LTC3700EMS#PBF [Linear]
LTC3700 - Constant Frequency Step-Down DC/DC Controller with LDO Regulator; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C;
| 型号： | LTC3700EMS#PBF |
| 厂家： | 
Linear |
| 描述： | LTC3700 - Constant Frequency Step-Down DC/DC Controller with LDO Regulator; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C 稳压器 控制器 |
| 文件： | 总16页 (文件大小：211K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3700
Constant Frequency
Step-Down DC/DC Controller
with LDO Regulator
U
FEATURES
DESCRIPTIO
The LTC®3700 is a constant frequency current mode step-
down (buck) DC/DC controller with excellent AC and DC
load and line regulation. The on-chip 150mA low dropout
(LDO) linear regulator can be powered from the buck
controller’s input supply, its own independent input supply
or the buck regulator’s output. The buck controller incor-
poratesanundervoltagelockoutfeaturethatshutsdownthe
controller when the input voltage falls below 2.1V.
■
Dual Output Regulator in Tiny 10-Pin MSOP
■
High Efficiency: Up to 94%
■
Wide VIN Range: 2.65V to 9.8V
Constant Frequency 550kHz Operation
■
■
150mA LDO Regulator with Current Limit and
Thermal Shutdown Protection
■
High Output Currents Easily Achieved
Burst Mode® Operation at Light Load
■
■
Low Dropout: 100% Duty Cycle
Thebuckregulatorprovidesa±2.5%outputvoltageaccu-
racy. It consumes only 210µA of quiescent current in nor-
maloperationwiththeLDOconsuminganadditional50µA.
In shutdown, a mere 10µA (combined) is consumed.
■
Current Mode Operation for Excellent Line and Load
Transient Response
■
0.8V Reference Allows Low Output Voltages
■
Low Quiescent Current: 260µA Total
■
■
Forapplicationswhereefficiencyisaprimeconsideration,
thebuckcontrollerisconfiguredforBurstModeoperation
which enhances efficiency at low output current. To fur-
ther maximize the life of a battery source, the external
P-channel MOSFET is turned on continuously in dropout
(100% duty cycle). High constant operating frequency of
550kHz allows the use of a small external inductor.
Shutdown Mode Draws Only 10µA Supply Current
Common Power Good Output for Both Supplies
U
APPLICATIO S
■
Notebook Computers
Portable Instruments
One or Two Li-Ion Battery-Powered Applications
, LTC and LT are registered trademarks of Linear Technology Corporation.
■
■
The LDO is protected by both current limit and thermal
shutdown circuits.
Burst Mode is a registered trademark of Linear Technology Corporation.
The LTC3700 is available in a tiny 10-pin MSOP.
U
TYPICAL APPLICATIO
V
IN1
5V
V
IN2
3.3V
Buck Efficiency vs Load Current
C1
10µF
10V
C3
R1
0.068Ω
10µF
10V
V
V
IN2
90
86
82
78
74
70
66
62
58
54
50
IN
V
R
= 1.8V
SENSE
OUT
V
= 3.3V
V
–
OUT2
IN
SENSE
PGATE
LDO
= 0.068Ω
2.5V AT
150mA
169k
L1
10µH
C4
M1
V
2.2µF
V
FB2
OUT1
1.8V
16V
V
= 5V
78.7k
IN
AT 1A
100k
LTC3700
D1
V
= 4.2V
IN
C2
+
47µF
80.6k
6V
C1, C3: TAIYO YUDEN EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
C4: MURATA GRM42-6X7R225K016AL
D1: MOTOROLA MBRM120T3
L1: COILTRONICS UP1B-100
M1: Si3443DV
V
PGOOD
/RUN GND
FB
10k
I
TH
220pF
3700 F01
R1: DALE 0.25W
1
10
100
1000
LOAD CURRENT (mA)
3700 F01a
Figure 1. High Efficiency 5V to 1.8V/1A Buck with 3.3V to 2.5V/150mA LDO
3700f
1
LTC3700
W W U W
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
Buck Input Supply Voltage (VIN) ................–0.3V to 10V
SENSE–, PGATE Voltages............. –0.3V to (VIN + 0.3V)
VFB, ITH/RUN Voltages ..............................–0.3V to 2.4V
PGATE Peak Output Current (<10µs) ....................... 1A
LDO Input Supply Voltage (VIN2) .................–0.3V to 6V
LDO, VFB2 Voltages..................... –0.3V to (VIN2 + 0.3V)
PGOOD Voltage .........................................–0.3V to 10V
