller
V
IN
5V TO 28V
5V
V
TG1
SW1
BG1
CC
+
0.8µH
0.002Ω
0.002Ω
22µF
×2
LTC3738
10µF
BOOST1
BOOST2
BOOST3
+
–
SENSE1
SENSE1
0.1µF
V
IN
SW3 SW2 SW1
POWER GOOD INDICATOR
MODE SELECTION/SYNC IN
TG2
PGOOD
FCB/SYNC
0.8µH
V
OUT
SW2
BG2
0.8375V TO 1.6000V
PLLFLTR
TSNS
VR_HOTB
OUTEN
THERMAL INPUT
VR HOT INDICATOR
ON/OFF
PGND
+
SENSE2
–
680pF
SENSE2
V
IN
I
TG3
SW3
BG3
TH
0.8µH
0.002Ω
5k
SS
0.1µF
SGND
EAIN
AVP
+
–
SENSE3
SENSE3
100pF
+
470µF
×4
+
–
IN
IN
VID0-VID5
6 VID BITS
3738 TA01
3738f
1
LTC3738
W W U W
U W
U
ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
(Note 1)
TOP VIEW
Topside Driver Voltages (BOOSTN)............38V to –0.3V
Switch Voltage (SWN)...................................32V to –5V
Boosted Driver Voltage (BOOSTN – SWN)....7V to –0.3V
Peak Output Current <1ms (TGN, BGN) ..................... 5A
Supply Voltage (VCC), PGOOD, VR_HOTB
Pin Voltages ................................................7V to –0.3V
OUTEN, SS, PLLFLTR,
FCB/SYNC Voltages ................................... VCC to –0.3V
ITH Voltage................................................2.4V to –0.3V
IN+, IN–, VID0-VID5, TSNS .............................. 5V to 0V
AVP, SENSE+, SENSE– .................................. 1.9V to 0V
Operating Ambient Temperature Range....... 0°C to 85°C
Junction Temperature (Note 2)............................. 125°C
Storage Temperature Range ..................–65°C to 125°C
38 37 36 35 34 33 32
FCB/SYNC
PLLFLTR
1
2
3
4
5
6
7
8
9
31 TG1
30 SW1
–
IN
BOOST2
29
28
+
IN
TG2
AVP
27 SW2
EAIN
V
CC
26
39
+
SENSE1
25 BG1
24 PGND
23 BG2
22 BG3
21 SW3
–
SENSE1
+
SENSE2
–
SENSE2 10
–
SENSE3 11
+
20
SENSE3 12
TG3
13 14 15 16 17 18 19
UHF PACKAGE
38-LEAD (7mm × 5mm) PLASTIC QFN
TJMAX = 125°C, θJA = 34°C/W
EXPOSED PAD (PIN 39) IS SGND MUST BE SOLDERED TO PCB
ORDER PART
NUMBER
UHF PART
MARKING
LTC3738CUHF
3738
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = VOUTEN = VSS = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loop
+
V
V
Regulated Voltage at IN
(Note 3); VID Code = 110101, V = 1.2V
1.166
1.163
1.175
1.175
1.184
1.187
V
V
REGULATED
SENSEMAX
MATCH
ITH
●
●
Maximum Current Sense Threshold
V
V
= 0.5V, V Open,
65
62
75
75
85
88
mV
mV
EAIN
ITH
– V
– V
,
– = 0.8V, 1.85V
SENSE1
,
SENSE2
SENSE3
I
Current Match
Worst-Case Error at V
–5
5
%
SENSE(MAX)
V
Output Voltage Load Regulation
(Note 3)
LOADREG
Measured in Servo Loop, ∆I Voltage = 1.2V to 0.7V
Measured in Servo Loop, ∆I Voltage = 1.2V to 2V
●
●
0.1
–0.1
0.5
–0.5
%
%
TH
TH
V
Output Voltage Line Regulation
V
= 4.5V to 7V
CC
0.03
3.05
1.5
%/V
mmho
MHz
V
REFLNREG
m
g
g
Transconductance Amplifier g
I
I
= 1.2V, Sink/Source 25µA (Note 3)
TH
m
Transconductance Amplifier GBW
Forced Continuous Threshold
FCB Bias Current
= 1.2V, (g • Z , Z = Series 1k-100kΩ-1nF)
m L L
mOL
TH
V
●
0.58
0.60
0.2
0.62
0.7
FCB
FCB
I
V
= 0.65V
µA
FCB
3738f
2
LTC3738
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = VOUTEN = VSS = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
Pulse Skip Threshold
Undervoltage SS Reset
Measured at FCB pin
V
– 1.5 V – 0.7 V – 0.3
V
V
PSTH
CC
CC
CC
UVR
V
CC
Lowered Until the SS Pin is Pulled Low
3.3
3.8
4.5
I
Input DC Supply Current
Normal Mode
Shutdown
(Note 4)
Q
V
= 5V
2.5
20
mA
µA
CC
V
= 0V
100
0.8
OUTEN
V
OUTEN Pin ON Threshold
Soft-Start Charge Current
SS Pin Arming Threshold
V
V
V
, Ramping Positive
0.4
0.6
–1.5
3.8
V
µA
V
OUTEN
OUTEN
I
= 1.9V
–0.8
–2.5
4.5
SS
SS
SS
V
, Ramping Positive Until Short-Circuit
SSARM
Latch-Off is Armed
V
SS Pin Latch-Off Threshold
SS Discharge Current
V
, Ramping Negative
3.3
–1.5
1.5
V
µA
µA
µA
SSLO
SCL
SS
I
I
I
Soft-Short Condition V
= 0.3V, V = 4.5V
–5
95
EAIN
SS
Shutdown Latch Disable Current
Total SENSE Pins Source Current
V
EAIN
= 0.3V, V = 4.5V
5
SDLHO
SENSE
SS
+
–
+
–
SENSE1 , SENSE1 , SENSE2 , SENSE2 ,
SENSE3 , SENSE3 All Equal 1.2V
In Dropout
90
120
+
–
DF
Maximum Duty Factor
98.5
%
MAX
TG t t
Top Gate Rise Time
Top Gate Fall Time
C
LOAD
C
LOAD
= 3300pF
= 3300pF
30
40
90
90
ns
ns
R, F
BG t
t
Bottom Gate Rise Time
Bottom Gate Fall Time
C
LOAD
C
LOAD
= 3300pF
= 3300pF
30
20
90
90
ns
ns
R,
F
TG/BG t
BG/TG t
Top Gate Off to Bottom Gate On Delay All Controllers, C
Synchronous Switch-On Delay Time
= 3300pF Each Driver
= 3300pF Each Driver
60
ns
ns
ns
1D
LOAD
Bottom Gate Off to Top Gate On Delay All Controllers, C
Top Switch-On Delay Time
60
2D
LOAD
t
Minimum On-Time
Tested with a Square Wave (Note 5)
120
ON(MIN)
VID Parameters
VID
VID
Maximum Low Level Input Voltage
Minimum High Level Input Voltage
VRM9 Enable Threshold at VID5
VID0 to VID4
0.4
V
V
IL
0.8
3.5
IH
VID9/VID10
ATTEN
V
– 1
4.5
V
CC
(Note 6)
●
–0.25
0.25
%
ERR
Power Good Output Indication
V
PGOOD Voltage Output Low
PGOOD Output Leakage
PGOOD Trip Thesholds
I
= 2mA
= 5V
0.1
0.3
V
PGL
PGOOD
I
V
±1
µA
PGOOD
PGOOD
V
with Respect to Set Output Voltage,
DIFFOUT
V
V
V
V
Ramping Negative
Ramping Positive
VID Code = 110101
PGOOD Goes Low After V
–7
7
–10
10
–14
14
%
%
PGTHNEG
PGTHPOS
DIFFOUT
DIFFOUT
Delay
UVDLY
t
Power Good Blanking
After VID Changes Outside PGOOD Window
100
µs
PGBLNK
3738f
3
LTC3738
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = VOUTEN = VSS = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX UNITS
Oscillator and Phase-Locked Loop
f
f
f
Nominal Frequency
Lowest Frequency
V
V
V
= 1.2V
= 0V
325
190
480
365
220
550
1.2
415
250
630
kHz
kHz
kHz
V
NOM
LOW
HIGH
PLLFLTR
PLLFLTR
PLLFLTR
Highest Frequency
= 2.4V
R
FCB Clock Input Threshold
FCB
I
Phase Detector Output Current
Sinking Capability
Sourcing Capability
PLL LPF
f
f
< f
20
20
µA
µA
PLLIN
PLLIN
OSC
OSC
> f
R
Controller 2-Controller 1 Phase
Controller 3-Controller 1 Phase
120
240
Deg
Deg
RELPHS
No_CPU Detection
No-CPU Shutdown Latency
Differential Amplifier
t
After All VID Bits = “1”
0.5
0
1
µs
NOCPU
A
V
Differential Gain
1.000
0.5
V/V
mV
V
V
+
–
Input Offset Voltage
IN = IN = 1.2V, Input Referred
OS
CM
Common Mode Input Voltage Range
Common Mode Rejection Ratio
Gain Bandwidth Product
Maximum High Output Voltage
Input Resistance
5
+
–
CMRR
GBP
0V < IN = IN < 5V, Input Referred
70
2
dB
MHz
V
V
V
– 0.8
CC
O(MAX)
+
R
Measured at IN Pin
160
kΩ
IN
Active Voltage Positioning
+
I
I
Sinking Current Ability of AVP Pin
Sourcing Current Ability of AVP Pin
IN = 1.2V
0.250
1.4
mA
mA
mV
SINK
+
IN = 1.2V
SOURCE
+
V
– V
Max Voltage Drops V
to VO
IN = 1.2V, V = 60mV
180
AVP
O(MAX)
AVP
IN
Thermal Detection
V
V
V
Thermal Comparator Trip Threshold
Hysteresis
1.67
0.21
V
V
V
INT
INT_HYS
TH_TH
Internal Detection Enable Threshold
3.4
V
– 1
CC
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 5: The minimum on-time condition corresponds to an inductor peak-
to-peak ripple current of ≥40% of I (see minimum on-time
MAX
considerations in the Applications Information Section).
Note 2: T is calculated from the ambient temperature T and power
J
A
dissipation P according to the following formula:
Note 6: ATTEN specification is in addition to the output voltage
accuracy specified at VID code 110101.
D
ERR
T = T + (P × 34°C/W)
J
A
D
Note7:ThisICincludesovertemperatureprotectionthatisintendedtoprotect
the device during momentary overload conditions. Junction temperature will
exceed125°Cwhenovertemperatureprotectionisactive.Continuousoperation
above the specified maximum operating junction temperature may impair
device reliability.
Note 3: The IC is tested in a feedback loop that includes the differential
amplifier driving the VID DAC into the error amplifier and servoing the
resultant voltage to the midrange point for the error amplifier (V = 1.2V).
