LTC3608 [Linear]
18V, 8A Monolithic Synchronous Step-Down DC/DC Converter; 18V ， 8A单片同步降压型DC / DC转换器
| 型号： | LTC3608 |
| 厂家： | 
Linear |
| 描述： | 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter |
| 文件： | 总24页 (文件大小：371K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3608
18V, 8A Monolithic
Synchronous Step-Down
DC/DC Converter
DESCRIPTION
FEATURES
TheLTC®3608isahighefficiency,monolithicsynchronous
step-downDC/DCconverterthatcandeliverupto8Aoutput
current from a 4V to 18V (20V maximum) input supply. It
uses a valley current control architecture to deliver very
low duty cycle operation at high frequency with excellent
transient response. The operating frequency is selected
by an external resistor and is compensated for variations
n
8A Output Current
n
Wide V Range = 4V to 18V
IN
n
Internal N-Channel MOSFETs
True Current Mode Control
Optimized for High Step-Down Ratios
n
n
n
t
≤ 100nsec
ON(MIN)
n
n
n
n
n
n
n
n
n
n
n
Extremely Fast Transient Response
Stable with Ceramic C
in V and V
.
OUT
IN
OUT
1ꢀ 0ꢁ.V Voltage Reference
Power Good Output Voltage Monitor
Adjustable On-Time/Switching Frequency
Adjustable Current Limit
Programmable Soft-Start
Output Overvoltage Protection
Optional Short-Circuit Shutdown Timer
The LTC3608 can be configured for discontinuous or
forced continuous operation at light load. Forced continu-
ous operation reduces noise and RF interference while
discontinuous mode provides high efficiency by reducing
switching losses at light loads.
Fault protection is provided by internal foldback current
limiting,anoutputovervoltagecomparatorandanoptional
short-circuit shutdown timer. Soft-start capability for sup-
ply sequencing is accomplished using an external timing
capacitor.Theregulatorcurrentlimitisuserprogrammable.
A power good output voltage monitor indicates when
the output is in regulation. The LTC3608 is available in a
compact 7mm × 8mm QFN package.
Low Shutdown I : 15μA
Available in a 7mm × 8mm 52-Lead QFN Package
Q
APPLICATIONS
n
Point of Load Regulation
Distributed Power Systems
n
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents including
5481178, 6100678, 6580258, 5847554, 6304066.
TYPICAL APPLICATION
High Efficiency Step-Down Converter
187k
Efficiency and Power Loss
vs Load Current
V
OUT
V
I
0.1μF
ON
ON
100
95
90
85
80
75
70
65
60
55
50
10000
1000
100
10
V
IN
4V TO 18V
RUN/SS
V
IN
EFFICIENCY
10μF
×3
100pF
LTC3608
0.8μH
V
2.5V
8A
OUT
SW
1500pF
11.3k
0.22μF
100μF
×2
I
TH
BOOST
POWER LOSS
SGND INTV
CC
30.1k
9.53k
FCB
V
OUT
EXTV = 5V
= 12V
IN
V
= 2.5V
4.7μF
V
RNG
CC
PGND
1
PGOOD
0.01
0.1
1
10
EXTV
CC
V
FB
LOAD CURRENT (A)
3608 TA01b
3608 TA01a
3608fa
1
LTC3608
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
Input Supply Voltage (SV , PV , I )....... 20V to –0.3V
IN
IN ON
Boosted Topside Driver Supply Voltage
(BOOST) ................................................ 26V to –0.3V
SW Voltage............................................... 20V to –5V
PV
PV
PV
PV
PV
PV
PV
1
2
3
4
5
6
7
40 PGND
39 PGND
38 PGND
37 PGND
36 PGND
35 PGND
34 PGND
33 SW
IN
IN
IN
IN
IN
IN
IN
INTV , EXTV , (BOOST – SW), RUN/SS,
CC
CC
53
PV
PGOOD Voltages...................................... 7V to –0.3V
55
SW
IN
FCB, V , V
TH FB
Operating Junction Temperature Range
Voltages............ INTV + 0.3V to –0.3V
ON RNG
CC
I , V Voltages....................................... 2.7V to –0.3V
SW 8
NC 9
32 INTV
31 INTV
CC
CC
(Notes 2, 4)........................................ –40°C to 125°C
Storage Temperature Range................... –55°C to 125°C
SGND 10
BOOST 11
RUN/SS 12
30 SV
IN
54
SGND
29 EXTV
28 NC
CC
V
13
ON
SGND 14
27 SGND
WKG PACKAGE
52-LEAD (7mm × 8mm) QFN MULTIPAD
T
= 125°C, θ = 29°C/W
JA
JMAX
ORDER INFORMATION
LEAD FREE FINISH
LTC3608EWKG#PBF
LTC3608IWKG#PBF
TAPE AND REEL
PART MARKING*
LTC3608WKG
LTC3608WKG
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to 125°C
LTC3608EWKG#TRPBF
LTC3608IWKG#TRPBF
52-Lead (7mm × 8mm) Plastic QFN
52-Lead (7mm × 8mm) Plastic QFN
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
3608fa
2
LTC3608
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°Cꢁ VIN = 15V unless otherwise notedꢁ
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loop
SV
Operating Input Voltage Range
4
18
V
IN
I
Input DC Supply Current
Normal
Q
900
15
2000
30
μA
μA
Shutdown Supply Current
V
Feedback Reference Voltage
I
TH
I
TH
= 1.2V, –40°C to 85°C (Note 3)
= 1.2V, –40°C to 125°C (Note 3)
0.594
0.590
0.600
0.600
0.606
0.610
V
V
FB
l
ΔV
ΔV
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
Feedback Input Current
V
= 4V to 18V, I = 1.2V (Note 3)
0.002
–0.05
–5
%/V
%
FB(LINEREG)
FB(LOADREG)
IN
TH
I
TH
= 0.5V to 1.9V (Note 3)
= 0.6V
–0.3
50
I
FB
V
nA
mS
V
FB
l
l
g
Error Amplifier Transconductance
Forced Continuous Threshold
Forced Continuous Pin Current
On-Time
I
TH
= 1.2V (Note 3)
1.4
1.7
2
m(EA)
V
0.54
0.6
0.66
–2
FCB
I
t
V
= 0.6V
FCB
–1
μA
FCB
ON
I
ON
I
ON
= 60μA, V = 1.5V
220
280
110
340
ns
ns
ON
= 60μA, V = 0V
ON
t
t
I
Minimum On-Time
I
I
= 180μA, V = 0V
60
100
500
ns
ns
ON(MIN)
ON
ON
ON
Minimum Off-Time
Maximum Valley Current
= 30μA, V = 1.5V
320
OFF(MIN)
ON
l
l
V
V
= 0.5V, V = 0.56V, FCB = 0V
5
8
11
16
A
A
VALLEY(MAX)
RNG
RNG
FB
= 0V, V = 0.56V, FCB = 0V
FB
I
Maximum Reverse Valley Current
V
RNG
V
RNG
= 0.5V, V = 0.64V, FCB = 0V
3.5
5.5
5.5
7.5
7.5
9.5
A
A
VALLEY(MIN)
FB
= 0V, V = 0.64V, FCB = 0V
FB
ΔV
Output Overvoltage Fault Threshold
RUN Pin Start Threshold
7
10
1.5
4
13
2
%
V
FB(OV)
l
V
V
V
0.8
RUN/SS(ON)
RUN/SS(LE)
RUN/SS(LT)
RUN/SS(C)
RUN/SS(D)
RUN Pin Latchoff Enable Threshold
RUN Pin Latchoff Threshold
Soft-Start Charge Current
RUN/SS Pin Rising
RUN/SS Pin Falling
4.5
4.2
–3
3
V
3.5
–1.2
1.8
3.4
3.5
V
I
I
V
V
= 0V
–0.5
0.8
μA
μA
V
RUN/SS
RUN/SS
Soft-Start Discharge Current
Undervoltage Lockout
= 4.5V, V = 0V
FB
l
l
V
V
INTV Falling
3.9
4
IN(UVLO)
CC
Undervoltage Lockout Release
INTV Rising
V
IN(UVLOR)
CC
R
Top Switch On-Resistance
Bottom Switch On-Resistance
10
8
19
14
mΩ
mΩ
DS(ON)
Internal V Regulator
CC
l
l
V
Internal V Voltage
6V < V < 18V, V = 4V
EXTVCC
4.7
4.5
5
5.5
2
V
%
INTVCC
CC
IN
ΔV
Internal V Load Regulation
I
CC
I
CC
I
CC
= 0mA to 20mA, V = 4V
EXTVCC
–0.1
4.7
LDO(LOADREG)
CC
V
EXTV Switchover Voltage
= 20mA, V
= 20mA, V
Rising
= 5V
V
EXTVCC
CC
EXTVCC
EXTVCC
ΔV
ΔV
EXTV Switch Drop Voltage
150
500
300
mV
mV
EXTVCC
CC
EXTV Switchover Hysteresis
EXTVCC(HYS)
CC
PGOOD Output
ΔV
ΔV
ΔV
PGOOD Upper Threshold
PGOOD Lower Threshold
PGOOD Hysteresis
V
V
V
Rising
7
10
–10
1
13
–13
2.5
0.4
%
%
%
V
FBH
FB
Falling
–7
FBL
FB
Returning
FB(HYS)
FB
V
PGOOD Low Voltage
I
= 5mA
0.15
PGL
PGOOD
3608fa
3
LTC3608
ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 3: The LTC3608 is tested in a feedback loop that adjusts V to
FB
achieve a specified error amplifier output voltage (I ). The specification at
TH
85°C is not tested in production. This specification is assured by design,
characterization, and correlation to testing at 125°C.