LDO Peak Output Current (< 10µs) ..................... 500mA
Storage Ambient Temperature Range ... –65°C to 150°C
Operating Temperature Range (Note 2) ... –40°C to 85°C
Junction Temperature (Note 3)............................. 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
TOP VIEW
LTC3700EMS
V
1
2
3
4
5
10
9
I
/RUN
IN2
TH
LDO
V
FB
–
V
8
SENSE
FB2
PGOOD
GND
7
6
V
IN
PGATE
MS PACKAGE
10-LEAD PLASTIC MSOP
MS PART MARKING
LTXN
TJMAX = 150°C, θJA = 230°C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The ● denotes specifications that apply over the full operating temperature
ELECTRICAL CHARACTERISTICS
range, otherwise specifications are at TA = 25°C. VIN = VIN2 = 4.2V unless otherwise specified. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Buck DC/DC Controller
Input DC Supply Current
Normal Operation
Sleep Mode
Shutdown
UVLO
Typicals at V = 4.2V (Note 4)
IN
2.65V ≤ V ≤ 9.8V
210
200
10
340
330
30
µA
µA
µA
µA
IN
2.65V ≤ V ≤ 9.8V
IN
2.65V ≤ V ≤ 9.8V, V /RUN = 0V
IN ITH
V
< UVLO Threshold
10
30
IN
Undervoltage Lockout Threshold
V
V
Falling
Rising
●
●
1.90
2.00
2.10
2.20
2.60
2.65
V
V
IN
IN
Shutdown Threshold (at I /RUN)
●
0.15
0.25
0.30
0.5
0.45
0.85
V
TH
Start-Up Current Source
V
/RUN = 0V
µA
ITH
Regulated Feedback Voltage
(Note 5), 0°C to 70°C
(Note 5), –40°C to 85°C
●
●
0.780
0.770
0.800
0.800
0.820
0.830
V
V
Output Voltage Line Regulation
Output Voltage Load Regulation
2.65V ≤ V ≤ 9.8V (Note 5)
0.1
mV/V
IN
I
I
/RUN Sinking 5µA (Note 5)
/RUN Sourcing 5µA (Note 5)
4
4
mV/µA
mV/µA
TH
TH
V
Input Current
(Note 5)
10
0.860
20
50
nA
V
FB
Overvoltage Protect Threshold
Overvoltage Protect Hysteresis
Oscillator Frequency
Measured at V
0.820
500
0.910
FB
mV
V
V
= 0.8V
= 0V
550
110
650
kHz
kHz
FB
FB
Gate Drive Rise Time
C
C
= 3000pF
= 3000pF
40
40
ns
ns
LOAD
LOAD
Gate Drive Fall Time
Peak Current Sense Voltage
Peak Current Sense Voltage in Burst Mode
(Note 6)
120
30
mV
mV
3700f
2
LTC3700
The ● denotes specifications that apply over the full operating temperature
ELECTRICAL CHARACTERISTICS
range, otherwise specifications are at TA = 25°C. VIN = VIN2 = 4.2V unless otherwise specified. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
LDO Regulator
V
Input Voltage
2.4
6
V
IN2
Input DC Supply Current
Typicals at V = 4.2V
IN2
Normal Operation with Buck Enabled
Normal Operation with Buck Undervoltage
Shutdown with Buck Enabled
2.4V ≤ V ≤ 6V
50
100
0
100
150
1
µA
µA
µA
µA
IN2
2.4V ≤ V ≤ 6V
IN2
2.4V ≤ V ≤ 6V, V
= 0V
= 0V
IN2
ITH/RUN
ITH/RUN
Shutdown with Buck Undervoltage
2.4V ≤ V ≤ 6V, V
8
24
IN2
Regulated Feedback Voltage
0°C ≤ T ≤ 70°C, I
= 1mA
LDO
●
●
0.780
0.765
0.800
0.800
0.830
0.835
V
V
A
–40°C ≤ T ≤ 85°C, I
= 1mA
A
LDO
Output Voltage Line Regulation
With Buck Enabled
With Buck Enabled
(Unity-Gain Feedback)
2.65V ≤ V ≤ 9.8V
0.05
4
4
mV/V
mV/V
mV/V
IN
2.4V ≤ V ≤ 6V, I
= 1mA
= 1mA
IN2
LDO
LDO
With Buck Undervoltage
2.4V ≤ V ≤ 6V, I
IN2
Output Voltage Load Regulation
1mA ≤ I
≤ 150mA
0.06
0
0.12
10
mV/mA
nA
LOAD
V
Input Current
FB2
LDO Short-Circuit Current
LDO Dropout
V
= 0V
150
200
mA
LDO
V
V
= 3.3V, I
= 6V, I
= 150mA
= 150mA
270
170
mV
mV
IN2
IN2
LDO
LDO
Overtemperature Trip Point
Overtemperature Hysteresis
PGOOD
(Note 7)
(Note 7)
150
5
°C
°C
Feedback Voltage PGOOD Threshold
PGOOD High-to-Low
(Note 8)
V
V
or V Falling
–12
–10
–7.5
7.5
%
%
FB
FB
FB2
or V Rising
12
FB2
PGOOD Low-to-High
V
V
or V Rising
–5.0
5.0
%
%
FB
FB
FB2
or V Falling
10
FB2
PGOOD On-Resistance
V
= 0V, V = V = 4.2V, V = 100mV
PGOOD
135
180
Ω
ITH/RUN
IN
IN2
Note 1: Absolute Maximum Ratings are those values beyond which the life
Note 4: Dynamic supply current is higher due to the gate charge being
of a device may be impaired.