ITH
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
3738f
4
LTC3738
U W
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
Efficiency vs IOUT
Efficiency vs VIN
Efficiency vs Frequency
100
90
92
91
92
91
90
89
V
= 12V
IN
FREQUENCY = 210kHz
V
= 5V
FCB/SYNC
V
= 5V
80
70
60
IN
I
= 20A
L
90
V
= OPEN
FCB/SYNC
V
= 10V
= 14V
IN
89
88
87
86
V
= 0V
FCB/SYNC
50
40
88
87
V
IN
I
= 50A
L
30
20
10
0
86
85
84
V
V
I
= 12V
V
V
= 12V
IN
OUT
IN
OUT
= 1.2V
= 20A
= 1.2V
FREQUENCY = 210kHz
10 100
INDUCTOR CURRENT (A)
LOAD
85
0.1
14
1
4
7
9
10 11 12 13
(V)
5
6
8
250
300
400
200
450
500
350
V
FREQUENCY (kHz)
IN
3738 G01
3733 G02
3738 G03
Reference Voltage
vs Temperature
Error Amplifier gm
vs Temperature
Maximum ISENSE Threshold
vs Temperature
610
605
600
595
590
4.0
3.5
3.0
2.5
2.0
85
80
75
70
65
V
= 1.85V
= 0.8V
O
V
O
–45
0
15 30 45 60 75 90
–45
0
15 30 45 60 75 90
–45
–30 –15
0
15 30 45 60 75 90
–30 –15
–30 –15
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3738 G04
3738 G05
3738 G06
Oscillator Frequency
vs Temperature
Undervoltage Reset Voltage
vs Temperature
Oscillator Frequency vs VPLLFLTR
600
550
500
450
400
350
300
250
200
150
100
550
500
450
400
350
300
250
200
5.0
4.5
4.0
3.5
3.0
V
= 5V
PLLFLTR
V
= 2.4V
PLLFLTR
V
= 1.2V
= 0V
PLLFLTR
V
PLLFLTR
–45
0
15 30 45 60 75 90
0
0.8
1.2
2.0
2.4
–45
–30 –15
0
15 30 45 60 75 90
–30 –15
0.4
1.6
(V)
TEMPERATURE (°C)
V
TEMPERATURE (°C)
PLLFLTR
3738 G07
3738 G08
3738 G09
3738f
5
LTC3738
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
U W
Short-Circuit Arming and Latchoff
vs Temperature
Shutdown Current
vs Temperature
Supply Current vs Temperature
2.9
2.7
2.5
2.3
2.1
1.9
65
60
55
50
45
40
35
30
25
5.0
4.5
4.0
3.5
3.0
2.5
2.0
ARMING
LATCHOFF
–45
0
15 30 45 60 75 90
–45
–30 –15
0
15 30 45 60 75 90
0
15 30 45 60 75 90
TEMPERATURE (°C)
–30 –15
–45
–30 –15
TEMPERATURE (°C)
TEMPERATURE (°C)
3738 G11
3738 G12
3738 G10
Maximum Current Sense
Threshold vs Duty Factor
SS Pull-Up Current
vs Temperature
Peak Current Threshold vs VITH
2.5
2.0
1.5
1.0
0.5
0
75
50
25
0
90
80
70
60
50
40
30
20
10
0
–10
–20
–45
0
15 30 45 60 75 90
0
40
60
80
100
0
1.2
(V)
1.6
2.0
2.4
–30 –15
20
0.4
0.8
TEMPERATURE (°C)
DUTY FACTOR (%)
V
ITH
3738 G13
3738 G14
3738 G15
Maximum Duty Factors
vs Temperature
Percentage of Nominal Output
vs Peak ISENSE (Foldback)
Total ISENSE Current vs VOUT
80
70
60
50
40
30
20
10
0
100
98
96
94
92
90
0
–20
V
= 0V
PLLFLTR
–40
–60
–80
–100
–120
–140
–160
–180
0
40 50 60 70 80 90 100
–45
0
15 30 45 60 75 90
0
0.2 0.4 0.6 0.8
1
2
10 20 30
–30 –15
1.2 1.4 1.6 1.8
(V)
PERCENTAGE OF NOMINAL OUTPUT VOLTAGE (%)
TEMPERATURE (°C)
V
OUT
3738 G16
3738 G17
3738 G18
3738f
6
LTC3738
U W
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
Maximum Current Threshold
Mismatch vs Temperature
(VAVP – VIN+) vs
–
(VSENSE+ – VSENSE
)
3.0
2.5
2.0
1.5
1.0
0.5
0
180
135
90
45
0
–45
–45
0
15 30 45 60 75 90
–30 –15
–15
0
15
+
30
–
45
60
TEMPERATURE (°C)
V
– V
(mV)
SENSE
SENSE
3738 G19
3738 G20
Shed Mode at 1A,
Light Load Current
Pulse Skip Mode at 1A,
Light Load Current
V
V
OUT
OUT
10mV/DIV
10mV/DIV
V
SW1
5V/DIV
V
SW1
5V/DIV
V
V
SW2
SW2
5V/DIV
5V/DIV
V
SW3
V
SW3
5V/DIV
5V/DIV
3738 G21
3738 G22
V
V
V
= 12V
2µs/DIV
V
V
V
= 12V
2µs/DIV
IN
IN
= 1.5V
= 1.5V
OUT
FCB
OUT
FCB
= V
= OPEN
CC
FREQUENCY = 210kHz
FREQUENCY = 210kHz
Continuous Mode at 1A,
Light Load Current
Load Transient with AVP
V
OUT
10mV/DIV
INTEL
SPEC
V
OUT
80mV
50mV/DIV
∆I
V
OUT
SW1
100A STEP
5V/DIV
dI/dt > 200A/µs
V
SW2
5V/DIV
V
SW3
3738 G24
5V/DIV
V
V
C
= 12V
IN
= 1.35V
OUT
OUT
3738 G23
= 10 × 330µF/2.5V
V
V
V
= 12V
2µs/DIV
IN
SANYO TPE POSCAP
= 1.5V
= GND
OUT
FCB
+ 18 × 22µF/X5R CERAMIC
FREQUENCY = 210kHz
3738f
7
LTC3738
U
U
U
PI FU CTIO S
FCB/SYNC (Pin 1): Forced Continuous Control Input. The
voltage applied to this pin sets the operating mode of the
controller. The forced continuous current mode is active
when the applied voltage is less than 0.6V. Pulse skip
mode operation will be active when the pin is allowed to
float and Stage Shedding mode will be active if the pin is
tied to the VCC pin. When an external clock is present, the
controller will be synchronized to the external clock and
forced continuous mode is selected internally. (Do not
apply voltage to this pin prior to the application of voltage
on the VCC pin.)
SS (Pin 13): Combination of Soft-Start and Short-Circuit
Detection Timer. A capacitor to ground at this pin sets the
ramp time to full current output as well as the time delay
prior to an output voltage short-circuit shutdown.
I
TH (Pin 14): Error Amplifier Output and Switching Regu-
lator Compensation Point. All three current comparator’s
thresholds increase with this control voltage.
TSNS (Pin 15): This pin selects external or internal ther-
mal detection. Tying this pin to VCC will enable the internal
thermal detector. When the voltage at this pin is less than
V
CC – 1.6V, the internal thermal detector is disabled and
PLLFLTR (Pin 2): The phase-locked loop’s lowpass filter
is tied to this pin. Alternatively, this pin can be driven with
an AC or DC voltage source to vary the frequency of the
internal oscillator. (Do not apply voltage to this pin prior to
the application of voltage on the VCC pin.)
IN+, IN– (Pins 4, 3): Inputs to a Precision, Unity-Gain
Differential Amplifier with Internal Precision Resistors.
This provides true remote sensing of both the positive and
negative load terminals for precise output voltage control.
this pin serves as the input to an internal comparator
which is referenced to VCC/3.
VR_HOTB (Pin 16): This open-collector output is pulled
low when voltage at the TSNS pin is less than VCC/3. If
TSNS is tied to VCC, this pin is pulled low when the internal
thermal detector is tripped.
PGND (Pin 24): Driver Power Ground. This pin connects
to the sources of the bottom N-channel external MOSFETs
and the (–) terminals of CIN.
AVP (Pin 5): Active Voltage Positioning Load Slope Pro-
gramming Pin. A resistor tied between this pin and IN+
sets the load slope.
BG1, BG2, BG3 (Pins 25, 23, 22): High Current Gate
DrivesfortheBottomN-ChannelMOSFETs.Voltageswing
at these pins is from ground to VCC.
EAIN (Pin 6): This is the input to the error amplifier which
compares the VID divided feedback voltage to the internal
0.6V reference voltage.
SENSE1+, SENSE2+, SENSE3+, SENSE1–, SENSE2–,
SENSE3– (Pins 7 to 12): The Inputs to Each Differential
Current Comparator. The ITH pin voltage and built-in
offsetsbetweentheSENSE– andSENSE+ pins, inconjunc-
tion with RSENSE, set the current trip threshold level.
VCC (Pin 26): Main Supply Pin. Because this pin supplies
both the controller circuit power as well as the high power
pulses supplied to drive the external MOSFET gates, this
pin needs to be very carefully and closely decoupled to the
IC’s PGND pin.
3738f
8
LTC3738
U
U
U
PI FU CTIO S
SW1, SW2, SW3 (Pins 30, 27, 21):Switch Node Connec-
tions to Inductors. Voltage swing at these pins is from a
Schottky diode (external) voltage drop below ground to
VIN (where VIN is the external MOSFET supply rail).
PGOOD (Pin 33): This open-drain output is pulled low
when the output voltage is outside the PGOOD tolerance
window. PGOOD is blanked during VID transitions for
approximately 100µs.
TG1,TG2,TG3(Pins31,28,20):HighCurrentGateDrives
for Top N-Channel MOSFETs. These are the outputs of the
floating drivers with a voltage swing equal to the boost
voltage source superimposed on the switch node voltage
SW.
VID0,VID1,VID2,VID3,VID4,VID5(Pins35,36,37,17,
18, 34): Output Voltage Programming Input Pins. When
VID5 is tied to VCC, the Intel VRM9 VID table is selected.
When voltage of VID5 is less than VCC – 2V, VID5 serves
as the fifth VID bit of VRM10.
BOOST1, BOOST2, BOOST3 (Pins 32, 29, 19): Positive
Supply Pins to the Topside Floating Drivers. Bootstrapped
capacitors, charged with external Schottky diodes and a
boostvoltagesourceareconnectedbetweentheBOOSTand
SWpins.VoltageswingattheBOOSTpinsisfromtheboost
source voltage (typically VCC) to this boost source voltage
+VIN (where VIN is the external MOSFET supply rail).
OUTEN (Pin 38): On/Off Control of the Controller.
SGND (Pin 39, Exposed Pad): Signal Ground. This pin
must be soldered to the ground plane.