Note 2: T is calculated from the ambient temperature T and power
Note 4: The LTC3608E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls. The LTC3608I is guaranteed
over the full –40°C to 125°C operating junction temperature range.
J
A
dissipation P as follows:
D
LTC3608: T = T + (P • 29°C/W)(θ is simulated per JESD51-7 high
J
A
D
JA
effective thermal conductivity test board)
= 1°C/W (θ is simulated when heat sink is applied at the bottom
θ
JC
JC
of the package.)
TYPICAL PERFORMANCE CHARACTERISTICS
Transient Response
Transient Response
Start-Up
V
V
OUT
OUT
200mV/DIV
200mV/DIV
RUN/SS
2V/DIV
I
L
I
L
5A/DIV
5A/DIV
V
OUT
1V/DIV
I
LOAD
I
LOAD
I
L
5A/DIV
5A/DIV
5A/DIV
3608 G03
3608 G01
3610 G02
40ms/DIV
20μs/DIV
20μs/DIV
V
V
= 12V
LOAD STEP 0A TO 8A
IN
OUT
V
V
= 12V
IN
OUT
= 2.5V
= 0.5Ω
V
V
= 12V
= 2.5V
IN
OUT
R
= 2.5V
LOAD
FCB = INTV
CC
FIGURE 6 CIRCUIT
FCB = 0V
FIGURE 6 CIRCUIT
FIGURE 6 CIRCUIT
Efficiency vs Load Current
Efficiency vs Input Voltage
Frequency vs Input Voltage
100
90
80
70
60
50
40
30
20
10
0
650
600
550
500
450
400
100
95
90
85
80
FCB = 0V
FCB = 5V
FIGURE 6 CIRCUIT
FIGURE 6 CIRCUIT
I
= 10A
LOAD
I
= 10A
= 1A
LOAD
V
V
V
V
V
= 5V
OUT
OUT
OUT
OUT
OUT
OUT
OUT
I
LOAD
= 3.3V
= 2.5V
= 2.5V
= 1.8V
= 1.2V
= 1V
I
= 1A
LOAD
V
V
V
= 12V
IN
FREQ = 550kHz
0.01 0.1
LOAD CURRENT (A)
1
10
5
10
15
20
5
10
15
20
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
3608 G04
3608 G06
3608 G05
3608fa
4
LTC3608
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency vs Load Current
Load Regulation
FIGURE 6 CIRCUIT
ITH Voltage vs Load Current
0.80
0.60
0.40
0.20
0
700
600
500
400
300
200
100
0
2.5
2.0
1.5
1.0
0.5
0
CONTINUOUS MODE
CONTINUOUS
MODE
DISCONTINUOUS MODE
–0.20
–0.40
–0.60
–0.80
DISCONTINUOUS
MODE
10
0
2
4
6
8
10
0
2
4
6
8
10
0
5
LOAD CURRENT (A)
LOAD CURRENT (A)
LOAD CURRENT (A)
3608 G07
3608 G08
3608 G09
Load Current
vs ITH Voltage and VRNG
On-Time vs ION Current
On-Time vs VON Voltage
25
20
15
10
5
1000
800
600
400
200
0
10000
1000
100
I
ON
= 30μA
V
VON
= 0V
V
=
RNG
1V
0.7V
0.5V
0
–5
–10
10
0
1
2
3
0
0.5
1.0
I
1.5
2.0
2.5
3.0
1
10
100
VOLTAGE (V)
V
ON
VOLTAGE (V)
I
CURRENT (μA)
TH
ON
3608 G11
3608 G12
3608 G10
Maximum Valley Current Limit
vs VRNG Voltage
Maximum Valley Current Limit
vs RUN/SS Voltage
On-Time vs Temperature
300
250
200
150
25
20
15
10
5
15
12
9
I
= 30μA
VON
ION
V
= 0V
6
100
50
0
3
0
–50 –25
0
25
50
75
100 125
0.5
0.6
0.7
0.8
0.9
1.0
1.65 1.90 2.15 2.40 2.65 2.90 3.15 3.40
RUN/SS VOLTAGE (V)
V
RNG
VOLTAGE (V)
TEMPERATURE (°C)
3608 G13
3608 G14
3608 G15
3608fa
5
LTC3608
TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Valley Current Limit
vs Temperature
Input Voltage
vs Maximum Valley Current
Maximum Valley Current Limit
in Foldback
20
15
10
5
15
10
5
18
16
14
12
10
8
6
0
0
4
0
0.1
0.2
0.3
(V)
0.4
0.5
0.6
4
8
12
16
20
–50 –25
0
25
50
75 100 125
V
TEMPERATURE (°C)
INPUT VOLTAGE (V)
FB
3608 G16
3608 G18
3608 G17
Feedback Reference Voltage
vs Temperature
Error Amplifier gm vs Temperature
2.0
1.8
1.6
1.4
1.2
1.0
0.62
0.61
0.60
0.59
0.58
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75 100 125
TEMPERATURE (°C)
TEMPERATURE (°C)
3608 G19
3608 G20
Input and Shutdown Currents
vs Input Voltage
INTVCC Load Regulation
1400
1200
1000
800
600
400
200
0
40
35
30
25
20
15
10
5
0.30
0.20
0.10
0
EXTV OPEN
CC
SHUTDOWN
–0.10
–0.20
–0.30
–0.40
EXTV = 5V
CC
0
0
5
10
INPUT VOLTAGE (V)
15
20
0
10
20
30
40
50
INTV LOAD CURRENT (mA)
CC
3608 G21
3608 G22
3608fa
6
LTC3608
TYPICAL PERFORMANCE CHARACTERISTICS
EXTVCC Switch Resistance
vs Temperature
IEXTVCC vs Frequency
10
8
20
18
16
14
12
10
8
V
IN
= 20V
6
4
6
2
4
2
0
0
400
500
600
700
800
900 1000
–50 –25
0
25
50
75 100 125
TEMPERATURE (°C)
FREQUENCY (kHz)
3608 G24
3608 G23
RUN/SS Pin Current
vs Temperature
FCB Pin Current vs Temperature
3
2
0
–0.25
–0.50
–0.75
PULL-DOWN CURRENT
1
0
–1.00
–1.25
–1.50
–1
PULL-UP CURRENT
0 25 50 75 100 125
–2
–50 –25
–50 –25
0
25
50
75
100 125
TEMPERATURE (°C)
TEMPERATURE (°C)
3608 G25
3608 G26
RUN/SS Pin Current
vs Temperature
Undervoltage Lockout Threshold
vs Temperature
5.0
4.5
4.0
3.5
4.0
3.5
3.0
2.5
LATCHOFF ENABLE
LATCHOFF THRESHOLD
3.0
2.0
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75 100 125
TEMPERATURE (°C)
TEMPERATURE (°C)
3608 G27
3608 G28
3608fa
7
LTC3608
PIN FUNCTIONS
PV (Pins 1, 2, 3, 4, 5, ., 7, 48, 49, 50, 51, 52, 53):
I (Pin18):CurrentControlThresholdandErrorAmplifier
IN
TH
Main Input Supply. Decouple this pin to power PGND with
Compensation Point. The current comparator threshold
increases with this control voltage. The voltage ranges
from 0V to 2.4V with 0.8V corresponding to zero sense
voltage (zero current).