delivered at the switching frequency.
Note 2: The LTC3700 is guaranteed to meet specifications from 0°C to
70°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls.
Note 5: The LTC3700 is tested in a feedback loop that servos V to the
output of the error amplifier.
Note 6: Peak current sense voltage is reduced dependent on duty cycle to
a percentage of value as given in Figure 2.
FB
Note 3: T is calculated from the ambient temperature T and power
J
A
Note 7: Guaranteed by design; not tested in production.
Note 8: PGOOD values are expressed as a percentage difference from the
respective “Regulated Feedback Voltage” as given in the table.
dissipation P according to the following formula:
D
T = T + (P • θ °C/W)
J
A
D
JA
3700f
3
LTC3700
U W
TYPICAL PERFOR A CE CHARACTERISTICS
BUCK DC/DC CONTROLLER
Normalized Oscillator Frequency
vs Temperature
VFB Voltage vs Temperature
805
10
8
V
TH
NO LOAD
= 4.2V
V
= 4.2V
IN
IN
804
803
802
801
800
799
798
797
796
795
I
/RUN = V
FB
6
4
2
0
–2
–4
–6
–8
–10
–55 –35 –15
5
25 45 65 85 105 125
–55 –35 –15
5
25 45 65 85 105 125
TEMPERATURE (°C)
TEMPERATURE (°C)
3700 G01
3700 G02
Undervoltage Lockout Trip
Voltage vs Temperature
Shutdown Threshold vs
Temperature
400
380
360
340
320
300
280
260
240
220
200
2.30
2.28
2.26
2.24
2.20
2.00
2.18
2.16
2.14
2.12
2.10
V
= 4.2V
IN
V
RISING
IN
IN
V
FALLING
–55 –35 –15
5
25 45 65 85 105 125
–55 –35 –15
5
25 45 65 85 105 125
TEMPERATURE (°C)
TEMPERATURE (°C)
3700 G04
3700 G03
Buck Supply Current
vs Input Voltage
Maximum (VIN – SENSE–) Voltage
vs Duty Cycle
250
240
230
220
210
200
190
180
170
160
150
130
120
110
100
90
I
/RUN = V
FB
IN2
= 25°C
TH
V
A
= 4.2V
IN
V
= 0V
T
= 25°C
T
A
80
70
60
50
2
6
8
9
20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
3
4
5
7
10
V
IN
INPUT VOLTAGE (V)
3700 G10
3700 G05
3700f
4
LTC3700
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TYPICAL PERFOR A CE CHARACTERISTICS
LDO REGULATOR
LDO Line Regulation (VFB2
Voltage vs Supply)
VFB2 Voltage vs Temperature
850
850
840
830
820
810
800
790
780
770
760
750
V
= 4.2V
FB2
T = 25°C
A
LDO = V
FB2
IN2
840
830
820
810
800
790
780
770
760
750
LDO = V
I
= 1mA
LOAD
I
= 1mA
LOAD
I
= 10µA
LOAD
I
= 10µA
LOAD
I
= 10mA
LOAD
I
= 10mA
LOAD
I
= 100mA
I
= 100mA
LOAD
LOAD
–55 –35 –15
5
25 45 65 85 105 125
2.4 2.85 3.3 3.75 4.2 4.65 5.1 5.55
INPUT VOLTAGE (V)
6
TEMPERATURE (°C)
V
IN2
3700 G06
3700 G07
LDO Pass FET RON vs Input
Voltage
PGOOD RON vs Input Voltage
4.0
3.7
3.4
3.1
2.8
2.5
2.2
1.9
1.6
1.3
1.0
300
270
240
210
180
150
120
90
V
= 0
V
V
T
= 0V
= 100mV
= 25°C
IN
IN2
PGOOD
A
ILDO = 100mA
T
= 25°C
A
60
30
0
2
2.5
3
3.5
4
4.5
5
5.5
6
2
3
4
5
6
7
8
9
10
V
INPUT VOLTAGE (V)
V
INPUT VOLTAGE (V)
IN
IN2
3700 G08
3700 G09
LDO Supply Current
vs Input Voltage
Load Transient Response
120
110
100
90
LDO = V
FB2
I
= 10µA
150
100
50
LDO
V = 0V
IN
T
= 25°C
A
I
LDO (mA)
50mA/DIV
0
80
70
∆VLDO
20mV/DIV
AC COUPLED
0
60
V
IN
= 9.8V
50
40
TA = 25°C
20µs/DIV
3700 G12
30
VIN2 = 3.3V
VLDO = 2.5V
20
C
LDO = 10µF
2
4
5
5.5
2.5
3
3.5
4.5
6
V
IN2
INPUT VOLTAGE (V)
3700 G11
3700f
5
LTC3700
U