3738f
9
LTC3738
U
U
W
FU CTIO AL DIAGRA
TSNS
PGOOD
0.66V
EAIN
–
+
2.4V
VR_HOTB
100µs
TCMP
–
+
BLANKING
–
+
2.5µA
V /3
CC
FCB/SYNC
VID TRANSITIONS
0.54V
MUX
+
INTERNAL
THERMAL
DETECTION
FCB
–
0.6V
PHASE DET
PLLFLTR
V
CC
V
IN
R
LP
DUPLICATE FOR SECOND AND THIRD
CONTROLLER CHANNELS
CLK1
D
B
BOOST
TG
C
LP
CLK2
CLK3
OSCILLATOR
DROP
OUT
DET
C
B
+
TOP
BOT
RS
LATCH
FORCED BOT
TOP ON
C
IN
–
+
IN
S
Q
SW
80k
80k
SWITCH
LOGIC
R
Q
–
V
CC
A1
+
BG
IN
FCB
80k
80k
+
PGND
R
SHDN
PRE-AVP
AVP
+
SENSE1
–
+
–
+
–
+
–
SENSE1
SENSE2
SENSE2
SENSE3
SENSE3
–
+
–
+
–
–
+
–
I
L
I
V
–
+
+
–
2
1
CC
+
R
AVP
+
SENSE
30k
3mV
R1
8k
SLOPE
COMP
ADDER
R
SENSE
–
C
OUT
SENSE
EAIN
V
FB
30k
5(V
FB
)
–
+
EA
0.600V
45k
45k
V
OUT
2.4V
OV
+
–
0.660V
I
TH
SHED
V
REF
0.600V
V
CC
V
C
CC
C
V
CC
R2 VARIABLE
SHDN
RST
RUN
SOFT-
START
INTERNAL
SUPPLY
R
1.5µA
C
+
5(V
FB
)
SGND
C
CC
VID
SELECTION
6V
VRM9/VRM10 VID DECODER
SS
OUTEN
C
SS
NO_CPU
1µs
100k
VID5 VID0 VID1 VID2 VID3 VID4
3738 F01
Figure 1
3738f
10
LTC3738
U
(Refer to Functional Diagram)
OPERATIO
Main Control Loop
Low Current Operation
The IC uses a constant frequency, current mode step-
down architecture. During normal operation, each top
MOSFET is turned on each cycle when the oscillator sets
the RS latch, and turned off when the main current
comparator, I1, resets each RS latch. The peak inductor
current at which I1 resets the RS latch is controlled by the
voltage on the ITH pin, which is the output of the error
amplifierEA. TheEAINpinreceivesaportionofthevoltage
feedback signal via the differential amplifier through the
internalVIDDACandiscomparedtotheinternalreference
voltage.Whentheloadcurrentincreases,itcausesaslight
decrease in the EAIN pin voltage relative to the 0.6V
reference, which in turn causes the ITH voltage to increase
until each inductor’s average current matches one third of
the new load current (assuming all three current sensing
resistors are equal). In pulse skip mode and Stage Shed-
ding mode, after each top MOSFET has turned off, the
bottom MOSFET is turned on until either the inductor
currentstartstoreverse, asindicatedbycurrentcompara-
tor I2, or the beginning of the next cycle.
The FCB/SYNC pin is a multifunction pin: 1) a logic input
toselectbetweenthreemodesofoperationand2)external
clock input pin for synchronization.
When the FCB/SYNC pin voltage is below 0.6V, the
controller performs as a continuous, PWM current mode
synchronous switching regulator. The top and bottom
MOSFETs are alternately turned on to maintain the output
voltage independent of direction of inductor current.
When the FCB/SYNC pin is below VCC – 1.5V, but greater
than 0.6V, the controller performs as a pulse skip mode
switching regulator. Pulse skip mode operation turns off
the synchronous MOSFET(s) when the inductor current
goes negative. Switching cycles will be skipped when the
output load current drops below 3% of the maximum
designed load current in order to maintain the output
voltage. Pulseskipoperationprovideslownoise, constant
frequency operation at light load conditions.
When the FCB/SYNC pin is tied to the VCC pin, Stage
Shedding mode is enabled. This mode provides constant
frequency, discontinuous current operation over the wid-
est possible output current range. At approximately 10%
of maximum designed load current, the second and third
output stages are shut off and the first controller alone is
active in discontinuous current mode. This “stage shed-
ding”optimizesefficiencybyeliminatingthegatecharging
losses and switching losses of the other two output
stages. Additional cycles will be skipped when the output
load current drops below 1% of maximum designed load
current in order to maintain the output voltage. This
constant frequency operation is more efficient than pulse
skip mode operation at very light load conditions.
The top MOSFET drivers are biased from floating boot-
strap capacitor CB, which is normally recharged during
each off cycle through an external Schottky diode. When
VIN decreases to a voltage close to VOUT, however, the
loop may enter dropout and attempt to turn on the top
MOSFET continuously. The dropout detector counts the
number of oscillator cycles that the bottom MOSFET
remains off and periodically forces a brief on period to
allow CB to recharge.
The main control loop is shut down by pulling the OUTEN
pinlow.PullingupOUTENallowsaninternal1.5µAcurrent
sourcetochargesoft-startcapacitorCSS attheSSpin.The
internal ITH voltage is clamped to the SS voltage while CSS
is slowly charged up. This “soft-start” clamping prevents
abrupt current from being drawn from the input power
source. When the OUTEN pin is low, all functions are kept
in a controlled state.
3738f
11
LTC3738
U
(Refer to Functional Diagram)
OPERATIO
Tying the FCB/SYNC pin to ground will force continuous
current operation. This is the least efficient operating
mode, but may be desirable in certain applications. The
output can source or sink current in this mode. When
forcing continuous operation and sinking current, this
current will be forced back into the main power supply,
potentially boosting the input supply to dangerous volt-
age levels.
Power Good
The PGOOD pin is connected to the drain of an internal
MOSFET. The MOSFET is turned on when the output
voltage exceeds the PGOOD ±10% tolerance window. The
PGOOD signal is blanked for approximately 100µs during
VID transitions. If a new VID transition occurs before the
previous blanking time expires, the timer is reset.
Short-Circuit Detection
Feeding a clock signal into the FCB/SYNC pin will syn-
chronize the LTC3738 to the external clock. See Fre-
quency Synchronization or Setup for more information.
The SS capacitor is used initially to limit the inrush current
from the input power source. Once the controllers have
been given time, as determined by the capacitor on the SS
pin, to charge up the output capacitors and provide full
load current, the SS capacitor is then used as a short-
circuit timeout circuit. If the output voltage falls to less
than62.5%ofitsnominaloutputvoltage, theSScapacitor
beginsdischarging,assumingthattheoutputisinasevere
overcurrentand/orshort-circuitcondition.Ifthecondition
lasts for a long enough period, as determined by the size
of the SS capacitor, the controller will be shut down until
the OUTEN pin voltage is recycled. This built-in latchoff
canbeoverriddenbyproviding>5µAatacomplianceof4V
to the SS pin. This current shortens the soft-start period
but prevents net discharge of the SS capacitor during a
severeovercurrentand/orshort-circuitcondition.Foldback
current limiting is activated when the output voltage falls
below 62.5% of its nominal level whether or not the short-
circuit latchoff circuit is enabled. Foldback current limit
can be overridden by clamping the EAIN pin such that the
voltage is held above the (62.5%)(0.6V) or 0.375V level
even when the actual output voltage is low.
Frequency Synchronization or Setup
The phase-locked loop allows the internal oscillator to be
synchronized to an external source using the FCB/SYNC
pin. The output of the phase detector at the PLLFLTR pin
is also the DC frequency control input of the oscillator
which operates over a 210kHz to 530kHz range corre-
spondingtoavoltageinputfrom0Vto2.4V.Whenlocked,
the PLL aligns the turn on of the top MOSFET to the rising
edge of the synchronizing signal and forced continuous
mode is set internally. When no frequency information is
suppliedtotheFCB/SYNCpin,PLLFLTRgoeslow,forcing
the oscillator to minimum frequency. A DC source can be
applied to the PLLFLTR pin to externally set the desired
operating frequency.
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both VOUT+ and VOUT– benefits regula-
tion in high current applications and/or applications hav-
ing electrical interconnection losses. This sensing also
isolates the physical power ground from the physical
signal ground preventing the possibility of troublesome
“ground loops” on the PC layout and prevents voltage
errors caused by board-to-board interconnects, particu-
larly helpful in VRM designs.
The SS capacitor will be reset if the input voltage, (VCC) is
allowed to fall below approximately 4V. The capacitor on
the pin will be discharged until the short-circuit arming
latch is disarmed. The SS capacitor will attempt to cycle
through a normal soft-start ramp up after the VCC supply
risesabove4V.Thiscircuitpreventspowersupplylatchoff
in the event of input power switching break-before-make
situations.
3738f
12
LTC3738
U
(Refer to Functional Diagram)
OPERATIO
Start-Up
Thermal Detection
The start-up of the LTC3738 is controlled by the voltage
ramp on the SS pin. The start-up is not completed until the
short-circuit arming latch is enabled. During start-up, the
foldback current limit is temporarily defeated and at the
same time no reverse inductor current is allowed. This is
helpful for situations where output voltage has been “pre-
biased” at some voltage before the controller is enabled.
This will prevent sinking current during start-up which
would otherwise pull current from the pre-biased output.
An accurate comparator and a thermal detector are inte-
grated into the LTC3738 for external or internal thermal
detection. Tying TSNS to VCC will enable an internal
thermal detector which generates a thermal event at or
above 120°C with 10°C hysteresis. When the voltage at
TSNS is less than VCC – 1.6V, the internal thermal detector
is disabled and this pin serves as the input to an accurate
comparator which is referenced to VCC/3 with a hysteresis
of VCC/24. A thermal event is generated when the voltage
at TSNS is less than VCC/3. VR_HOTB, an open-collector
outputpin, willbepulledlowwhenathermaleventoccurs.
VID Table and NO_CPU Detection
The LTC3738 has a VID block which is compatible with
VRM9andVRM10. TyingVID5toVCC willselecttheVRM9
table. When the voltage at VID5 is less than VCC – 1.5V, the
VRM10 table is selected and this pin serves as the VID5 bit
of VRM10. There is a built in –25mV output offset for the
VRM10 VID table and a –12.5mV output offset for the
VRM9 VID table.
Active Voltage Positioning
Load slope is programmable in the LTC3738 through
external resistors. The inductor current information for all
three channels is sensed and combined; the final result is
presented as a voltage drop between AVP and IN+. This
voltage drop is scaled through two external resistors
attached to IN+ and then added to the output voltage as the
compensation for load slope. The final load slope is
defined by the inductor current sense resistors and the
two external resistors mentioned above.