the input capacitance C
IN
SW (Pins 8, 33, 41, 42, 43, 44, 45, 4., 47, 55): Switch
Node Connection to the Inductor. The (–) terminal of the
bootstrapcapacitorC alsoconnectshere.Thispinswings
FCB (Pin 19): Forced Continuous Input. Tie this pin to
B
from a diode voltage drop below ground up to V .
ground to force continuous synchronous operation at low
IN
load,toINTV toenablediscontinuousmodeoperationat
CC
SGND(Pins10,14,15,20,2.,27,54):SignalGround.All
small-signalcomponentsandcompensationcomponents
should connect to this ground, which in turn connects to
PGND at one point.
low load or to a resistive divider from a secondary output
when using a secondary winding.
NC (Pins 9, 21, 24, 25, 28): No Connection.
BOOST (Pin 11): Boosted Floating Driver Supply. The
I
(Pin22):On-TimeCurrentInput.TiearesistorfromV
ON IN
(+) terminal of the bootstrap capacitor C connects here.
to this pin to set the one-shot timer current and thereby
set the switching frequency.
B
This pin swings from a diode voltage drop below INTV
CC
up to V + INTV .
IN
CC
V
(Pin 23): Error Amplifier Feedback Input. This pin
FB
RUN/SS (Pin 12): Run Control and Soft-Start Input. A
capacitor to ground at this pin sets the ramp time to full
output current (approximately 3s/μF) and the time delay
for overcurrent latchoff (see Applications Information).
Forcing this pin below 0.8V shuts down the device.
connects the error amplifier input to an external resistive
divider from V
.
OUT
EXTV (Pin29):ExternalV Input.WhenEXTV exceeds
CC
CC
CC
4.7V, an internal switch connects this pin to INTV and
CC
shuts down the internal regulator so that controller and
V
(Pin13):On-TimeVoltageInput. Voltagetrippointfor
gate drive power is drawn from EXTV . Do not exceed
ON
CC
the on-time comparator. Tying this pin to the output volt-
7V at this pin and ensure that EXTV < V .
CC IN
age or an external resistive divider from the output makes
SV (Pin 30): Supply pin for internal PWM controller.
IN
the on-time proportional to V . The comparator input
OUT
INTV (Pins 31, 32): Internal 5V Regulator Output. The
defaults to 0.7V when the pin is grounded and defaults to
CC
driver and control circuits are powered from this voltage.
Decouple this pin to power ground with a minimum of
4.7μF low ESR tantalum or ceramic capacitor.
2.4V when the pin is tied to INTV . Tie this pin to INTV
CC
CC
in high V
applications to use a lower R value.
OUT
ON
PGOOD (Pin 1.): Power Good Output. Open drain logic
output that is pulled to ground when the output voltage
is not within 10% of the regulation point.
PGND (Pins 34, 35, 3., 37, 38, 39, 40): Power Ground.
Connect this pin closely to the (–) terminal of C
and
VCC
the (–) terminal of C .
IN
V
(Pin 17): Current Limit Range Input. The voltage at
RNG
this pin adjusts maximum valley current and can be set
from 0.5V to 0.7V by a resistive divider from INTV . It
defaults to 0.7V if the V
results in a typical 19A current limit.
CC
pin is tied to ground which
RNG
3608fa
8
LTC3608
FUNCTIONAL DIAGRAM
R
ON
SV
IN
V
ON
13
I
FCB
19
EXTV
29
ON
CC
22
30
4.7V
PV
IN
0.7V
2.4V
1μA
+
–
1, 2, 3, 4, 5, 6,
7, 48, 49, 50,
51, 52, 53
0.6V
REF
C
IN
0.6V
5V
REG
INTV
CC
+
–
31, 32
F
BOOST
11
V
I
VON
ION
t
=
(10pF)
R
S
ON
C
B
Q
FCNT
M1
ON
L1
20k
D
B
SW
+
–
+
–
V
OUT
8, 33, 41, 42,
43, 44, 45,
46, 47, 55
SWITCH
LOGIC
I
I
REV
CMP
SHDN
OV
+
1.4V
0.7V
1
C
OUT
M2
C
VCC
V
RNG
17
PGND
×
34, 35, 36, 37,
38, 39, 40
(0.5 TO 2)
16
PGOOD
R2
0.54V
240k
+
–
1V
Q2 Q4
UV
OV
Q6
I
THB
V
FB
23
Q3 Q1
R1
+
–
SGND
10, 14, 15,
20, 26, 27, 54
+
–
0.66V
0.8V
RUN
SHDN
SS
–
+
1.2μA
6V
EA
×3.3
–
+
27
NC
9, 21, 24,
25, 28
0.6V
0.4V
18
12
3608 FD
I
TH
RUN/SS
C
SS
3608fa
9
LTC3608
OPERATION
Main Control Loop
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback volt-
age exits a 10% window around the regulation point.
Furthermore, in an overvoltage condition, M1 is turned
off and M2 is turned on and held on until the overvoltage
condition clears.
The LTC3608 is a high efficiency monolithic synchronous,
step-down DC/DC converter utilizing a constant on-time,
current mode architecture. It operates from an input volt-
age range of 4V to 18V (20V maximum) and provides a
regulated output voltage at up to 8A of output current. The
internal synchronous power switch increases efficiency
and eliminates the need for an external Schottky diode. In
normal operation, the top MOSFET is turned on for a fixed
interval determined by a one-shot timer OST. When the
top MOSFET is turned off, the bottom MOSFET is turned
Foldback current limiting is provided if the output is
shorted to ground. As V drops, the buffered current
FB
threshold voltage I
is pulled down by clamp Q3 to
THB
a 1V level set by Q4 and Q6. This reduces the inductor
valley current level to one sixth of its maximum value as
on until the current comparator I
trips, restarting the
V
FB
approaches 0V.
CMP
one-shottimerandinitiatingthenextcycle.Inductorcurrent
is determined by sensing the voltage between the PGND
and SW pins using the bottom MOSFET on-resistance.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2μA current source to charge
The voltage on the I pin sets the comparator threshold
TH
up an external soft-start capacitor C . When this voltage
SS
corresponding to inductor valley current. The error am-
reaches1.5V,thecontrollerturnsonandbeginsswitching,
plifier EA adjusts this voltage by comparing the feedback
but with the I voltage clamped at approximately 0.6V
TH
signal V from the output voltage with an internal 0.6V
FB
below the RUN/SS voltage. As C continues to charge,
SS
reference. If the load current increases, it causes a drop
the soft-start current limit is removed.
in the feedback voltage relative to the reference. The I
TH
voltage then rises until the average inductor current again
INTV /EXTV Power
CC
CC
matches the load current.