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PIN FUNCTIONS
VIN2 (Pin 1): LDO Input Supply Pin. Must be closely
decoupled to GND (Pin 5).
VIN (Pin 7): Buck Input Supply Pin. Must be closely
decoupled to GND (Pin 5).
LDO (Pin 2): LDO Output Pin. Must be closely decoupled
to GND (Pin 5) with a low ESR ceramic capacitor ≥2.2µF.
SENSE– (Pin 8): The Negative Input to the Current Com-
parator of the Buck. Monitors switch current of external
P-Channel MOSFET.
V
FB2 (Pin 3): LDO Feedback Voltage. Receives the feed-
back voltage from an external resistor divider between
LDO (Pin 2) and GND (Pin 5).
VFB (Pin 9): Buck Feedback Voltage. Receives the feed-
back voltage from an external resistor divider between
buck output and GND (Pin 5).
PGOOD (Pin 4): Open-Drain Power Good Output. This pin
willpulltogroundifeithervoltageoutputofthebuckorthe
LDO [sensed at VFB (Pin 9) and VFB2 (Pin 3), respectively]
is out of range. When both voltage outputs are valid, this
pin will go to a high impedance state.
ITH/RUN (Pin 10): This pin performs two functions. It
serves as the error amplifier compensation point for the
buck, as well as a common run control input for both the
buck and the LDO. The current comparator threshold of
the buck increases with this voltage. Nominal voltage
range for this pin is 0.7V to 1.9V. Forcing this pin below
0.3V causes both the buck and the LDO to be shut down.
In shutdown all functions are disabled, the PGATE pin is
held high and the LDO output will go to a high impedance
state.
GND(Pin5):CommonGroundPinforBothBuckandLDO.
PGATE (Pin 6): Gate Drive for Buck’s External P-Channel
MOSFET. This pin swings from 0V to VIN.
3700f
6
LTC3700
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FUNCTIONAL DIAGRA
–
V
SENSE
8
PGOOD
4
V
V
IN
FB2
3
IN2
7
1
0.86V
–
+
LDO
2
V
FB
PGOOD
LDO
V
FB2
0.74V
0.8V
OVERTEMPERATURE
SHDN
DETECT
+
ICMP
–
V
IN
RS1
PGATE
6
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
SLOPE
COMP
R
Q
S
OSC
–
FREQ
OVP
FOLDBACK
BURST
CMP
+
+
–
0.3V
0.15V
+
SHORT-CIRCUIT
DETECT
V
+
REF
SLEEP
60mV
–
V
IN
EAMP
V
REF
0.8V
+
–
0.5µA
V
FB
I
/RUN
9
+
–
10
TH
V
IN
V
V
IN2
IN
0.3V
0.3V
+
SHDN
UV
SHDN
CMP
VOLTAGE
REFERENCE
V
REF
0.8V
–
GND
5
UNDERVOLTAGE
LOCKOUT
1.2V
3700 FD
3700f
7
LTC3700
U
(Refer to Functional Diagram)
OPERATIO
Main Control Loop (Buck Controller)
Dropout Operation
TheLTC3700isaconstantfrequencycurrentmodeswitch-
ing regulator. During normal operation, the external
P-channel power MOSFET is turned on each cycle when
the oscillator sets the RS latch (RS1) and turned off when
the current comparator (ICMP) resets the latch. The peak
inductor current at which ICMP resets the RS latch is
controlled by the voltage on the ITH/RUN pin, which is the
output of the error amplifier EAMP. An external resistive
divider connected between VOUT and ground allows the
EAMPtoreceiveanoutputfeedbackvoltageVFB.Whenthe
load current increases, it causes a slight decrease in VFB
relative to the 0.8V reference, which in turn causes the
ITH/RUN voltage to increase until the average inductor
current matches the new load current.