The LTC3738 detects the presence of CPU by monitoring
the VID bits. If a VID0-VID4 all “1” condition is detected,
the controller acknowledges a NO_CPU fault. If this fault
condition persists for more than 1µs, the SS pin is pulled
low and the controller is shut down. The LTC3738 will
attempt a normal start-up when the NO_CPU fault is
removed.
3738f
13
LTC3738
W U U
U
APPLICATIO S I FOR ATIO
The basic application circuit is shown on the first page of
this data sheet. External component selection is driven by
the load requirement, and normally begins with the selec-
tion of an inductance value based upon the desired
operating frequency, inductor current and output voltage
ripple requirements. Once the inductors and operating
frequency have been chosen, the current sensing resis-
tors can be calculated. Next, the power MOSFETs and
Schottky diodes are selected. Finally, CIN and COUT are
selected according to the required voltage ripple require-
ments. The circuit shown on the first page of this data
sheet can be configured for operation up to a MOSFET
supply voltage of 28V (limited by the external MOSFETs).
Inductor Value Calculation and Output Ripple Current
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge and transition losses. In addition to
this basic tradeoff, the effect of inductor value on ripple
current and low current operation must also be consid-
ered. The PolyPhase approach reduces both input and
output ripple currents while optimizing individual output
stagestorunatalowerfundamentalfrequency,enhancing
efficiency.
Operating Frequency
Theinductorvaluehasadirecteffectonripplecurrent.The
inductor ripple current ∆IL per individual section, N,
decreases with higher inductance or frequency and in-
The IC uses a constant frequency architecture with the
frequency determined by an internal capacitor. This ca-
pacitor is charged by a fixed current plus an additional
current which is proportional to the voltage applied to the
PLLFLTR pin. Refer to the Phase-Locked Loop and Fre-
quency Synchronization and Setup sections for additional
information.
creases with higher VIN or VOUT
:
VOUT VOUT
⎛
⎞
⎟
∆IL =
1−
⎜
fL ⎝
V ⎠
IN
where f is the individual output stage operating frequency.
A graph for the voltage applied to the PLLFLTR pin versus
frequency is given in Figure 2. As the operating frequency
isincreasedthegatechargelosseswillbehigher,reducing
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 530kHz.
InaPolyPhaseconverter,thenetripplecurrentseenbythe
output capacitor is much smaller than the individual
inductor ripple currents due to the ripple cancellation. The
details on how to calculate the net output ripple current
can be found in Application Note 77.
550
450
350
250
150
Figure 3 shows the net ripple current seen by the output
capacitors for the different phase configurations. The
outputripplecurrentisplottedforafixedoutputvoltageas
the duty factor is varied between 10% and 90% on the
x-axis. The output ripple current is normalized against the
inductor ripple current at zero duty factor. The graph can
be used in place of tedious calculations. As shown in
Figure 3, the zero output ripple current is obtained when:
0
0.5
1.0
1.5
2.0
2.5
VOUT
V
IN
k
N
=
where k = 1, 2,...,N – 1
PLLFLTR PIN VOLTAGE (V)
3738 F02
Figure 2. Operating Frequency vs VPLLFLTR
3738f
14
LTC3738
W U U
APPLICATIO S I FOR ATIO
U
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Power MOSFET and Schottky Diode Selection
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY FACTOR (V /V
)
OUT IN
AtleasttwoexternalpowerMOSFETsmustbeselectedfor
each of the three output sections: One N-channel MOSFET
for the top (main) switch and one or more N-channel
MOSFET(s) for the bottom (synchronous) switch. The
number, type and “on” resistance of all MOSFETs selected
takeintoaccountthevoltagestep-downratioaswellasthe
actualposition(mainorsynchronous)inwhichtheMOSFET
will be used. A much smaller and much lower input
capacitance MOSFET should be used for the top MOSFET
in applications that have an output voltage that is less than
3738 F03
Figure 3. Normalized Peak Output Current
vs Duty Factor [IRMS = 0.3(IO(P-P)
]
Sothenumberofphasesusedcanbeselectedtominimize
the output ripple current and therefore the output ripple
voltage at the given input and output voltages. In applica-
tions having a highly varying input voltage, additional
phases will produce the best results.
Accepting larger values of ∆IL allows the use of low
inductances but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
∆IL = 0.4(IOUT)/N, where N is the number of channels and
IOUT is the total load current. Remember, the maximum
∆IL occurs at the maximum input voltage. The individual
inductor ripple currents are constant determined by the
inductor, input and output voltages.
1/3oftheinputvoltage. InapplicationswhereVIN >>VOUT
,
the top MOSFETs’ “on” resistance is normally less impor-
tant for overall efficiency than its input capacitance at
operating frequencies above 300kHz. MOSFET manufac-
turers have designed special purpose devices that provide
reasonably low “on” resistance with significantly reduced
inputcapacitanceforthemainswitchapplicationinswitch-
ing regulators.
The peak-to-peak MOSFET gate drive levels are set by the
voltage, VCC, requiring the use of logic-level threshold
MOSFETs in most applications. Pay close attention to the
BVDSS specification for the MOSFETs as well; many of the
logic-level MOSFETs are limited to 30V or less.
Inductor Core Selection
Once the value for the inductors is known, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of ferrite, molyper-
malloyorKoolMµ® cores. Actualcorelossisindependent
of core size for a fixed inductor value, but it is very
dependent on inductance selected. As inductance in-
creases, core losses go down. Unfortunately, increased
inductance requires more turns of wire and therefore
copper losses will increase.
Selection criteria for the power MOSFETs include the “on”
resistance RSD(ON), input capacitance, input voltage and
maximum output current.
MOSFET input capacitance is a combination of several
components but can be taken from the typical “gate
charge” curve included on most data sheets (Figure 4).
Kool Mµ is a registered trademark of Magnetics, Inc.
3738f
15
LTC3738
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U
APPLICATIO S I FOR ATIO
V
IN
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
MILLER EFFECT
V
GS
V
V
DS
a
b
2
V
GS
VOUT
V
IN
I
MAX
N
⎛
⎜
⎝
⎞
⎟
⎠
Q
IN
PMAIN
=
1+ δ R
+
(
)
DS(ON)
C
= (Q – Q )/V
B A DS
3738 F04
MILLER
2 IMAX
2N
V
R
C
•
(
DR)(
)
Figure 4. Gate Charge Characteristic
IN
MILLER
The curve is generated by forcing a constant input current
into the gate of a common source, current source loaded
stage and then plotting the gate voltage versus time. The
initialslopeistheeffectofthegate-to-sourceandthegate-
to-drain capacitance. The flat portion of the curve is the
result of the Miller capacitance effect of the drain-to-
source capacitance as the drain drops the voltage across
the current source load. The upper sloping line is due to
the drain-to-gate accumulation capacitance and the gate-
to-source capacitance. The Miller charge (the increase in
coulombsonthehorizontalaxisfromatobwhilethecurve
is flat) is specified for a given VDS drain voltage, but can be
adjusted for different VDS voltages by multiplying by the
ratio of the application VDS to the curve specified VDS
values. A way to estimate the CMILLER term is to take the
change in gate charge from points a and b on a manufac-
turers data sheet and divide by the stated VDS voltage
specified. CMILLER is the most important selection criteria
fordeterminingthetransitionlossterminthetopMOSFET
but is not directly specified on MOSFET data sheets. CRSS
and COS are specified sometimes but definitions of these
parameters are not included.
⎡
⎢
⎣
⎤
⎥
⎦
1
1
+
f
( )
V – VTH(MIN) VTH(MIN) ⎥
⎢ CC
2
V – VOUT
I
MAX
⎛
⎜
⎝
⎞
⎟
⎠
IN
PSYNC
=
1+ δ R
DS(ON)
(
)
V
IN
N
where N is the number of output stages, δ is the tempera-
ture dependency of RDS(ON), RDR is the effective top driver
resistance (approximately2ΩatVGS =VMILLER), VIN isthe
drain potential and the change in drain potential in the
particular application. VTH(MIN) is the data sheet specified
typical gate threshold voltage specified in the power
MOSFET data sheet. CMILLER is the calculated capacitance
using the gate charge curve from the MOSFET data sheet
and the technique described above.
BothMOSFETshaveI2RlosseswhilethetopsideN-channel
equationincludesanadditionaltermfortransitionlosses,
which peak at the highest input voltage. For VIN < 12V, the
high current efficiency generally improves with larger
MOSFETs, while for VIN > 12V, the transition losses
rapidly increase to the point that the use of a higher
RDS(ON) device with lower CRSS actually provides higher
efficiency. The synchronous MOSFET losses are greatest
athighinputvoltagewhenthetopswitchdutyfactorislow
or during a short circuit when the synchronous switch is
on close to 100% of the period.
When the controller is operating in continuous mode the
duty cycles for the top and bottom MOSFETs are given by:
VOUT
V
IN
Main SwitchDuty Cycle =
The term (1 + δ ) is generally given for a MOSFET in the
form of a normalized RDS(ON) vs temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
⎛ V – VOUT
⎝
⎞
IN
Synchronous SwitchDuty Cycle =
⎜
⎟
V
IN
⎠
3738f
16
LTC3738
W U U
APPLICATIO S I FOR ATIO
U
The Schottky diodes shown in the Typical Application on
the first page of this data sheet conduct during the dead
time between the conduction of the two large power
MOSFETs. This prevents the body diode of the bottom
MOSFET from turning on, storing charge during the dead
time and requiring a reverse recovery period which could
cost as much as several percent in efficiency. A 2A to 8A
Schottky is generally a good compromise for both regions
of operation due to the relatively small average current.
Larger diodes result in additional transition losses due to
their larger junction capacitance.
These worst-case conditions are commonly used for de-
signbecauseevensignificantdeviationsdonotoffermuch
relief. Note that capacitor manufacturer’s ripple current
ratings are often based on only 2000 hours of life. This
makes it advisable to further derate the capacitor or to
choose a capacitor rated at a higher temperature than re-
quired. Several capacitors may also be paralleled to meet
size or height requirements in the design. Always consult
the capacitor manufacturer if there is any question.
The Figure 5 graph shows that the peak RMS input current
is reduced linearly, inversely proportional to the number N
of stages used. It is important to note that the efficiency
loss is proportional to the input RMS current squared and
therefore a 3-stage implementation results in 90% less
power loss when compared to a single phase design. Bat-
tery/input protection fuse resistance (if used), PC board
trace and connector resistance losses are also reduced by
the reduction of the input ripple current in a PolyPhase
system. The required amount of input capacitance is
further reduced by the factor, N, due to the effective in-
crease in the frequency of the current pulses.
CIN and COUT Selection
InputcapacitanceESRrequirementsandefficiencylosses
are reduced substantially in a multiphase architecture
because the peak current drawn from the input capacitor
is effectively divided by the number of phases used and
power loss is proportional to the RMS current squared. A
3-stage, single output voltage implementation can reduce
input path power loss by 90%.