PowerforthetopandbottomMOSFETdriversandmostof
theinternalcontrollercircuitryisderivedfromtheINTV
At light load, the inductor current can drop to zero and
become negative. This is detected by current reversal
CC
pin. The top MOSFET driver is powered from a floating
bootstrap capacitor C . This capacitor is recharged from
comparator I
which then shuts off M2 (see Func-
REV
B
tionalDiagram),resultingindiscontinuousoperation.Both
INTV through an external Schottky diode D when
CC
B
switcheswillremainoffwiththeoutputcapacitorsupplying
the load current until the I voltage rises above the zero
the top MOSFET is turned off. When the EXTV pin is
CC
TH
grounded, an internal 5V low dropout regulator supplies
current level (0.8V) to initiate another cycle. Discontinu-
ous mode operation is disabled by comparator F when
the FCB pin is brought below 0.6V, forcing continuous
synchronous operation.
the INTV power from V . If EXTV rises above 4.7V,
CC
IN
CC
the internal regulator is turned off, and an internal switch
connects EXTV to INTV . This allows a high efficiency
CC
CC
sourceconnectedtoEXTV , suchasanexternal5Vsup-
CC
ply or a secondary output from the converter, to provide
Theoperatingfrequencyisdeterminedimplicitlybythetop
MOSFET on-time and the duty cycle required to maintain
regulation.Theone-shottimergeneratesanon-timethatis
proportionaltotheidealdutycycle,thusholdingfrequency
the INTV power. Voltages up to 7V can be applied to
CC
EXTV for additional gate drive. If the input voltage is
CC
low and INTV drops below 3.5V, undervoltage lockout
CC
circuitry prevents the power switches from turning on.
approximately constant with changes in V . The nominal
IN
frequency can be adjusted with an external resistor R .
ON
3608fa
10
LTC3608
APPLICATIONS INFORMATION
The basic LTC3608 application circuit is shown on the
frontpageofthisdatasheet.Externalcomponentselection
is primarily determined by the maximum load current.
The LTC3608 uses the on-resistance of the synchronous
powerMOSFETfordeterminingtheinductorcurrent.The
desiredamountofripplecurrentandoperatingfrequency
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improvesefficiencybyreducingMOSFETswitchinglosses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
alsodeterminestheinductorvalue.Finally,C isselected
IN
The operating frequency of LTC3608 applications is de-
termined implicitly by the one-shot timer that controls the
for its ability to handle the large RMS current into the
converterandC ischosenwithlowenoughESRtomeet
OUT
on-time t of the top MOSFET switch. The on-time is set
the output voltage ripple and transient specification.
ON
by the current into the I pin and the voltage at the V
ON
ON
V
and PGOOD
pin according to:
ON
V
The LTC3608 has an open-drain PGOOD output that
indicates when the output voltage is within 10% of the
tON
=
VON (10pF)
IION
regulation point. The LTC3608 also has a V pin that
ON
Tying a resistor R from V to the I pin yields an
allows the on-time to be adjusted. Tying the V pin high
ON
IN
ON
ON
on-time inversely proportional to V . The current out of
resultsinlowervaluesforR whichisusefulinhighV
IN
ON
OUT
the I pin is
applications. The V pin also provides a means to adjust
ON
ON
the on-time to maintain constant frequency operation in
V
IN
ION
=
applications where V
changes and to correct minor
OUT
RON
frequency shifts with changes in load current.
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
V
Pin and I Adjust
RNG
LIMIT
The V
pin is used to adjust the maximum inductor
RNG
VOUT
VVON RON(10pF)
f =
[HZ ]
valley current, which in turn determines the maximum
average output current that the LTC3608 can deliver. The
maximum output current is given by:
Toholdfrequencyconstantduringoutputvoltagechanges,
tie the V pin to V or to a resistive divider from V
I
= I
+ 1/2 ΔI ,
ON
OUT
OUT
OUT(MAX)
VALLEY(MAX) L
when V
> 2.4V. The V pin has internal clamps that
OUT
ON
The I
CurrentLimitvsV
Characteristics.
is shown in the figure “Maximum Valley
RNG
VALLEY(MAX)
limit its input to the one-shot timer. If the pin is tied below
0.7V,theinputtotheone-shotisclampedat0.7V.Similarly,
if the pin is tied above 2.4V, the input is clamped at 2.4V.
Voltage”intheTypicalPerformance
An external resistor divider from INTV can be used to
set the voltage on the V
In high V
applications, tying V to INTV so that the
CC
OUT
ON CC
pin from 0.5V to 1V, or it can
comparator input is 2.4V results in a lower value for R .
RNG
ON
be simply tied to ground force a default value equivalent
to 0.7V. When setting current limit ensure that the junc-
tion temperature does not exceed the maximum rating of
Figures 1a and 1b show how R relates to switching
frequency for several common output voltages.
ON
125°C. Do not float the V
pin.
RNG
3608fa
11
LTC3608
APPLICATIONS INFORMATION
1000
loadcurrentincreases.Bylengtheningtheon-timeslightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I pin to the V pin and V . The values
TH
ON
OUT
V
OUT
= 3.3V
required will depend on the parasitic resistances in the
V
= 1.5V
V
OUT
= 2.5V
OUT
specific application. A good starting point is to feed about
25% of the voltage change at the I pin to the V pin
TH
ON
as shown in Figure 2a. Place capacitance on the V pin
ON
to filter out the I variations at the switching frequency.
TH
The resistor load on I reduces the DC gain of the error
TH
100
100
1000
(kΩ)
10000
amp and degrades load regulation, which can be avoided
R
ON
3608 F01a
by using the PNP emitter follower of Figure 2b.
Figure 1aꢁ Switching Frequency vs RON (VON = 0V)
Minimum Off-time and Dropout Operation
The minimum off-time t
is the smallest amount of
OFF(MIN)
1000
time that the LTC3608 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 320ns.
The minimum off-time limit imposes a maximum duty
V
= 12V
OUT
V
= 5V
OUT
cycle of t /(t + t
). If the maximum duty cycle
ON ON
OFF(MIN)
V
= 3.3V
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
OUT
t
ON + tOFF(MIN)
100
VIN(MIN) = VOUT
100
1000
(kΩ)
10000
tON
R
ON
3608 F01b
A plot of Maximum Duty Cycle vs Frequency is shown in
Figure 3.
Figure 1bꢁ Switching Frequency vs RON (VON = INTVCC
)
Because the voltage at the I pin is about 0.7V, the cur-
ON
Setting the Output Voltage
rent into this pin is not exactly inversely proportional to
V , especially in applications with lower input voltages.
IN
The LTC3608 develops a 0.6V reference voltage between
To correct for this error, an additional resistor R
con-
ON2
the feedback pin, V , and the signal ground as shown in
FB
nectedfromtheI pintothe5VINTV supplywillfurther
ON
CC
Figure 6. The output voltage is set by a resistive divider
stabilize the frequency.
according to the following formula:
5V
0.7V
R2
R1
ꢀ
ꢁ
ꢃ
ꢄ
RON2
=
RON
VOUT = 0.6V 1+
ꢂ
ꢅ
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
Toimprovethefrequencyresponse,afeedforwardcapaci-
tor C1 may also be used. Great care should be taken to
route the V line away from noise sources, such as the
FB
inductor or the SW line.
3608fa
12
LTC3608
APPLICATIONS INFORMATION
R
VON1
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
30k
V
V
ON
OUT
C
VON
R
VON2
100k
0.01μF
LTC3608
TH
R
C
I
A reasonable starting point is to choose a ripple current
C
C
that is about 40% of I
. The largest ripple current
OUT(MAX)
occurs at the highest V . To guarantee that ripple current
IN
(2a)
does not exceed a specified maximum, the inductance
should be chosen according to:
R
VON1
3k
V
V
OUT
ON
C
ꢁ
ꢄ ꢁ
ꢄ
R
VON
VON2
10k
VOUT
f ꢀI
VOUT
0.01μF
10k
L =
1ꢇ
LTC3608
TH
ꢃ
ꢆ ꢃ
ꢆ
INTV
CC
V
R
C
ꢂ
ꢅ ꢂ
ꢅ
L(MAX)
IN(MAX)
Q1
2N5087
I
C
C
3608 F02
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
affordthecorelossfoundinlowcostpowderedironcores.
A variety of inductors designed for high current, low volt-
ageapplicationsareavailablefrommanufacturerssuchas
Sumida, Panasonic, Coiltronics, Coilcraft and Toko.