When the input supply voltage decreases towards the
output voltage, the rate of change of inductor current
during the ON cycle decreases. This reduction means that
the external P-channel MOSFET will remain on for more
thanoneoscillatorcyclesincetheinductorcurrenthasnot
ramped up to the threshold set by EAMP. Further reduc-
tion in input supply voltage will eventually cause the
P-channel MOSFET to be turned on 100%, i.e., DC. The
outputvoltagewillthenbedeterminedbytheinputvoltage
minus the voltage drop across the MOSFET, the sense
resistor and the inductor.
Undervoltage Lockout
TopreventoperationoftheP-channelMOSFETbelowsafe
input voltage levels, an undervoltage lockout is incorpo-
rated into the buck input supply. When the input supply
voltage drops below approximately 2.1V, the P-channel
MOSFET and all circuitry is turned off except the under-
voltage block, which draws only several microamperes.
ThemaincontrolloopisshutdownbypullingtheITH/RUN
pin low. Releasing ITH/RUN allows an internal 0.5µA
current source to charge up the external compensation
network. When the ITH/RUN pin reaches 0.3V, the main
control loop is enabled with the ITH/RUN voltage then
pulled up to its zero current level of approximately 0.7V.
Astheexternalcompensationnetworkcontinuestocharge
up, the corresponding output current trip level follows,
allowing normal operation.
Short-Circuit Protection
Whentheoutputisshortedtoground, thefrequencyofthe
oscillator will be reduced to about 110kHz. This lower
frequency allows the inductor current to safely discharge,
thereby preventing current runaway. The oscillator’s fre-
quency will gradually increase to its designed rate when
the feedback voltage again approaches 0.8V.
Comparator OVP guards against transient overshoots
>7.5% by turning off the external P-channel power
MOSFET and keeping it off until the fault is removed.
Burst Mode Operation
Overvoltage Protection
The buck enters Burst Mode operation at low load cur-
rents. In this mode, the peak current of the inductor is set
as if VITH/RUN = 1V (at low duty cycles) even though the
voltage at the ITH/RUN pin is at a lower value. If the
inductor’saveragecurrentisgreaterthantheloadrequire-
ment, the voltage at the ITH/RUN pin will drop. When the
ITH/RUN voltage goes below 0.85V, the sleep signal goes
high, turning off the external MOSFET. The sleep signal
goes low when the ITH/RUN voltage goes above 0.925V
and the buck resumes normal operation. The next oscilla-
tor cycle will turn the external MOSFET on and the switch-
ing cycle repeats.
As a further protection, the overvoltage comparator in the
buck will turn the external MOSFET off when the feedback
voltage has risen 7.5% above the reference voltage of
0.8V. This comparator has a typical hysteresis of 20mV.
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
V
10 R
ITH – 0.7
IPK
=
(
)
SENSE
3700f
8
LTC3700
U
(Refer to Functional Diagram)
OPERATIO
when the buck is operating below 40% duty cycle. How-
ever, once the duty cycle exceeds 40%, slope com-
pensation begins and effectively reduces the peak induc-
torcurrent. Theamountofreductionisgivenbythecurves
in Figure 2.
The LDO is protected by both current limit and thermal
shutdown circuits. Current limit is set such that the output
voltagewillstartdroppingoutwhentheloadcurrentreaches
approximately 200mA. With a short-circuited LDO output,
the device will limit the sourced current to approximately
225mA. The thermal shutdown circuit has a typical trip
point of 150°C with a typical hysteresis of 5°C. In thermal
shutdown, the LDO pass device is turned off.
Soft-Start
An internal default soft-start circuit is employed at power
up and/or when coming out of shutdown. The soft-start
circuit works by internally clamping the voltage at the ITH/
RUN pin to the corresponding zero-current level and
graduallyraisingtheclampvoltagesuchthattheminimum
time required for the programmed switch current to reach
its maximum is approximately 0.5msec. After the soft-
start circuit has timed out, it is disabled until the part is put
in shutdown again or the input supply is cycled.