In continuous mode, the source current of each top
N-channelMOSFETisasquarewaveofdutycycleVOUT/VIN.
A low ESR input capacitor sized for the maximum RMS
current must be used. The details of a close form equation
can be found in Application Note 77. Figure 5 shows the
input capacitor ripple current for different phase configu-
rations with the output voltage fixed and input voltage
varied. The input ripple current is normalized against the
DC output current. The graph can be used in place of
tedious calculations. The minimum input ripple current
can be achieved when the product of phase number and
output voltage, N(VOUT), is approximately equal to the
input voltage VIN or:
Ceramic capacitors are becoming very popular for small
designs but several cautions should be observed. “X7R”,
“X5R” and “Y5V” are examples of a few of the ceramic
materials used as the dielectric layer, and these different
dielectrics have very different effect on the capacitance
value due to the voltage and temperature conditions
0.6
0.5
1-PHASE
0.4
2-PHASE
3-PHASE
4-PHASE
0.3
6-PHASE
VOUT
V
IN
k
N
=
where k = 1, 2,...,N – 1
0.2
0.1
So the phase number can be chosen to minimize the input
capacitor size for the given input and output voltages.
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
In the graph of Figure 5, the local maximum input RMS
capacitor currents are reached when:
DUTY FACTOR (V /V
)
OUT IN
3738 F05
Figure 5. Normalized Input RMS Ripple Current
vs Duty Factor for One to Six Output Stages
VOUT 2k – 1
=
where k = 1, 2,...,N
V
IN
N
3738f
17
LTC3738
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APPLICATIO S I FOR ATIO
applied. Physically, if the capacitance value changes due
to applied voltage change, there is a concommitant piezo
effect which results in radiating sound! A load that draws
varying current at an audible rate may cause an attendant
varying input voltage on a ceramic capacitor, resulting in
an audible signal. A secondary issue relates to the energy
flowing back into a ceramic capacitor whose capacitance
value is being reduced by the increasing charge. The
voltage can increase at a considerably higher rate than the
constant current being supplied because the capacitance
value is decreasing as the voltage is increasing! Ceramic
capacitors,whenproperlyselectedandusedhowever,can
provide the lowest overall loss due to their extremely low
ESR.
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo and the Panasonic SP
surface mount types have a good (ESR)(size) product.
Once the ESR requirement for COUT has been met, the
RMS current rating generally far exceeds the IRIPPLE(P-P)
requirement. Ceramic capacitors from AVX, Taiyo Yuden,
Murata and Tokin offer high capacitance value and very
low ESR, especially applicable for low output voltage
applications.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handlingrequirementsoftheapplication.Aluminumelec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. New special polymer sur-
face mount capacitors offer very low ESR also but have
muchlowercapacitivedensityperunitvolume.Inthecase
oftantalum,itiscriticalthatthecapacitorsaresurgetested
for use in switching power supplies. Several excellent
choices are the AVX TPS, AVX TPSV, the KEMET T510
series of surface-mount tantalums or the Panasonic SP
series of surface mount special polymer capacitors avail-
able in case heights ranging from 2mm to 4mm. Other
capacitor types include Sanyo POS-CAP, Sanyo OS-CON,
Nichicon PL series and Sprague 595D series. Consult the
manufacturer for other specific recommendations.
The selection of COUT is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The steady-state output ripple (∆VOUT) is determined by:
⎛
1
⎞
∆VOUT ≈ ∆IRIPPLE ESR +
⎜
⎟
⎝
8NfCOUT
⎠
where f = operating frequency of each stage, N is the
number of output stages, COUT = output capacitance and
∆IL = ripple current in each inductor. The output ripple is
highest at maximum input voltage since ∆IL increases
withinputvoltage.Theoutputripplewillbelessthan50mV
at max VIN with ∆IL = 0.4IOUT(MAX) assuming:
RSENSE Selection for Output Current
Once the frequency and inductor have been chosen,
RSENSE1, RSENSE2, RSENSE3 are determined based on the
required peak inductor current. The current comparator
has a maximum threshold of 75mV/RSENSE and an input
common mode range of SGND to (1.1) • VCC. The current
comparator threshold sets the peak inductor current,
yielding a maximum average output current IMAX equal to
the peak value less half the peak-to-peak ripple current,
∆IL.
COUT required ESR < N • RSENSE
and
COUT > 1/(8Nf)(RSENSE
)
The emergence of very low ESR capacitors in small,
surfacemountpackagesmakesverysmallphysicalimple-
mentations possible. The ability to externally compensate
the switching regulator loop using the ITH pin allows a
much wider selection of output capacitor types. The
impedance characteristics of each capacitor type is sig-
nificantly different than an ideal capacitor and therefore
requires accurate modeling or bench evaluation during
design.
Allowing a margin for variations in the IC and external
component values yields:
50mV
IMAX
RSENSE = N
Manufacturers such as Nichicon, United Chemicon and
Sanyoshouldbeconsideredforhighperformancethrough-
The IC works well with values of RSENSE from 0.001Ω to
0.02Ω.
3738f
18
LTC3738
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APPLICATIO S I FOR ATIO
U
VCC Decoupling
common, tightly coupled pair of PC traces. The differen-
tial amplifier rejects common mode signals capacitively
orinductivelyradiatedintothefeedbackPCtracesaswell
as ground loop disturbances. The differential amplifier
output signal is divided down through the VID DAC and
is compared with the internal, precision 0.6V voltage
reference by the error amplifier.
TheVCC pinsuppliespowernotonlytotheinternalcircuits
of the controller but also to the top and bottom gate
drivers and therefore must be bypassed very carefully to
ground with a ceramic capacitor, type X7R or X5R (de-
pending upon the operating temperature environment) of
at least 1µF immediately next to the IC and preferably an
additional 10µF placed very close to the IC due to the
extremely high instantaneous currents involved. The total
capacitance, taking into account the voltage coefficient of
ceramic capacitors, should be 100 times as large as the
total combined gate charge capacitance of ALL of the
MOSFETs being driven. Good bypassing close to the IC is
necessary to supply the high transient currents required
by the MOSFET gate drivers while keeping the 5V supply
quiet enough so as not to disturb the very small-signal
high bandwidth of the current comparators.
The amplifier has a 0 to VCC common mode input range
and an output swing range of 0 to VCC – 1.2V. The output
uses an NPN emitter follower with 160kΩ feedback
resistance.
Output Voltage
Selection of the VRM9 or VRM10 VID table is through the
VID5pin.TyingVID5toVCC willselecttheVRM9 VIDtable.
If the VRM9 VID table is selected (Table 1), output voltage
in 25mV increments is produced from 1.1V to 1.85V.
There is a built-in –12.5mV DC offset for the output
voltage.
Topside MOSFET Driver Supply (CB, DB)
Externalbootstrapcapacitors,CB,connectedtotheBOOST
pins, supply the gate drive voltages for the topside
MOSFETs. Capacitor CB in the Functional Diagram is
charged though diode DB from VCC when the SW pin is
low. When one of the topside MOSFETs turns on, the
driver places the CB voltage across the gate-source of the
desiredMOSFET.ThisenhancestheMOSFETandturnson
the topside switch. The switch node voltage, SW, rises to
VIN and the BOOST pin follows. With the topside MOSFET
IftheVRM10VIDtableisselected(Table2),outputvoltage
in 12.5mV increments is produced from 0.8375V to 1.6V.
There is a built-in –25mV DC offset for output voltage.
Active Voltage Position Control
TheLTC3738sensesinductorcurrentinformationthrough
monitoring voltage drops on the sense resistor RSENSE of
all three channels. The voltage drops are added together
and applied as VPRE-AVP between the AVP and IN+ pins,
which are connected through resistor RPRE-AVP. Then
VPRE-AVP is scaled through RAVP and added to output
voltage as the compensation for the load voltage drop. In
summary, the load slope is:
on, the boost voltage is above the input supply (VBOOST
=
VCC + VIN). The value of the boost capacitor CB needs to be
30 to 100 times that of the total input capacitance of the
topsideMOSFET(s).ThereversebreakdownofDB mustbe
greater than VIN(MAX).
Differential Amplifier
⎛
RAVP
RPRE−AVP
⎞
R
SENSE
•
V/A
⎜
⎟
⎝
⎠
The IC has a true remote voltage sense capability. The
sensing connections should be returned from the load,
back to the differential amplifier’s inputs through a
The recommended value for RAVP is 90Ω to 100Ω.
3738f
19
LTC3738
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APPLICATIO S I FOR ATIO
Thermal Control
Table 1. VRM9 VID Table
PROCESSOR PINS (0 = L0W, 1 = HIGH)
V
CC(CORE)
When external thermal detection is enabled, the TSNS pin
serves as the input to an accurate comparator which is
referenced to VCC/3 and has a hysteresis of VCC/24.
VR_HOTB is pulled low when the voltage at TSNS is less
than VCC/3. In this case, the input of TSNS is an analog
signal. If necessary, lowpass filter the signal before feed-
ing it into the pin to avoid a false thermal trip.
VID4
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
VID3
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
VID2
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
VID1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
VID0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
(V )
DC
Output Off
1.100
1.125
1.150
1.175
1.200
1.225
1.250
1.275
1.300
1.325
1.350
1.375
1.400
1.425
1.450
1.475
1.500
1.525
1.550
1.575
1.600
1.625
1.650
1.675
1.700
1.725
1.750
1.775
1.800
1.825
1.850
WhenVR_HOTBisreported,theoperationofLTC3738will
not be affected, although there is another thermal sensor
inside the IC for self protection. When the temperature of
the IC is around 140°C, the LTC3738 will shut down and
not start-up again until this overtemperature situation has
beenremoved. Thisselfshutdownfeatureisnottestedbut
is guaranteed by design.
ON/OFF Control
The OUTEN pin provides simple ON/OFF control for the
LTC3738. Driving the OUTEN pin above 0.8V permits the
controller to start operating. Pulling OUTEN below 0.4V
puts the LTC3738 into low current shutdown (IQ ≈ 50µA).
Soft-Start Function
The SS pin provides two functions: 1) soft-start and 2) a
defeatable short-circuit latch off timer. Soft-start reduces
the input power sources’ surge currents by gradually
increasingthecontroller’scurrentlimit(proportionaltoan
internal buffered and clamped VITH). The latchoff timer
prevents very short, extreme load transients from tripping
the overcurrent latch. A small pull-up current (>5µA)
supplied to the SS pin will prevent the overcurrent latch
from operating. The following explanation describes how
this function operates.