(2b)
Figure 2ꢁ Correcting Frequency Shift with Load Current Changes
2.0
C and C
Selection
IN
OUT
1.5
The input capacitance C is required to filter the square
wavecurrentatthedrainofthetopMOSFET.UsealowESR
capacitor sized to handle the maximum RMS current.
IN
DROPOUT
REGION
1.0
0.5
0
VOUT
V
VOUT
IN
IRMS ≅IOUT(MAX)
–1
V
IN
This formula has a maximum at V = 2V , where
IN
OUT
0
0.25
0.50
0.75
1.0
I
= I
/2. This simple worst-case condition is
RMS
OUT(MAX)
DUTY CYCLE (V /V
)
OUT IN
3608 F03
commonly used for design because even significant de-
viations do not offer much relief. Note that ripple current
ratings from capacitor manufacturers are often based on
only 2000 hours of life which makes it advisable to derate
the capacitor.
Figure 3ꢁ Maximum Switching Frequency vs Duty Cycle
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
The selection of C
is primarily determined by the
OUT
ESR required to minimize voltage ripple and load step
transients. The output ripple ΔV
bounded by:
is approximately
OUT
ꢁ
ꢃ
ꢂ
ꢄ
ꢆ
ꢅ
ꢁ
OUT ꢄ
V
VOUT
f L
ꢀI =
1ꢇ
L
ꢃ
ꢆ
V
ꢂ
ꢅ
IN
ꢂ
ꢅ
ꢇ
1
ꢀVOUT ꢁ ꢀIL ESR+
ꢄ
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
8fC
ꢃ
OUT ꢆ
3608fa
13
LTC3608
APPLICATIONS INFORMATION
Since ΔI increases with input voltage, the output ripple
Discontinuous Mode Operation and FCB Pin
L
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfied, the capacitance is adequate
for filtering and has the necessary RMS current rating.
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when in-
ductor current reverses. The load current at which current
reverses and discontinuous operation begins depends on
the amplitude of the inductor ripple current and will vary
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramiccapacitorsareallavailableinsurfacemountpack-
ages. Special polymer capacitors offer very low ESR but
havelowercapacitancedensitythanothertypes.Tantalum
capacitors have the highest capacitance density but it is
important to only use types that have been surge tested
foruseinswitchingpowersupplies.Aluminumelectrolytic
capacitors have significantly higher ESR, but can be used
incost-sensitiveapplicationsprovidingthatconsideration
is given to ripple current ratings and long term reliability.
Ceramic capacitors have excellent low ESR characteris-
tics but can have a high voltage coefficient and audible
piezoelectriceffects.ThehighQofceramiccapacitorswith
traceinductancecanalsoleadtosignificantringing. When
used as input capacitors, care must be taken to ensure
that ringing from inrush currents and switching does not
pose an overvoltage hazard to the power switches and
controller. Todampeninputvoltagetransients, addasmall
5μFto50μFaluminumelectrolyticcapacitorwithanESRin
the range of 0.5Ω to 2Ω. High performance through-hole
capacitors may also be used, but an additional ceramic
capacitor in parallel is recommended to reduce the effect
of their lead inductance.
with changes in V . Tying the FCB pin below the 0.6V
IN
threshold forces continuous synchronous operation, al-
lowing current to reverse at light loads and maintaining
high frequency operation.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to maintain a
flyback winding output when the primary is operating
in discontinuous mode. The secondary output V
is
OUT2
normally set as shown in Figure 4 by the turns ratio N
of the transformer. However, if the controller goes into
discontinuous mode and halts switching due to a light
primary load current, then V
will droop. An external
OUT2
resistor divider from V
to the FCB pin sets a minimum
OUT2
voltage V
below which continuous operation is
has risen above its minimum:
OUT2(MIN)
forced until V
OUT2
R4
R3
ꢀ
ꢁ
ꢃ
ꢄ
VOUT2(MIN) = 0.6V 1+
ꢂ
ꢅ
Fault Conditions: Current Limit and Foldback
The LTC3608 has a current mode controller which inher-
ently limits the cycle-by-cycle inductor current not only
in steady state operation but also in transient. To further
limit current in the event of a short circuit to ground, the
LTC3608 includes foldback current limiting. If the output
fallsbymorethan25%,thenthemaximumsensevoltageis
progressively lowered to about one sixth of its full value.
Top MOSFET Driver Supply (C , D )
B
B
AnexternalbootstrapcapacitorC connectedtotheBOOST
B
pinsuppliesthegatedrivevoltageforthetopsideMOSFET.
This capacitor is charged through diode D from INTV
B
CC
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V and the BOOST pin rises
IN
to approximately V + INTV . The boost capacitor needs
IN
CC
INTV Regulator and EXTV Connection
CC
CC
to store about 100 times the gate charge required by the
top MOSFET. In most applications an 0.1μF to 0.47μF, X5R
or X7R dielectric capacitor is adequate.
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
3608fa
14
LTC3608
APPLICATIONS INFORMATION
SW
GND
IN4148
40 39 38 37 36 35 34 33 32 31 30 29 28 27
V
OUT2
+
C
SEC
1μF
41
42
43
44
45
46
47
48
49
50
51
52
26
25
24
23
22
21
20
19
18
17
16
15
SW
SW
SW
SW
SW
SW
SW
SGND
NC
V
OUT1
T1
+
1:N
C
OUT
NC
V
FB
I
ON
NC
SGND
FCB
R4
OPTIONAL EXTV
LTC3608
V
IN
CC
PV
PV
PV
PV
PV
IN
CONNECTION
5V < V < 7V
+
OUT2
I
TH
C
IN
IN
IN
IN
IN
R3
V
RNG
PGOOD
SGND
= PGND
= SGND
3608 F04
1
2
3
4
5
6
7
8
9 10 11 12 13 14
SGND
SW
Figure 4ꢁ Secondary Output Loop and EXTVCC Connection
withintheLTC3608.TheINTV pincansupplyupto50mA
3. EXTV connected to an output derived boost network.
CC
CC
RMS and must be bypassed to ground with a minimum of
4.7μF tantalum or ceramic capacitor. Good bypassing is
necessary to supply the high transient currents required
by the MOSFET gate drivers.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The
system will start-up using the internal linear regulator
until the boosted output supply is available.
The EXTV pin can be used to provide MOSFET gate drive
CC
Soft-Start and Latchoff with the RUN/SS Pin
and control power from the output or another external
The RUN/SS pin provides a means to shut down the
LTC3608 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
source during normal operation. Whenever the EXTV
CC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV
pin to INTV . INTV power is supplied from EXTV
CC
CC
LTC3608 into a low quiescent current shutdown (I <
Q
CC
CC
30μA). Releasing the pin allows an internal 1.2μA current
until this pin drops below 4.5V. Do not apply more than
7V to the EXTV pin and ensure that EXTV ≤ V . The
source to charge up the external timing capacitor C . If
SS
CC
CC
IN
RUN/SS has been pulled all the way to ground, there is a
following list summarizes the possible connections for
EXTV :
delay before starting of about:
CC
1.5V
1.2μA
1. EXTV grounded. INTV is always powered from the
CC
CC
tDELAY
=
C = 1.3s/μF C
SS SS
(
)
internal 5V regulator.
2. EXTV connectedtoanexternalsupply.Ahighefficiency
CC
supply compatible with the MOSFET gate drive require-
ments (typically 5V) can improve overall efficiency.
3608fa
15
LTC3608
APPLICATIONS INFORMATION
INTV
When the voltage on RUN/SS reaches 1.5V, the LTC3608
CC
R
begins operating with a clamp on I of approximately
TH
*
SS
V
IN
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
TH
RUN/SS
3.3V OR 5V
RUN/SS
*
D2*
is raised until its full 2.4V range is available. This takes an
additional 1.3s/μF, during which the load current is folded
back until the output reaches 75% of its final value.