Frequency compensation of the LDO is accomplished by
forcing the dominant pole at the output. For stability, a low
ESR ceramic capacitor ≥2.2µF is required from LDO to
GND. For improved transient response, particularly at
heavy loads, it is recommended to use the largest value of
capacitor available in the same size considered.
Both the buck and the LDO share the same internally
generated bandgap reference voltage for their feedback
reference. When both input supplies are present, the
internal reference is powered by the buck input supply
(VIN). For this reason, line regulation for the LDO output is
specified both with respect to VIN and VIN2 if the buck is
present and with respect only to VIN2 if the buck is
disabled. The same is true for VIN2 supply current, which
will be higher when the buck is disabled by the current
draw of the internal reference.
LDO Regulator
The 150mA low dropout (LDO) regulator on the LTC3700
employs an internal P-channel MOSFET pass device be-
tween its input supply (VIN2) and the LDO output pin. The
pass FET has an on-resistance of approximately 1.5Ω
(with VIN2 = 4.2V) with a strong dependence on input
supply voltage. The dropout voltage is simply the FET on-
resistance multiplied by the load current when in dropout.
110
100
90
80
70
60
50
I
= 0.4I
PK
RIPPLE
40
30
20
10
AT 5% DUTY CYCLE
= 0.2I
I
RIPPLE
PK
AT 5% DUTY CYCLE
V
= 4.2V
IN
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
3700 F02
Figure 2. Maximum Output Current vs Duty Cycle
3700f
9
LTC3700
U
W U U
APPLICATIONS INFORMATION
ThebasicLTC3700applicationcircuitisshownin Figure 1.
External component selection for the buck is driven by the
load requirement and begins with the selection of L1 and
RSENSE (= R1). Next, the power MOSFET, M1 and the
output diode D1 are selected followed by CIN (= C1) and
COUT (= C2).
SF
RSENSE
=
(10)(IOUT )(100)
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the use
of a smaller inductor for the same amount of inductor
ripplecurrent. However, thisisattheexpenseofefficiency
due to an increase in MOSFET gate charge losses.
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
Withthecurrentcomparatormonitoringthevoltagedevel-
oped across RSENSE, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent the buck can provide is given by:
The inductance value also has a direct effect on ripple
current. The ripple current, IRIPPLE, decreases with higher
inductance or frequency and increases with higher VIN or
VOUT. The inductor’s peak-to-peak ripple current is given
by:
0.12
RSENSE
IRIPPLE
IOUT
=
−
2
V − VOUT VOUT + VD
IN
IRIPPLE
=
where IRIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section).
f(L)
V + VD
IN
wherefistheoperatingfrequency.Acceptinglargervalues
of IRIPPLE allows the use of low inductances, but results in
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is
IRIPPLE =0.4(IOUT(MAX)).Remember,themaximumIRIPPLE
occurs at the maximum input voltage.
A reasonable starting point for setting ripple current is
IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it
becomes:
1
RSENSE
=
for Duty Cycle < 40%
(10)(IOUT
)
However,foroperationthatisabove40%dutycycle,slope
compensation effect has to be taken into consideration to
selecttheappropriatevaluetoprovidetherequiredamount
of current. Using Figure 2, the value of RSENSE is:
3700f
10
LTC3700
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APPLICATIONS INFORMATION
In Burst Mode operation on the LTC3700, the ripple
current is normally set such that the inductor current is
continuous during the burst periods. Therefore, the peak-
to-peak ripple current must not exceed:
Molypermalloy (from Magnetics, Inc.) is a very good, low
losscorematerialfortoroids,butitismoreexpensivethan
ferrite. A reasonable compromise from the same manu-
facturer is Kool Mµ. Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause they generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount that do
not increase the height significantly are available.
0.03
RSENSE
IRIPPLE
≤
This implies a minimum inductance of:
Power MOSFET Selection
V − VOUT VOUT + VD
IN
LMIN
=
An external P-channel power MOSFET must be selected
for use with the LTC3700. The main selection criteria for
the power MOSFET are the threshold voltage VGS(TH) and
the “on” resistance RDS(ON), reverse transfer capacitance
CRSS and total gate charge.