An internal 1.5µA current source charges up the CSS
capacitor. As the voltage on SS increases from 0V to 2.4V,
3738f
20
LTC3738
Table 2. VRM10 VID Table
PROCESSOR PINS (0 = LOW, 1 = HIGH)
V
PROCESSOR PINS (0 = LOW, 1 = HIGH)
V
OUT
OUT
(V)
(V)
VID4
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
VID3
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
VID2
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
VID1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
VID0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
VID5
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
VID4
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
VID3
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
VID2
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
VID1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
VID0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
VID5
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0.8375
0.8500
0.8625
0.8750
0.8875
0.9000
0.9125
0.9250
0.9375
0.9500
0.9625
0.9750
0.9875
1.0000
1.0125
1.0250
1.0375
1.0500
1.0625
1.0750
1.0875
Off*
1.2125
1.2250
1.2375
1.2500
1.2625
1.2750
1.2875
1.3000
1.3125
1.3250
1.3375
1.3500
1.3625
1.3750
1.3875
1.4000
1.4125
1.4250
1.4375
1.4500
1.4625
1.4750
1.4875
1.5000
1.5125
1.5250
1.5375
1.5500
1.5625
1.5750
1.5875
1.6000
Off*
1.1000
1.1125
1.1250
1.1375
1.1500
1.1625
1.1750
1.1875
1.2000
*Output disabled—same as deasserting the Output Enable input
the internal current limit is increased from 0V/RSENSE to
75mV/RSENSE. The output current limit ramps up slowly,
taking 1.6s/µF to reach full current. The output current
thus ramps up slowly, eliminating the starting surge
current required from the input power supply.
The SS pin has an internal 6V zener clamp (see the
Functional Diagram).
Fault Conditions: Overcurrent Latchoff
The SS pin also provides the ability to latch off the
controllerswhenanovercurrentconditionisdetected.The
SS capacitor is used initially to limit the inrush current of
2.4V – 0V
tIRAMP
=
CSS = 1.6s/µF C
SS
(
)
1.5µA
all three output stages. After the controllers have been
3738f
21
LTC3738
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APPLICATIO S I FOR ATIO
The value of the soft-start capacitor CSS may need to be
scaled with output current, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
given adequate time to charge up the output capacitor and
provide full load current, the SS capacitor is used for a
short-circuit timer. If the output voltage falls to less than
62.5% of its nominal value, the SS capacitor begins
discharging on the assumption that the output is in an
overcurrent condition. If the condition lasts for a long
enough period, as determined by the size of the SS
capacitor, the controller will be shut down until the RUN
pin voltage is recycled. If the overload occurs during start-
up, the time can be approximated by:
C
SS > (COUT )(VOUT) (10–4) (RSENSE
)
The minimum recommended soft-start capacitor of
CSS = 0.1µF will be sufficient for most applications.
Current Foldback
In certain applications, it may be desirable to defeat the
internal current foldback function. A negative impedance
is experienced when powering a switching regulator.
That is, the input current is higher at a lower VIN and
decreases as VIN is increased. Current foldback is de-
signed to accommodate a normal, resistive load having
increasing current draw with increasing voltage. The EAIN
pin should be artificially held 62.5% above its nominal
operating level of 0.6V, or 0.375V in order to prevent the
IC from “folding back” the peak current level. A suggested
circuit is shown in Figure 7.
t
LO1 >> (CSS • 0.6V)/(1.5µA) = 4 • 105 (CSS)
If the overload occurs after start-up, the voltage on the SS
capacitor will continue charging and will provide addi-
tional time before latching off:
t
LO2 >> (CSS • 3V)/(1.5µA) = 2 • 106 (CSS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor to the SS pin from VCC as
shown in Figure 6. When VCC is 5V, a 200k resistance will
prevent the discharge of the SS capacitor during an
overcurrent condition but also shortens the soft-start
period, so a larger SS capacitor value will be required.
The emitter of Q1 will hold up the EAIN pin to a voltage in
the absence of VOUT that will prevent the internal sensing
circuitry from reducing the peak output current. Remov-
ing the function in this manner eliminates the external
MOSFET’s protective feature under short-circuit condi-
tions. This technique will also prevent the short-circuit
latchoff function from turning off the part during a short-
circuit event and the output current will only be limited to
Why should you defeat overcurrent latchoff? During the
prototypingstageofadesign,theremaybeaproblemwith
noise pick-up or poor layout causing the protection circuit
to latch off the controller. Defeating this feature allows
troubleshooting of the circuit and PC layout. The internal
foldback current limiting still remains active, thereby
protecting the power supply system from failure. A deci-
sion can be made after the design is complete whether to
rely solely on foldback current limiting or to enable the
latchoff feature by removing the pull-up resistor.
N • 75mV/RSENSE
.
V
V
CC
CC
LTC3738
V
SS PIN
Q1
CC
R
SS
EAIN
CALCULATE FOR
0.375V TO 0.55V
3738 F07
C
SS
3738 F06
Figure 7. Foldback Current Elimination
Figure 6. Defeating Overcurrent Latchoff
3738f
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LTC3738
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APPLICATIO S I FOR ATIO
Undervoltage Reset
U
R
LP
10k
PHASE
DETECTOR/
OSCILLATOR
2.4V
In the event that the input power source to the IC (VCC)
drops below 4V, the SS capacitor will be discharged to
ground and the controller will be shut down. When VCC
rises above 4V, the SS capacitor will be allowed to re-
chargeandinitiateanothersoft-startturn-onattempt.This
may be useful in applications that switch between two
supplies that are not diode connected, but note that this
cannot make up for the resultant interruption of the
regulated output.
C
LP
EXTERNAL
OSC
PLLFLTR
OSC
FCB/SYNC
DIGITAL
PHASE/
FREQUENCY
DETECTOR
3738 F08
Phase-Locked Loop and Frequency Synchronization
Figure 8. Phase-Locked Loop Block Diagram
The IC has a phase-locked loop comprised of an internal
voltage controlled oscillator and phase detector. This
allows the top MOSFET of output stage 1’s turn-on to be
locked to the rising edge of an external source. The
frequency range of the voltage controlled oscillator is
±50% around the center frequency fO. A voltage applied to
the PLLFLTR pin of 1.2V corresponds to a frequency of
approximately 350kHz. The nominal operating frequency
range of the IC is 210kHz to 530kHz.
stable operating point, the phase comparator output is
open and the filter capacitor CLP holds the voltage. The IC
FCB/SYNC pin must be driven from a low impedance
source such as a logic gate located close to the pin. When
usingmultipleICsforaphase-lockedsystem,thePLLFLTR
pin of the master oscillator should be biased at a voltage
that will guarantee the slave oscillator(s) ability to lock
onto the master’s frequency. A voltage of 1.7V or below
applied to the master oscillator’s PLLFLTR pin is recom-
mended in order to meet this requirement. The resultant
operating frequency will be approximately 500kHz for
1.7V.
The phase detector used is an edge sensitive digital type
that provides zero degrees phase shift between the
external and internal oscillators. This type of phase
detector will not lock the internal oscillator to harmonics
of the input frequency. The PLL hold-in range, ∆fH, is
equal to the capture range, ∆fC:
The loop filter components (CLP, RLP) smooth out the
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
components CLP and RLP determine how fast the loop
acquires lock. Typically RLP =10k and CLP ranges from
0.01µF to 0.1µF.
∆fH = ∆fC = ±0.5 fO
The output of the phase detector is a complementary pair
of current sources charging or discharging the external
filter components on the PLLFLTR pin. A simplified block
diagram is shown in Figure 8.
Minimum On-Time Considerations
If the external frequency (fPLLIN) is greater than the oscil-
lator frequency, fOSC, current is sourced continuously,
pulling up the PLLFLTR pin. When the external frequency
is less than fOSC, current is sunk continuously, pulling
down the PLLFLTR pin. If the external and internal fre-
quencies are the same, but exhibit a phase difference, the
currentsourcesturnonforanamountoftimecorrespond-
ing to the phase difference. Thus, the voltage on the
PLLFLTR pin is adjusted until the phase and frequency of
the external and internal oscillators are identical. At this
Minimum on-time, tON(MIN), is the smallest time duration
that the IC is capable of turning on the top MOSFET. It is
determined by internal timing delays and the gate charge
of the top MOSFET. Low duty cycle applications may
approach this minimum on-time limit and care should be
taken to ensure that:
VOUT
tON MIN
<
(
)
V f
IN( )
3738f
23
LTC3738
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APPLICATIO S I FOR ATIO
Ifthedutycyclefallsbelowwhatcanbeaccommodatedby
the minimum on-time, the IC will begin to skip every other
cycle, resulting in half-frequency operation. The output
voltage will continue to be regulated, but the ripple current
and ripple voltage will increase.
time, VOUT can be monitored for excessive overshoot or
ringing, which would indicate a stability problem. The
availability of the ITH pin not only allows optimization of
control loop behavior, but also provides a DC coupled
and AC filtered closed-loop response test point. The DC
step, risetimeandsettlingatthistestpointtrulyreflects
the closed-loop response. Assuming a predominantly
second order system, phase margin and/or damping
factor can be estimated using the percentage of overshoot
seen at this pin. The bandwidth can also be estimated by
examining the rise time at the pin. The ITH external com-
ponents shown in the Figure 1 circuit will provide an
adequate starting point for most applications.
The minimum on-time for the IC is generally about 120ns.
However, as the peak sense voltage decreases the mini-
mum on-time gradually increases. This is of particular
concern in forced continuous applications with low ripple
current at light loads. If the duty cycle drops below the
minimum on-time limit in this situation, a significant
amount of cycle skipping can occur with correspondingly
larger current and voltage ripple.
The ITH series RC-CC filter sets the dominant pole-zero
loop compensation. The values can be modified slightly
(from 0.2 to 5 times their suggested values) to maximize
transient response once the final PC layout is done and the
particular output capacitor type and value have been
determined. The output capacitors need to be decided
upon because the various types and values determine the
loop feedback factor gain and phase. An output current
pulse of 20% to 80% of full load current having a rise time
of<2µswillproduceoutputvoltageandITH pinwaveforms
that will give a sense of the overall loop stability without
breakingthefeedbackloop. Theinitialoutputvoltagestep,
resulting from the step change in output current, may not
bewithinthebandwidthofthefeedbackloop,sothissignal
cannot be used to determine phase margin. This is why it
is better to look at the ITH pin signal which is in the
feedback loop and is the filtered and compensated control
loop response. The gain of the loop will be increased by
increasing RC and the bandwidth of the loop will be
increased by decreasing CC. If RC is increased by the same
factor that CC is decreased, the zero frequency will be kept
the same, thereby keeping the phase the same in the most
critical frequency range of the feedback loop. The output
voltage settling behavior is related to the stability of the
closed-loop system and will demonstrate the actual over-
all supply performance.