R
SS
D1
2N7002
C
SS
C
SS
3608 F05
After the controller has been started and given adequate
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
time to charge up the output capacitor, C is used as a
SS
short-circuittimer.AftertheRUN/SSpinchargesabove4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8μA current then
(5a)
(5b)
Figure 5ꢁ RUN/SS Pin Interfacing with Latchoff Defeated
beginsdischargingC . Ifthefaultconditionpersistsuntil
SS
Efficiency Considerations
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
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. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3608 circuits:
Theovercurrentprotectiontimerrequiresthatthesoft-start
timing capacitor C be made large enough to guarantee
SS
that the output is in regulation by the time C has reached
SS
the 4V threshold. In general, this will depend upon the
size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor
can be estimated from:
2
1. DC I R losses. These arise from the resistance of the
internal resistance of the MOSFETs, inductor and PC
board traces and cause the efficiency to drop at high
outputcurrents.Incontinuousmodetheaverageoutput
currentflowsthroughL,butischoppedbetweenthetop
–4
C
SS
> C
V
R
(10 [F/V s])
OUT OUT SENSE
2
andbottomMOSFETs. TheDCI RlossforoneMOSFET
can simply be determined by [R
Generally 0.1μF is more than sufficient.
+ R ] • I .
DS(ON)
L O
Overcurrent latchoff operation is not always needed or de-
sired. Loadcurrentisalreadylimitedduringashort-circuit
bythecurrentfoldbackcircuitryandlatchoffoperationcan
prove annoying during troubleshooting. The feature can
be overridden by adding a pull-up current greater than
5μA to the RUN/SS pin. The additional current prevents
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated re-
gion during switch node transitions. It depends upon
the input voltage, load current, driver strength and
MOSFET capacitance, among other factors. The loss
is significant at input voltages above 20V and can be
estimated from:
the discharge of C during a fault and also shortens the
SS
soft-start period. Using a resistor to V as shown in Fig-
IN
–1
2
ure 5a is simple, but slightly increases shutdown current.
Transition Loss ≅ (1.7A ) V
I
C
f
IN OUT RSS
Connecting a resistor to INTV as shown in Figure 5b
CC
3. INTV current. This is the sum of the MOSFET driver
CC
eliminates the additional shutdown current, but requires
and control currents. This loss can be reduced by sup-
a diode to isolate C . Any pull-up network must be able
SS
plying INTV current through the EXTV pin from a
CC
CC
to pull RUN/SS above the 4.2V maximum threshold of the
latchoffcircuitandovercomethe4μAmaximumdischarge
current.
high efficiency source, such as an output derived boost
network or alternate supply if available.
3608fa
16
LTC3608
APPLICATIONS INFORMATION
4. C loss. The input capacitor has the difficult job of
Selecting a standard value of 1.2μH results in a maximum
ripple current of:
IN
filtering the large RMS input current to the regulator. It
2
must have a very low ESR to minimize the AC I R loss
2.5V
550kHz 1.2μH
2.5V
12V
ꢁ
ꢂ
ꢄ
ꢅ
ꢀIL =
1–
= 3A
and sufficient capacitance to prevent the RMS current
from causing additional upstream losses in fuses or
batteries.
ꢃ
ꢆ
(
)(
)
Next, set up V
voltage and check the I
. Tying V
RNG
LIMIT RNG
to 0.5V will set the typical current limit to 11A, and tying
Other losses, including C
ESR loss, Schottky diode D1
OUT
V
to GND will result in a typical current around 16A.
RNG
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
C
IN
is chosen for an RMS current rating of about 5A at
85°C. The output capacitors are chosen for a low ESR
of 0.002Ω to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
Whenmakingadjustmentstoimproveefficiency, theinput
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
ΔV
= ΔI
(ESR)
L(MAX)
OUT(RIPPLE)
= (3A) (0.002Ω) = 6mV
Checking Transient Response
However, a 0A to 8A load step will cause an output change
of up to:
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 load current. When
ΔV
= ΔI
(ESR) = (8A) (0.002Ω) = 16mV
LOAD
OUT(STEP)
An optional 22μF ceramic output capacitor is included
to minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 6.
a load step occurs, V
immediately shifts by an amount
OUT
equal to ΔI
(ESR), where ESR is the effective series
LOAD
resistance of C . ΔI
also begins to charge or dis-
OUT
LOAD
chargeC generatingafeedbackerrorsignalusedbythe
OUT
PC Board Layout Checklist
regulator to return V
this recovery time, V
to its steady-state value. During
can be monitored for overshoot
OUT
OUT
When laying out a PC board follow one of the two sug-
gested approaches. The simple PC board layout requires
a dedicated ground plane layer. Also, for higher currents, a
multilayerboardisrecommendedtohelpwithheatsinking
of power components.
or ringing that would indicate a stability problem. The I
TH
pin external components shown in Figure 6 will provide
adequate compensation for most applications. For a
detailed explanation of switching control loop theory see
Application Note 76.
• The ground plane layer should not have any traces and
it should be as close as possible to the layer with the
LTC3608.
Design Example
As a design example, take a supply with the following
• Place C and C
all in one compact area, close to
IN
OUT
specifications: V = 5V to 20V (12V nominal), V
=
IN
OUT
the LTC3608. It may help to have some components
2.5V 5%, I
= 8A, f = 550kHz. First, calculate the tim-
OUT
on the bottom side of the board.
ing resistor with V = V
:
ON
OUT
• Keep small-signal components close to the LTC3608.
2.5V
RON
=
≈187k
550kHz 10pF (2.4V)
)(
(
)
•
Ground connections (including LTC3608 SGND and
PGND) should be made through immediate vias to
the ground plane. Use several larger vias for power
components.
and choose the inductor for about 40% ripple current at
the maximum V :
IN
2.5V
550kHz 0.4 8A
)( )(
2.5V
20V
ꢁ
ꢂ
ꢄ
ꢅ
L =
1ꢀ
=1.24µH
ꢃ
ꢆ
(
)
3608fa
17
LTC3608
APPLICATIONS INFORMATION
INTV
CC
EXTV
CC
C4
0.01μF
V
IN
C
R
F1
1Ω
F
C
4.7μF
6.3V
VCC
0.47μF
25V
SW
PGND
SGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
V
OUT
41
26
25
24
23
22
21
20
19
18
17
16
15
(OPTIONAL)
C2
SW
SW
SW
SW
SW
SW
SW
PV
SGND
2.5V AT
8A
R1
9.5k
1%
R2
30.1k
1%
L1
42
43
44
45
46
47
48
49
50
51
52
NC
NC
C1
+
C5
22μF
6.3V
C
OUT1
0.8μH
100μF
R
187k
1%
ON
(OPTIONAL)
×2
V
OUT
V
FB
(OPTIONAL)
I
V
ON
GND
IN
C
ON
0.01μF
NC
SGND
FCB
(OPTIONAL)
LTC3608
C
V
C1
1500pF
IN
V
IN
5V TO 18V
R5
IN
GND
11.3k
PV
PV
PV
PV
I
TH
IN
+
C
C6
10μF
35V
IN
V
IN
IN
IN
RNG
10μF
35V
3×
PGOOD
SGND
R3
0Ω
(OPTIONAL)
R
PG1
100k
C3
C
C2
100pF
INTV
CC
(OPTIONAL)
1
2
3
4
5
6
7
B
8
9
10 11 12 13 14
SGND
R
VON
0Ω
SW
C
C
: TAIYO YUDEN GMK325BJ106MM-B
: TDKC2012X5ROJ226M
IN
OUT
V
OUT
INTV
CC
L1: CDEP85NP-R80MC-50
R
C
SS1
510k
B1
C5: MURATA GRM31CR60J226KE19
D
0.22μF
CMDSH-3
0.1μF
V
IN
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
SW
C
SS
0.1μF
3608 F06
(OPTIONAL)
= PGND
= SGND
Figure .ꢁ Design Example: 5V to 18V Input to 2ꢁ5V/8A at 550kHz
• Useacompactplanefortheswitchnode(SW)toimprove
cooling of the MOSFETs and to keep EMI down.
• Segregate the signal and power grounds. All small
signal components should return to the SGND pin at
one point, which is then tied to the PGND pin.