V + VD
IN
0.03
RSENSE
f
(Use VIN(MAX) = VIN)
A smaller value than LMIN could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
Since the LTC3700 is designed for operation down to low
inputvoltages,asublogiclevelthresholdMOSFET(RDS(ON)
guaranteed at VGS = 2.5V) is required for applications that
workclosetothisvoltage.WhentheseMOSFETsareused,
make sure that the input supply to the buck is less than the
absolute maximum VGS rating, typically 8V.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ® cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, increased inductance requires
more turns of wire and therefore copper losses will in-
crease. Ferrite designs have very low core losses and are
preferred at high switching frequencies, so design goals
canconcentrateoncopperlossandpreventingsaturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
The required minimum RDS(ON) of the MOSFET is gov-
erned by its allowable power dissipation. For applications
that may operate the LTC3700 in dropout, i.e., 100% duty
cycle, at its worst case the required RDS(ON) is given by:
PP
RDS(ON)
=
2
DC=100%
I
(
1+ δp
(
)
)
OUT(MAX)
where PP is the allowable power dissipation and δp is the
temperature dependency of RDS(ON). (1 + δp) is generally
given for a MOSFET in the form of a normalized RDS(ON) vs
temperature curve, but δp = 0.005/°C can be used as an
approximation for low voltage MOSFETs.
Kool Mµ is a registered trademark of Magnetics, Inc.
3700f
11
LTC3700
U
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APPLICATIONS INFORMATION
In applications where the maximum duty cycle is less than
100% and the buck is in continuous mode, the RDS(ON) is
governed by:
where PD is the allowable power dissipation and will be
determined by efficiency and/or thermal requirements.
A fast switching diode must also be used to optimize
efficiency. Schottky diodes are a good choice for low
forwarddropandfastswitchingtimes. Remembertokeep
lead length short and observe proper grounding (see
Board Layout Checklist) to avoid ringing and increased
dissipation.
PP
RDS(ON)
2
DC I
1+ δp
(
(
)
)
OUT
where DC is the maximum operating duty cycle of the
buck.
CIN and COUT Selection
Output Diode Selection
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (VOUT + VD)/
(VIN + VD). To prevent large voltage transients, a low ESR
input capacitor sized for the maximum RMS current must
beused. ThemaximumRMScapacitorcurrentisgivenby:
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As VIN approaches VOUT
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short-circuited. Under this condition the diode must
safelyhandleIPEAK atcloseto100%dutycycle. Therefore,
itisimportanttoadequatelyspecifythediodepeakcurrent
and average power dissipation so as not to exceed the
diode ratings.
1/2
]
V
V − V
OUT
(
)
[
OUT IN
CIN Required IRMS ≈ IMAX
V
IN
This formula has a maximum value at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monlyusedfordesignbecauseevensignificantdeviations
donotoffermuchrelief.Notethatcapacitormanufacturer’s
ripplecurrentratingsareoftenbasedon2000hoursoflife.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC3700, ceramic capacitors
can also be used for CIN. Always consult the manufacturer
if there is any question.
Under normal load conditions, the average current con-
ducted by the diode is:
V − VOUT
V + VD
IN
IN
ID=
IOUT
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
PD
VF ≈
ISC(MAX)
3700f
12
LTC3700
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APPLICATIONS INFORMATION
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surfacemountconfigurations. Inthecaseoftantalum, itis
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS, AVX TPSV and KEMET T510 series of surface mount
tantalum, available in case heights ranging from 2mm to
4mm. Other capacitor types include Sanyo OS-CON,
Nichicon PL series and Panasonic SP.
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 output ripple (∆VOUT) is approximated by:
1
∆VOUT ≈IRIPPLE ESR +
8fCOUT
where f is the operating frequency, COUT is the output
capacitance and IRIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since ∆IL increases with input voltage.
Low Supply Voltage Operation
Manufacturers such as Nichicon, United Chemicon and
Sanyoshouldbeconsideredforhighperformancethrough-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR (size)
product of any aluminum electrolytic at a somewhat
higher price. Once the ESR requirement for COUT has been
met, the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement.
Although the LTC3700 can function down to 2.1V (typ),
the maximum allowable output current is reduced when
VIN decreases below 3V. Figure 3 shows the amount of
changeasthesupplyisreduceddownto2.2V. Alsoshown
in Figure 3 is the effect of VIN on VREF as VIN goes below
2.3V.