If an application can operate close to the minimum on-
timelimit, aninductormustbechosenthatislowenough
in value to provide sufficient ripple amplitude to meet the
minimum on-time requirement. As a general rule, keep
the inductor ripple current equal to or greater than 30%
of IOUT(MAX) at VIN(MAX)
.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
whereL1, L2, etc. aretheindividuallossesasapercentage
of input power.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in DC (resistive) load
current. When a load step occurs, VOUT shifts by an
amount equal to ∆ILOAD • ESR, where ESR is the effective
series resistance of COUT. ∆ILOAD also begins to charge or
discharge COUT, generating the feedback error signal that
forces the regulator to adapt to the current change and
return VOUT to its steady-state value. During this recovery
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
dischargedbypasscapacitorsareeffectivelyputinparallel
with COUT, causing a rapid drop in VOUT. No regulator can
3738f
24
LTC3738
W U U
APPLICATIO S I FOR ATIO
U
Next verify the minimum on-time is not violated. The
minimum on-time occurs at maximum VCC:
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistanceislowanditisdrivenquickly. IfCLOAD isgreater
than2%ofCOUT , theswitchrisetimeshouldbecontrolled
so that the load rise time is limited to approximately
1000 •RSENSE • CLOAD. Thusa 250µF capacitor and a2mΩ
RSENSE resistor would require a 500µs rise time, limiting
the charging current to about 1A.
VOUT
IN(MAX)( )
1.3V
20V 400kHz
tON MIN
=
=
= 162ns
(
)
V
f
(
)
The output voltage will be set by the VID code according
to Table 1.
The power dissipation on the topside MOSFET can be
estimated.UsingaSiliconixSi7390DPforexample,RDS(ON)
= 13.5mΩ, CMILLER = 2.1nC/15V = 140pF. At maximum
input voltage with T(estimated) = 50°C:
Design Example (Using Three Phases)
As a design example, assume VIN = 12V(nominal), VIN =
20V(max), VOUT = 1.3V, IMAX = 45A, f = 400kHz and the
AVP slope is 1mV/A. The inductance value is chosen first
based upon a 30% ripple current assumption. The highest
value of ripple current in each output stage occurs at the
maximum input voltage.
1.3V
20V
2
PMAIN
≈
15 1+ 0.005 50°C − 25°C
( )
(
)(
)
]
[
⎛
⎞
45A
2 3
( )( )
2
0.0135Ω + 20
2Ω 140pF
)( )
( )
(
⎜
⎟
⎝
⎠
VOUT
⎛
VOUT
V
IN
⎞
L =
1−
1
1
⎛
⎜
⎝
⎞
⎠
⎜
⎟
+
400kHz = 0.51W
f ∆I ⎝
( )
⎠
(
)
⎟
5V – 1.8V 1.8V
1.3V
1.3V
20V
⎛
⎜
⎝
⎞
⎟
⎠
=
1−
using a Siliconix Si7356DP as bottom side MOSFET.
400kHz 30% 15A
(
)( )(
)
The worst-case power dissipation by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction tem-
perature rise is:
≥ 0.68µH
Using L = 0.6µH, a commonly available value results in
34% ripple current. The worst-case output ripple for the
three stages operating in parallel will be less than 11% of
the peak output current.
20V − 1.3V
2
PSYNC
=
15A 1.25 0.004Ω = 1.05W
) ( )(
(
)
20V
RSENSE1, RSENSE2 and RSENSE3 can be calculated by using
aconservativemaximumsensecurrentthresholdof65mV
and taking into account half of the ripple current:
Ashortcircuittogroundwillresultinafoldedbackcurrent
of:
⎛
⎞
150ns 20V
25mV
1
(
)
ISC
≈
+
= 7.5A
65mV
⎜
⎟
2 + 3 mΩ 2
0.6µH
(
)
⎝
⎠
RSENSE
=
= 0.0037Ω
34%
2
⎛
⎝
⎞
⎟
⎠
15A 1+
⎜
with a typical value of RDS(ON) and d = (0.005/°C)(50°C) =
0.25.TheresultingpowerdissipatedinthebottomMOSFET
is:
Use a commonly available 0.003Ω sense resistor.
PSYNC = (7.5A)2(1.25)(0.004Ω) ≈ 0.28W
Take RAVP as recommended value 100Ω, the RPREAVP is:
which is less than one third of the normal, full load
conditions. Incidentally, since the load no longer dissi-
pates any power, total system power is decreased by over
90%. Therefore, the system actually cools significantly
100Ω
1mV/A
RPREAVP = 0.003Ω
= 300Ω
during a shorted condition!
3738f
25
LTC3738
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APPLICATIO S I FOR ATIO
PC Board Layout Checklist
6) The filter capacitors between the ITH and SGND pins
should be as close as possible to the pins of the IC.
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
IC.Theseitemsarealsoillustratedgraphicallyinthelayout
diagram of Figure 9. Check the following in the PC layout:
Figure 9 illustrates all branch currents in a three-phase
switching regulator. It becomes very clear after studying
the current waveforms why it is critical to keep the high
switchingcurrentpathstoasmallphysicalsize. Highelec-
tricandmagneticfieldswillradiatefromthese“loops”just
as radio stations transmit signals. The output capacitor
ground should return to the negative terminal of the input
capacitor and not share a common ground path with any
switchedcurrentpaths.Thelefthalfofthecircuitgivesrise
to the “noise” generated by a switching regulator. The
ground terminations of the synchronous MOSFETs and
Schottkydiodesshouldreturntothebottomplate(s)ofthe
inputcapacitor(s)withashortisolatedPCtracesincevery
highswitchedcurrentsarepresent.Aseparateisolatedpath
fromthebottomplate(s)oftheinputandoutputcapacitor(s)
should be used to tie in the IC power ground pin (PGND).
This technique keeps inherent signals generated by high
current pulses taking alternate current paths that have
finite impedances during the total period of the switching
regulator.ExternalOPTI-LOOPcompensationallowsover-
compensation for PC layouts which are not optimized but
this is not the recommended design procedure.
1)Arethesignalandpowergroundpathsisolated?Keepthe
SGND at one end of a printed circuit path thus preventing
MOSFETcurrentsfromtravelingundertheIC.TheICsignal
ground pin should be used to hook up all control circuitry
on one side of the IC, routing the copper through SGND,
undertheICcoveringthe“shadow”ofthepackage,connect-
ingtothePGNDpinandthencontinuingontothe(–)plates
of CIN and COUT. The VCC decoupling capacitor should be
placed immediately adjacent to the IC between the VCC pin
and PGND. A 1µF ceramic capacitor of the X7R or X5R type
is small enough to fit very close to the IC to minimize the ill
effectsofthelargecurrentpulsesdrawntodrivethebottom
MOSFETs. An additional 5µF to 10uF of ceramic, tantalum
or other very low ESR capacitance is recommended in or-
der to keep the internal IC supply quiet. The power ground
returns to the sources of the bottom N-channel MOSFETs,
anodes of the Schottky diodes and (–) plates of CIN, which
should have as short lead lengths as possible.
2) Does the IC IN+ pin connect to the (+) plates of COUT
?
A 30pF to 300pF feedforward capacitor between the
DIFFOUT and EAIN pins should be placed as close as
possible to the IC.
Simplified Visual Explanation of How a 3-Phase
Controller Reduces Both Input and Output RMS
Ripple Current
3) Are the SENSE– and SENSE+ printed circuit traces for
each channel routed together with minimum PC trace
spacing?ThefiltercapacitorsbetweenSENSE+andSENSE–
for each channel should be as close as possible to the pins
of the IC. Connect the SENSE– and SENSE+ pins to the
pads of the sense resistor as illustrated in Figure 10.
Theeffectofmultiphasepowersupplydesignsignificantly
reduces the amount of ripple current in both the input and
output capacitors. The RMS input ripple current is divided
by, and the effective ripple frequency is multiplied up by
the number of phases used (assuming that the input
voltage is greater than the number of phases used times
the output voltage). The output ripple amplitude is also
reduced by,andtheeffectiveripplefrequency isincreased
by the number of phases used. Figure 11 graphically
illustrates the principle.
4) Do the (+) plates of CIN connect to the drains of the
topside MOSFETs as closely as possible? This capacitor
provides the pulsed current to the MOSFETs.
5) Keep the switching nodes, SWITCH, BOOST and TG
away from sensitive small-signal nodes. Ideally the
SWITCH, BOOST and TG printed circuit traces should be
routedawayandseparatedfromtheICandthe“quiet”side
of the IC.
Theworst-caseinputRMSripplecurrentforasinglestage
design peaks at twice the value of the output voltage. The
worst-case input RMS ripple current for a two stage
design results in peaks at 1/4 and 3/4 of the input voltage,
3738f
26
LTC3738
W U U
APPLICATIO S I FOR ATIO
U
L1
SW1
R
SENSE1
D1
L2
V
V
OUT
IN
SW2
R
SENSE2
R
IN
+
+
R
L
D2
C
OUT
C
IN
BOLD LINES INDICATE HIGH,
SWITCHING CURRENT LINES.
KEEP LINES TO A MININMUM
LENGTH
L3
SW3
R
SENSE3
D3
3738 F09
Figure 9. Branch Current Waveforms
INDUCTOR
LTC3738
SENSE
+
SENSE
1000pF
RESISTOR
–
SENSE
3738 F10
OUTPUT CAPACITOR
Figure 10. Kelvin Sensing RSENSE
3738f
27
LTC3738
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APPLICATIO S I FOR ATIO
SINGLE PHASE
subtract current from the (VCC – VOUT)/L charging current
resultingfromthestagewhichhasitstopMOSFETon. The
output ripple current for a 3-phase design is:
SW V
I
CIN
VOUT
f L
( )( )
IP-P
=
1– 3DC V > 3VOUT
IN
(
)
I
COUT
The ripple frequency is also increased by three, further
reducingtherequiredoutputcapacitancewhenVCC<3VOUT
as illustrated in Figure 3.
TRIPLE PHASE
SW1 V
SW2 V
SW3 V
Efficiency Calculation
To estimate efficiency, the DC loss terms include the input
and output capacitor ESR, each MOSFET RDS(ON), induc-
tor resistance RL, the sense resistance RSENSE and the
forward drop of the Schottky rectifier at the operating
output current and temperature. Typical values for the
design example given previously in this data sheet are:
I
L1
I
L2
I
L3
I
CIN
I
Main MOSFET RDS(ON) = 13.5mΩ (18mΩ at 90°C)
COUT
3738 F11
Sync MOSFET RDS(ON) = 4mΩ (5.3mΩ at 90°C)
CINESR = 20mΩ
COUTESR = 3mΩ
RL = 2mΩ
RSENSE = 3mΩ
VSCHOTTKY = 0.8V at 15A (0.7V at 90°C)
VOUT = 1.3V
VIN = 12V
IMAX = 45A
δ = 0.5%°C
Figure 11. Single and Polyphase Current Waveforms
and the worst-case input RMS ripple current for a three
stage design results in peaks at 1/6, 1/2, and 5/6 of the
input voltage. The peaks, however, are at ever decreasing
levels with the addition of more phases. A higher effective
duty factor results because the duty factors “add” as long
as the currents in each stage are balanced. Refer to AN19
for a detailed description of how to calculate RMS current
for the single stage switching regulator.