• Use planes for V and V
to maintain good voltage
OUT
IN
filtering and to keep power losses low.
•
Connect the input capacitor(s) C close to the IC. This
IN
capacitor carries the MOSFET AC current.
• Flood all unused areas on all layers with copper. Flood-
ing with copper reduces the temperature rise of power
components. Connect these copper areas to any DC
• Keep the high dV/dT SW, BOOST and TG nodes away
from sensitive small-signal nodes.
net (V , V , GND or to any other DC rail in your
IN OUT
• Connect the INTV decoupling capacitor C
closely
CC
VCC
system).
to the INTV and PGND pins.
CC
When laying out a printed circuit board without a ground
plane, use the following checklist to ensure proper opera-
tion of the controller. These items are also illustrated in
Figure 7.
• Connect the top driver boost capacitor C closely to
B
the BOOST and SW pins.
• Connect the V pin decoupling capacitor C closely to
IN
F
the V and PGND pins.
IN
3608fa
18
LTC3608
APPLICATIONS INFORMATION
C
VCC
SW
40 39 38 37 36 35 34 33 32 31 30 29 28 27
41
42
43
44
45
46
47
48
49
50
51
52
26
25
24
23
22
21
20
19
18
17
16
15
SW
SW
SW
SW
SW
SW
SW
PV
SGND
NC
C
OUT
R1
R2
NC
V
FB
R
ON
I
ON
V
OUT
NC
SGND
FCB
LTC3608
C
C1
IN
R
C
PV
PV
PV
PV
I
TH
IN
V
RNG
IN
IN
IN
C
IN
PGOOD
SGND
C
C2
1
2
3
4
5
6
7
8
9 10 11 12 13 14
V
IN
C
B
C
SS
D
B
3608 F07
Figure 7ꢁ LTC3.08 Layout Diagram
3608fa
19
LTC3608
TYPICAL APPLICATIONS
3ꢁ.V Input to 1ꢁ5V/8A at 750kHz
V
IN2
= 5V
INTV
CC
EXTV
CC
C4
0.01μF
C
F
0.47μF
25V
C
4.7μF
6.3V
VCC
SW
PGND
SGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
V
OUT
41
42
43
44
45
46
47
48
49
50
51
52
26
25
24
23
22
21
20
19
18
17
16
15
(OPTIONAL)
C2
SW
SW
SW
SW
SW
SW
SW
PV
SGND
NC
1.5V AT
8A
R1
20.5k
1%
R2
30.1k
1%
L1
C1
(OPTIONAL)
+
+
C5
22μF
6.3V
C
OUT1
0.2μH
100μF
NC
R
113k
1%
ON
×2
V
OUT
V
I
FB
(OPTIONAL)
V
ON
GND
IN
C
ON
0.01μF
NC
(OPTIONAL)
LTC3608
SGND
FCB
C
V
C1
3300pF
IN
V
IN
3.6V
R5
IN
GND
6.19k
PV
PV
PV
PV
I
TH
IN
C
C6
10μF
10V
IN
39.2k
V
IN
IN
IN
RNG
10μF
INTV
CC
3×
PGOOD
SGND
11k
(OPTIONAL)
R
C
PG1
C2
100pF
100k
INTV
CC
1
2
3
4
5
6
7
8
9 10 11 12 13 14
SGND
V
OUT
C
C
: TAIYO YUDEN TMK432BJ106MM
: TDKC4532X5ROJ107M
IN
OUT1
C
B1
0.22μF
V
OUT
L1: CDEP85NP-R20MC-50
INTV
CC
R
SS1
510k
0.1μF
C5: TAIYO YUDEN JMK316BJ226ML-T
V
IN
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
C
SS
0.1μF
3608 TA02
(OPTIONAL)
= PGND
= SGND
Transient Response
Efficiency Curve
100
95
90
85
80
75
70
65
60
55
50
DCM
CCM
I
L
5A/DIV
V
OUT
200mV/DIV
3608 TA02a
200mV
V
= 3.6V
IN
LOAD STEP 1A-8A
FREQ = 750kHz
V
IN
V
OUT
= 3.6V
= 1.5V
100
1000
1000
10000
FCB = 0V
LOAD CURRENT (A)
3608 TA02b
3608fa
20
LTC3608
TYPICAL APPLICATIONS
5V to 18V Input to 1ꢁ2V/8A at 550kHz
R
F1
1Ω
INTV
CC
V
IN2
EXTV
CC
C4
0.01μF
C
F
0.47μF
25V
C
4.7μF
6.3V
VCC
SW
PGND
SGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
V
OUT
41
42
43
44
45
46
47
48
49
50
51
52
26
25
24
23
22
21
20
19
18
17
16
15
(OPTIONAL)
R2
SW
SW
SW
SW
SW
SW
SW
PV
SGND
NC
1.2V AT
8A
R1
30k
1%
L1
C1
(OPTIONAL)
30.1k
1%
C2
+
+
C5
22μF
6.3V
C
OUT1
0.5μH
100μF
NC
R
187k
1%
ON
×2
V
OUT
V
I
FB
(OPTIONAL)
V
ON
GND
IN
C
ON
0.01μF
NC
(OPTIONAL)
LTC3608
SGND
FCB
C
V
C1
IN
V
IN
5V TO 18V
R5
1500pF
IN
GND
7.68k
PV
PV
PV
PV
I
TH
IN
C
C6
10μF
35V
IN
V
IN
IN
IN
RNG
10μF
25V
3×
PGOOD
SGND
(OPTIONAL)
R
C
PG1
100k
C2
100pF
INTV
CC
1
2
3
4
5
6
7
8
9 10 11 12 13 14
SGND
V
OUT
R
VON
C5: TAIYO YUDEN JMK316BJ226ML-T
C
B1
0.22μF
V
C
C
: TAIYO YUDEN TMK432BJ106MM
: TDKC4532X5R107M
OUT
IN
OUT1
INTV
CC
R
SS1
510k
0.1μF (OPTIONAL)
C
VON
L1: CDEP85NP-R50MC-125
D
B
V
CMDSH-3
IN
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
C
SS
0.1μF
3608 TA03
(OPTIONAL)
= PGND
= SGND
Transient Response
Efficiency vs Load Current
90
85
80
75
70
65
60
55
50
V
= 12V
IN
FREQ = 550kHz
I
L
5A/DIV
V
OUT
200mV/DIV
DCM
OCM
3608 TA03a
200mV
LOAD STEP 1A-8A
V
IN
V
OUT
= 12V
100
1000
1000
10000
= 1.2V
FCB = 0V
LOAD CURRENT (A)
3608 TA03b
3608fa
21
LTC3608
TYPICAL APPLICATIONS
5V to 18V Input to 1ꢁ8V/8A All Ceramic 1MHz
R
F1
1Ω
INTV
CC
V
IN
EXTV
CC
C4
0.01μF
C
0.1μF
25V
F
C
4.7μF
6.3V
VCC
SW
PGND
SGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
V
OUT
41
42
43
44
45
46
47
48
49
50
51
52
26
25
24
23
22
21
20
19
18
17
16
15
(OPTIONAL)
C2
SW
SW
SW
SW
SW
SW
SW
PV
SGND
NC
1.8V AT
8A
R1
10k
1%
R2
30.1k
1%
L1
C1
(OPTIONAL)
+
C5
22μF
6.3V
C
OUT1
0.47μH
100μF
NC
R
102k
1%
ON
×2
V
OUT
V
I
FB
(OPTIONAL)
V
ON
GND
IN
C
ON
0.01μF
NC
(OPTIONAL)
LTC3608
SGND
FCB
C
V
C1
1500pF
IN
V
IN
5V TO 18V
R5
IN
5.76k
PV
PV
PV
PV
I
TH
IN
C
IN
V
IN
IN
IN
RNG
10μF
25V
3×
PGOOD
SGND
R
C
PG1
C2
100pF
100k
INTV
CC
1
2
3
4
5
6
7
8
9 10 11 12 13 14
SGND
V
OUT
C
: TDKC3225XROJ107M
OUT
C
B1
0.22μF
V
L1: VISHAY IHLP2525-R47
C5: TAIYO YUDEN JMK316BJ226ML-T
OUT
INTV
CC
R
SS1
0.1μF
510k
D
B
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
V
CMDSH-3
IN
C
SS
0.1μF
3608 TA04
(OPTIONAL)
= PGND
= SGND
Transient Response
Efficiency vs Load Current