105
V
REF
100
95
90
85
80
75
V
ITH
2.0
2.2
2.4
2.6
2.8
3.0
INPUT VOLTAGE (V)
3700 F03
Figure 3. Line Regulation of VREF and VITH
3700f
13
LTC3700
U
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APPLICATIONS INFORMATION
Setting Output Voltage (Buck Controller)
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
The buck develops a 0.8V reference voltage between the
feedback (Pin 9) terminal and ground (see Figure 4). By
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the overall output voltage. The
regulated output voltage is determined by:
Foldback current limiting is implemented by adding di-
odes DFB1 and DFB2 between the output and the ITH/RUN
pin as shown in Figure 5. In a hard short (VOUT = 0V), the
current will be reduced to approximately 50% of the
maximum output current.
R2
R1
VOUT1 = 0.8 1+
Setting Output Voltage (LDO Regulator)
The LDO develops a 0.8V reference voltage between VFB2
(Pin 3) and ground (see Figure 6), similar to the buck
controller. The regulated output voltage VOUT2 is given by:
Formostapplications, an80kresistorissuggestedforR1.
To prevent stray pickup, locate resistors R1 and R2 close
to LTC3700.
R4
VOUT2 = 0.8 1+
R3
Foldback Current Limiting
As described in the Output Diode Selection, the worst-
case dissipation occurs with a short-circuited output
when the diode conducts the current limit value almost
continuously. To prevent excessive heating in the diode,
Formostapplications, an80kresistorissuggestedforR3.
To prevent stray pickup, locate resistors R3 and R4 close
to LTC3700.
V
OUT1
LTC3700
/RUN V
V
OUT1
R2
R1
+
R2
R1
10
9
LTC3700
I
TH
FB
D
D
9
FB1
FB2
V
FB
3700 F05
3700 F04
Figure 5. Foldback Current Limiting
Figure 4. Setting Output Voltage (Buck Controller)
2
3
LDO
LTC3700
V
OUT2
R4
R3
V
FB2
3700 F06
Figure 6. Setting Output Voltage (LDO Regulator)
3700f
14
LTC3700
U
PACKAGE DESCRIPTION
MS Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.2 – 3.45
(.126 – .136)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.497 ± 0.076
(.0196 ± .003)
REF
0.50
0.305 ± 0.038
(.0120 ± .0015)
TYP
(.0197)
10 9
8
7 6
BSC
RECOMMENDED SOLDER PAD LAYOUT
3.00 ± 0.102
(.118 ± .004)
NOTE 4
4.90 ± 0.15
(1.93 ± .006)
DETAIL “A”
0° – 6° TYP
0.254
(.010)
GAUGE PLANE
1
2
3
4 5
0.53 ± 0.01
(.021 ± .006)
0.86
(.034)
REF
1.10
(.043)
MAX
DETAIL “A”
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.13 ± 0.076
(.005 ± .003)
MSOP (MS) 0802
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
3700f
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.
15
LTC3700
U
TYPICAL APPLICATIO
5V Input Supply to 3.3V/1A High Efficiency Output and 2.5V/150mA Low Noise Output
V
IN1
5V
7
1
R1
0.05Ω
C1
10µF
16V
V
V
IN2
IN
8
6
2
3
V
–
OUT2
SENSE
PGATE
LDO
2.5V AT
150mA
169k
L1
15µH
C3
M1
V
V
2.2µF
FB2
OUT1
3.3V
16V
78.7k
AT 1A
249k
LTC3700
D1
+
C2
47µF
6V
C1: TAIYO YUDEN EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
C3: MURATA GRM42-6X7R225K016AL
D1: MOTOROLA MBRS130LT3
L1: COILTRONICS UP1B150
M1: Si3443DV
80.6k
9
4
5
V
PGOOD
/RUN GND
FB
10
I
TH
10k
220pF
R1: DALE 0.25W
3700 TA01
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PART NUMBER
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DC/DC Controller in ThinSOT
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at Light Load Current
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2.65V ≤ V ≤ 8.5V, 0.8V ≤ V
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Up to 97% Efficiency, 4V ≤ V ≤ 36V,
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0.8V ≤ V
≤ (0.9)(V ), I
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2.5V ≤ V ≤ 9.8V, 550kHz, 90% Efficiency
IN
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2.65V ≤ V ≤ 6V, 700mA Output Current, 8-Lead MSOP
IN
LTC3406/LTC3406B 600mA (I ), 1.5MHz Synchronous Step-Down Converter
V = 2.5V to 5.5V, 95% Efficiency, ThinSOT,
IN
B Version: Burst Mode Defeat
OUT
No R
and ThinSOT are trademarks of Linear Technology Corporation.
SENSE
3700f
LT/TP 0203 2K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
LINEAR TECHNOLOGY CORPORATION 2001
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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