Figure 5 illustrates the RMS input current drawn from the
input capacitance versus the duty cycle as determined by
the ration of input and output voltage. The peak input RMS
current level of the single phase system is reduced by 2/3
in a 3-phase solution due to the current splitting between
the three stages.
N = 3
f = 400kHz
The main MOSFET is on for the duty factor VOUT/VIN and
the synchronous MOSFET is on for the rest of the period
or simply (1 – VOUT/VIN). Assuming the ripple current is
small, the AC loss in the inductor can be made small if a
good quality inductor is chosen. The average current,
OUT is used to simplify the calaculations. The equation
below is not exact but should provide a good technique
for the comparison of selected components and give a
The output ripple current is reduced significantly when
compared to the single phase solution using the same
inductance value because the VOUT/L discharge currents
term from the stages that has their bottom MOSFETs on
I
3738f
28
LTC3738
W U U
APPLICATIO S I FOR ATIO
U
This totals 0.14W at VIN = 8V, 0.315W at VIN = 12V and
0.875W at VIN = 20V.
result that is within 10% to 20% of the final application.
The temperature of the MOSFET’s die temperature may
require interative calculations if one is not familiar typical
performance. A maximum operating junction tempera-
ture of 90° to 100°C for the MOSFETs is recommended
for high reliability applications.
Total of all three synchronous MOSFET’s AC loss:
V
V
IN
VDSSPEC
IN
(3)QG
(f) = (3)(16nC)
(400kHz)
VDSSPEC
Common output path DC loss:
This totals 0.085W at VIN = 8V, 0.128W at VIN = 12V and
0.213W at VIN = 20V. The bottom MOSFET does not
experience the Miller capacitance dissipation issue that
the main switch does because the bottom switch turns on
when its drain is close to ground.
2
I
⎛
⎝
⎞
⎟
⎠
MAX
N
PCOMPATH ≈ N
R +R
+ COUTESR Loss
(
)
⎜
L
SENSE
This totals 3.375W + COUTESR loss.
The Schottky rectifier loss assuming 50ns nonoverlap
time:
Total of all three main MOSFET’s DC loss:
2
⎛ VOUT ⎞ I
2 • 3(0.7V)(15A)(50ns)(400kHz)
This totals 1.26W.
⎛
⎜
⎝
⎞
⎟
⎠
MAX
N
PMAIN = N
1+ δ RDS(ON) + CINESR Loss
(
)
⎜
⎟
⎝ V
⎠
IN
This totals 1.3W + CINESR loss.
The total output power is (1.3V)(45A) = 58.5W and the
total input power is approximately 67W so the % loss of
each component is as follows:
Total of all three synchronous MOSFET’s DC loss:
2
Main switch AC loss (VIN = 12V) 0.315W 0.47%
⎛
VOUT ⎞ I
⎛
MAX
⎞
⎟
⎠
PSYNC = N 1–
1+ δ R
DS(ON)
(
)
⎜
⎟
⎜
⎝
⎝
V
IN
⎠
N
Main switch DC loss
1.3W
0.128W 0.2%
3.2W 4.8%
3.375W 5.05%
1.9%
Synchronous switch AC loss
Synchronous switch DC loss
Power path loss
This totals 3.2W.
Total of all three main MOSFET’s AC loss:
45A
(2)(3)
P
MAIN ≈ 3(V )2
(2Ω)(140pF)
IN
The numbers above represent the values at VIN = 12V.
1
1
⎛
⎜
⎝
⎞
⎟
⎠
+
(400kHz) = 6.3W
5V – 1.8V 1.8V
3738f
29
LTC3738
U
TYPICAL APPLICATIO
65A Power Supply for VRM10
V
CC
5V
47k
PGOOD
VID0 IN
VID1 IN VID5 IN
1Ω
VID2 IN
V
ON/0FF
CC
100pF
0.1µF
51k
V
V
OUTEN VID2 VID1 VID0 VID5 PGOOD BOOST1
IN
V
OUT
FCB/SYNC
TG1
M1
M2
L1
L2
0.002Ω
10k
PLLFLTR
SW1
+
10µF
6.3V
×3
V
CC
R
–
AVP
C
C
D1
IN
BOOST2
TG2
OUT
100Ω
+
0.1µF
S1
S2
–
–
+
S1
IN
AVP
SW2
30pF
IN
R
220Ω
V
IN
7V TO 21V
PREAVP
10Ω ×6
EAIN
V
M3
M4
CC
+
0.002Ω
10µF
35V
×5
LTC3738
(EXPOSED PAD IS SGND)
IN
68µF
+
+
S1
SENSE1
BG1
PGND
BG2
1µF
10µF
1000pF
1000pF
1000pF
25V
–
+
–
S1
S2
D2
SENSE1
+
+
S2
SENSE2
–
–
+
–
S2
S3
S3
SENSE2
BG3
–
SENSE3
SW3
V
IN
+
M5
M6
L3
SENSE3
TG3
0.002Ω
3738 TA02
SS
I
TSNS VR_HOTB VID3 VID4 BOOST3
0.1µF
TH
D3
0.1µF
100pF
200Ω
V
VID4 IN
+
–
CC
S3
S3
VID3 IN
2.2k
V
CC
V
CC
2200pF
V
V
: 7V TO 21V
OUT
SWITCHING FREQUENCY: 300kHz
C
C
: SANYO OS-CON 25SP68M
L1 TO L3: 0.6µH PULSE PG0006.601 OR TOKO FDA1055 0.56µH
M1, M3, M5: Si7390DP ×1 OR HAT2168H ×1
M2, M4, M6: Si7356DP ×2 OR HAT2165H ×2
IN
IN
: 0.8V TO 1.55V, 65A
: 330µF/2.5V ×10 SANYO POSCAP 2R5TPE330M9
OUT
D1 TO D3: MBRS340T3
Block Diagram—6-Phase LTC3731/LTC3738 Supply
3-PHASE LTC3731
CLKOUT
V
OUT
CLK
60°
V
IN
VRM9/VRM10
90A TO 120A
FCB/SYNC
3-PHASE LTC3738
3738 TA03
3738f
30
LTC3738
U
PACKAGE DESCRIPTIO
UHF Package
38-Lead Plastic QFN (7mm × 5mm)
(Reference LTC DWG # 05-08-1701)
0.70 ± 0.05
5.50 ± 0.05
(2 SIDES)
4.10 ± 0.05
(2 SIDES)
3.20 ± 0.05
(2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
5.20 ± 0.05 (2 SIDES)
6.10 ± 0.05 (2 SIDES)
7.50 ± 0.05 (2 SIDES)
RECOMMENDED SOLDER PAD LAYOUT
3.15 ± 0.10
(2 SIDES)
0.75 ± 0.05
5.00 ± 0.10
(2 SIDES)
0.435 0.18
0.18
37 38
0.00 – 0.05
PIN 1
TOP MARK
(SEE NOTE 6)
1
2
0.23
5.15 ± 0.10
(2 SIDES)
7.00 ± 0.10
(2 SIDES)
0.40 ± 0.10
0.200 REF 0.25 ± 0.05
0.50 BSC
R = 0.115
TYP
(UH) QFN 0303
0.200 REF
0.00 – 0.05
0.75 ± 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE
OUTLINE M0-220 VARIATION WHKD
2. DRAWING NOT TO SCALE
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3. ALL DIMENSIONS ARE IN MILLIMETERS
3738f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
31
LTC3738
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
Reduces C and C , Power Good Output Signal, Synchronizable,
LTC1628/LTC1628-PG/ 2-Phase, Dual Output Synchronous Step-Down
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up to 20A, 0.8V ≤ V
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Mobile Pentium® III Processors, 550kHz,
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LT®1709/
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High Efficiency, 2-Phase Synchronous Step-Down
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Accurate Current Sharing, 3.5V ≤ V ≤ 36V
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High Efficiency Synchronous Step-Down
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Output Fault Protection, 16-Pin SSOP
High Efficiency Synchronous Controller with 5-Bit Mobile Output Fault Protection, 24-Pin SSOP,
VID Control 3.5V ≤ V ≤ 36V
IN
No R Current Mode Synchronous Step-Down
Up to 97% Efficiency, 4V ≤ V ≤ 36V, 0.8V ≤ V
≤ (0.9)(V ),
OUT IN
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IN
Controller
I
up to 20A
OUT
LTC1929/
LTC1929-PG
2-Phase Synchronous Controllers
Up to 42A, Uses All Surface Mount Components,
No Heat Sinks, 3.5V ≤ V ≤ 36V
IN
LTC3708
2-Phase, Dual DC/DC Synchronous Buck Controller with Tracks Two or More Supplies, Fast Transient Response, No R
Output Tracking
SENSE
LTC3711
No R
Current Mode Synchronous Step-Down
Up to 97% Efficiency, Ideal for Pentium III Processors,
0.925V ≤ V ≤ 2V, 4V ≤ V ≤ 36V, I up to 20A
SENSE
Controller with Digital 5-Bit Interface
OUT
IN
OUT
LTC3717
LTC3719
DDR/QDR Memory Termination Regulator
V
= 0.5V , ±20A, ±0.65% V
Accuracy
OUT
IN
OUT
2-Phase, 5-Bit VID Current Mode, High Efficiency
Synchronous Step-Down Controller
AMD Hammer-K8 Processors, Wide V Range: 4V to 36V Operation
IN
LTC3729
LTC3731
LTC3732
LTC3733
LTC3734
LTC3735
20A to 200A, 550kHz PolyPhase Synchronous Controller Expandable from 2-Phase to 12-Phase, Uses all Surface Mount
Components, V up to 36V
IN
3-Phase, 600kHz Synchronous Buck
Switching Regulator Controller
Expandable from 3-Phase to 12-Phase, Uses all Surface Mount
Components, V up to 36V
IN
3-Phase, 5-Bit VID, 600kHz Synchronous Buck
Switching Regulator Controller
VRM9.0 and VRM9.1 (VID = 1.1V to 1.85V)
3-Phase, 5-Bit VID, 600kHz Synchronous Buck
Switching Regulator Controller
AMD OpteronTM (VID = 0.8V to 1.55V)
Single Phase DC/DC Controller for Intel IMVP-4
Compatible Processors
6-Bit IMVP-4 VID: 0.7V ≤ V
≤ 1.708V, 4V ≤ V ≤ 30V, I ≤ 25A,
OUT
OUT
IN
Lossless Voltage Positioning
2-Phase DC/DC Controller for Pentium (Centrino)
Processors
40A, 0.7V ≤ V ≤1.708V, 4V ≤ V ≤ 30V
OUT
IN
No R
is a trademark of Linear Technology Corporation. Pentium is a registered trademark of Intel Corporation.
SENSE
Opteron is a trademark of AMD Corporation.
3738f
LT/TP 0604 1K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
32
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
©LINEAR TECHNOLOGY CORPORATION 2004
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