90
80
70
60
50
40
30
DCM
CCM
I
L
5A/DIV
V
OUT
200mV/DIV
3608 TA04a
200mV
LOAD STEP 1A-5A
V
IN
= 12V
V
IN
V
OUT
= 12V
100
1000
10000
= 1.8V
FCB = 0V
LOAD CURRENT (mA)
3608 TA04b
3608fa
22
LTC3608
PACKAGE DESCRIPTION
WKG Package
52-Lead QFN Multipad (7mm × 8mm)
(Reference LTC DWG # 05-08-1768 Rev Ø)
SEATING PLANE
0.00 – 0.05
A
7.00
BSC
2.625 REF
2.90 REF
0.50 BSC
PIN 1 ID
41
52
B
PAD 1
CORNER
4
40
1
2.025
± 0.10
3.20 ± 0.10
3.40 REF
3.40 REF
3.90 ± 0.10
7
2.925 ± 0.10
8.00
BSC
33
32
8
1.00 REF
9
10
NX b
4.275 ± 0.10
2.25 ± 0.10
27
14
0.580 ± 0.10
0.40 ± 0.10
26
19
15
aaa C 2x
TOP VIEW
0.90 ± 0.10
1.35
± 0.10
1.775
REF
0.25 ± 0.05
NX
0.08 C
9
// ccc C
BOTTOM VIEW
(BOTTOM METALLIZATION DETAILS)
8
MLP52 QFN REV Ø 0807
7.50 ± 0.05
2.90 REF
0.50 BSC
2.625 REF
NOTE:
1. DIMENSIONING AND TOLERANCING CONFORM TO ASME Y14.5M-1994
2. ALL DIMENSIONS ARE IN MILLIMETERS, ANGLES ARE IN DEGREES (°)
3. N IS THE TOTAL NUMBER OF TERMINALS
PIN 1
4
THE LOCATION OF THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING
CONVENTION CONFORMS TO JEDEC PUBLICATION 95 SPP-002
3.20 ± 0.10
3.40 REF
2.025
± 0.10
3.40 REF
5. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY
6. NJR REFER TO NON JEDEC REGISTERED
2.925 ± 0.10
3.90 ± 0.10
7
DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED
BETWEEN 0.20mm AND 0.30mm FROM THE TERMINAL TIP. IF THE TERMINAL
HAS THE OPTIONAL RADIUS ON THE OTHER END OF THE TERMINAL, THE
DIMENSION b SHOULD NOT BE MEASURED IN THAT RADIUS AREA.
1.00 REF
8.50 ± 0.05
8
9
COPLANARITY APPLIES TO THE TERMINALS AND ALL OTHER SURFACE
METALLIZATION
DRAWING SHOWN ARE FOR ILLUSTRATION ONLY
4.275 ± 0.10
2.25 ± 0.10
0.40 ± 0.10
SYMBOL TOLERANCE
PACKAGE
OUTLINE
aaa
bbb
ccc
0.15
0.10
0.10
1.35
± 0.10
0.25 ± 0.05
1.775
REF
RECOMMENDED SOLDER PAD LAYOUT
TOP VIEW
3608fa
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LTC3608
TYPICAL APPLICATION
14V to 18V Input to 12V/5A at 500kHz
C
VCC
4.7μF, 6.3V
EXTV
CC
C4
0.01μF
V
IN2
INTV
CC
R
C
F
F1
1Ω
0.1μF
25V
PGND
SW
SGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
V
OUT
41
26
25
24
23
22
21
20
19
18
17
16
15
(OPTIONAL)
C2
SW
SW
SW
SW
SW
SW
SW
PV
SGND
NC
12V AT
5A
R1
3.16k
1%
R2
60.4k
1%
L1
42
43
44
45
46
47
48
49
50
51
52
+
C1
(OPTIONAL)
C5
22μF
25V
C
OUT1
4.3μH
180μF
16V
NC
R
1M
1%
ON
V
OUT
V
I
FB
(OPTIONAL)
V
IN
ON
GND
C
ON
0.01μF
NC
(OPTIONAL)
LTC3608
SGND
FCB
C
V
C1
3300pF
IN
V
IN
14V TO 18V
R5
IN
GND
24.9k
PV
PV
PV
PV
I
TH
IN
+
C
C6
10μF
35V
IN
90.9k
V
IN
IN
IN
RNG
10μF
25V
3×
INTV
CC
PGOOD
SGND
10k
(OPTIONAL)
C
R
C2
100pF
PG1
100k
INTV
CC
C
C
: TAIYO YUDEN TMK432BJ106MM
IN
1
2
3
4
5
6
7
8
9
10 11 12 13 14
SGND
: SANYO 16SVP180MX
OUT
L1: CDEP85NP-4R3MC-88
C
B1
0.22μF
INTV
CC
R
SS1
510k
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.
(OPTIONAL)
C
INTV
VON
CC
V
IN
D
B
C
SS
CMDSH-3
= PGND
(OPTIONAL)
0.1μF
RUN/SS
3608 TA05
= SGND
Efficiency Curve
Transient Response
100
95
90
85
80
75
70
65
60
55
50
DCM
I
L
CCM
5A/DIV
V
OUT
200mV/DIV
3608 TA05a
200mV
V
= 18V
IN
FREQ = 500kHz
LOAD STEP 1A-8A
V
IN
V
OUT
= 18V
= 12V
FCB = 0V
100
1000
1000 10000
LOAD CURRENT (A)
3608 TA05b
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
LTC1778
No R
Current Mode Synchronous Step-Down Controller
Up to 97% Efficiency, V : 4V to 36V, 0.8V ≤ V
≤ (0.9)(V ), I
Up
SENSE
IN
OUT
IN OUT
to 20A
LTC3414
4A (I ), 4MHz, Synchronous Step-Down DC/DC Converter
95% Efficiency, V : 2.25V to 5.5V, V
= 0.8V, I = 64μA, I
:
SD
OUT
IN
OUT(MIN)
Q
<1μA, TSSOP20E Package
LTC3418
8A (I ), 4MHz, Synchronous Step-Down DC/DC Converter
95% Efficiency, V : 2.25V to 5.5V, V
= 0.8V, Thermally
OUT
IN
OUT(MIN)
Enhanced 38-Lead QFN Package
LTC3610
12A Current Mode Monolithic Synchronous Step-Down
Converter
Up to 24V Input (28V Maximum). Current Mode Extremely Fast
Transient Response
LTM4600HV
LTM4601HV
LTM4603HV
10A Complete Switch Mode Power Supply
12A Complete Switch Mode Power Supply
6A Complete Switch Mode Power Supply
92% Efficiency, V : 4.5V to 28V, V : 0.6V, True Current Mode
IN OUT
Control, Ultrafast Transient Response
92% Efficiency, V : 4.5V to 28V, V : 0.6V, True Current Mode
IN
OUT
Control, Ultrafast Transient Response
93% Efficiency, V : 4.5V to 28V, with PLL, Output Tracking and
IN
Margining with Ultrafast Transient Response
3608fa
LT 0808 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
© LINEAR TECHNOLOGY CORPORATION 2008
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
相关型号：
LTC3608EWKG#PBF
LTC3608 - 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter; Package: QFN; Pins: 52; Temperature Range: -40°C to 85°C
Linear
LTC3608EWKG#TRPBF
LTC3608 - 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter; Package: QFN; Pins: 52; Temperature Range: -40°C to 85°C
Linear
LTC3609EWKG#PBF
LTC3609 - 32V, 6A Monolithic Synchronous Step-Down DC/DC Converter; Package: QFN; Pins: 52; Temperature Range: -40°C to 85°C
Linear
©2020 ICPDF网 联系我们和版权申明