FEATURES
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LTC3785-1
10V, High Efficiency,
Buck-Boost Controller
with Power Good
DESCRIPTION
The LTC
®
3785-1 is a high power synchronous buck-boost
controller that drives all N-channel power MOSFETs from
input voltages above, below and equal to the output volt-
age. With an input range of 2.7V to 10V, the LTC3785-1 is
well suited for a wide variety of single or dual cell Li-Ion
or multicell alkaline/NiMH applications.
The operating frequency can be programmed from 100kHz
to 1MHz. The soft-start time and current limit are also
programmable. The soft-start capacitor doubles as the
fault timer which can program the IC to latch off or recycle
after a determined off time. Burst Mode operation is user
controlled and can be enabled by driving the mode pin
high. The LTC3785-1 includes a power good output that
indicates when the output voltage is within 7.5% of its
designed setpoint.
Protection features include foldback current limit,
short-circuit and overvoltage protection.
L,
LT, LTC ,LTM and Burst Mode are registered trademarks of Linear Technology Corporation.
No R
SENSE
is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
Single Inductor Architecture Allows V
IN
Above,
Below or Equal to V
OUT
Power Good Output Indicator
2.7V to 10V Input and Output Range
Up to 96% Efficiency
Up to 10A of Output Current
All N-channel MOSFETs, No R
SENSE
™
True Output Disconnect During Shutdown
Programmable Current Limit and Soft-Start
Optional Short-Circuit Shutdown Timer
Output Overvoltage and Undervoltage Protection
Programmable Frequency: 100kHz to 1MHz
Selectable Burst Mode
®
Operation
Available in 24-Lead (4mm
×
4mm) Exposed Pad
QFN Package
APPLICATIONS
n
n
n
n
Palmtop Computers
Handheld Instruments
Wireless Modems
Cellular Telephones
TYPICAL APPLICATION
V
OUT
V
IN
V
CC
215k
1.3k
270pF
FB
127k
1nF
V
C
RT
49.9k
MODE
PGOOD
RUN/SS
I
LSET
2.2nF
42.2k
CCM
GND
TG2
D
V
BST2
SW2
I
SSW2
BG2
C
37851 TA01a
4.7μF
V
IN
2.7V
TO 10V
100
Efficiency vs Input Voltage
V
OUT
= 3.3V
F
OSC
= 500kHz
I
SVIN
V
SENSE
TG1
A
V
BST1
0.22μF
SW1
I
SSW1
V
DRV
BG1
LTC3785-1
I
SVOUT
22μF
EFFICIENCY (%)
95
215k
127k
I
LOAD
= 2A
I
LOAD
= 1A
90
12k
4.7μH
B
V
OUT
3.3V
5A
85
2.5
4
5.5
V
IN
(V)
7
8.5
10
37851 TA01b
0.22μF
100μF
37851fa
1
LTC3785-1
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
V
BST1
I
SVIN
SW1
18 I
SSW1
17 BG1
25
16 V
DRV
15 BG2
14 I
SSW2
13 SW2
7
RT
8
MODE
9 10 11 12
I
SVOUT
V
BST2
PGOOD
TG2
TG1
V
CC
V
IN
RUN/SS 1
V
C
2
FB 3
V
SENSE
4
I
LSET
5
CCM 6
Input Supply Voltage (V
IN
) ......................... –0.3V to 11V
I
SVOUT
, I
SVIN
.............................................. –0.3V to 11V
SW1, SW2, I
SSW1
, I
SSW2
Voltage:
DC............................................................. –1V to 11V
Pulsed, <1μs ............................................. –2V to 12V
RUN/SS, MODE, CCM, V
DRV
, V
CC
Voltages .. –0.3V to 6V
PGOOD Voltage ............................................ –0.3V to 6V
V
BST1
Voltage ............................................. –0.3V to 16V
With Respect to SW1 ............................... –0.3V to 6V
V
BST2
Voltage ............................................. –0.3V to 16V
With Respect to SW2 ............................... –0.3V to 6V
Peak Driver Output Current < 10μs
(TG1, TG2, BG1, BG2).................................................3A
V
CC
Average Output Current.................................100mA
Operating Temperature Range (Note 2).... –40°C to 85°C
Junction Temperature ........................................... 125°C
Storage Temperature Range................... –65°C to 125°C
24 23 22 21 20 19
UF PACKAGE
24-LEAD (4mm 4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 37°C/W 4 LAYER BOARD
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3785EUF-1#PBF
TAPE AND REEL
LTC3785EUF-1#TRPBF
PART MARKING
37851
PACKAGE DESCRIPTION
24-Lead (4mm
×
4mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
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/
ELECTRICAL CHARACTERISTICS
PARAMETER
V
IN
Supply
Input Operating Voltage
Quiescent Current—Burst Mode Operation
Quiescent Current—Shutdown
Quiescent Current—Active
Error Amp
Feedback Voltage
Feedback Input Current
Error Amp Source Current
Error Amp Sink Current
Error Amp A
VOL
(Note 5)
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= I
SVOUT
= V
DRV
= V
BST1
= V
BST2
= 3.6V, R
T
= 49.9k, R
ILSET
= 59k.
CONDITIONS
l
MIN
2.7
TYP
MAX
10
UNITS
V
μA
μA
mA
V
nA
μA
μA
dB
37851fa
V
C
= 0V, MODE = 3.6V (Note 4)
RUN/SS = 0V, I
SVOUT
= 3.6V
MODE = 0V (Note 4)
l
86
15
0.8
1.200
1.225
1
–500
900
90
200
25
1.5
1.25
500
2
LTC3785-1
ELECTRICAL CHARACTERISTICS
PARAMETER
V
CC
Regulator
V
CC
Maximum Regulating Voltage
V
CC
Regulation Voltage
V
CC
Regulator Sink Current
Run/Soft-Start
RUN/SS Threshold
RUN/SS Input Current
RUN/SS Discharge Current
Current Limit
Current Limit Sense Threshold
Reverse Current Limit Sense Threshold
Input Current
I
SVIN
to I
SSW1
, R
ILSET
= 121k
I
SVIN
to I
SSW1
, R
ILSET
= 59k
I
SSW2
to I
SVOUT
, CCM > 2V
I
SSW2
to I
SVOUT
, CCM < 0.4V
I
SVIN
I
SVOUT
I
SSW1
, I
SSW2
l
l
l
l
l
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= I
SVOUT
= V
DRV
= V
BST1
= V
BST2
= 3.6V, R
T
= 49.9k, R
ILSET
= 59k.
CONDITIONS
V
IN
= 5V, I
VCC
= –20mA
V
IN
= 3.6V, I
VCC
= –20mA
I
SVOUT
= V
CC
= 5V
When IC is Enabled
When EA is at Maximum Boost Duty Cycle
RUN/SS = 0V
During Current Limit
20
55
–50
l
l
l
MIN
4.15
3.3
TYP
4.35
3.5
800
MAX
4.55
3.6
UNITS
V
V
μA
0.35
0.7
1.9
–1
20
60
105
–110
–15
80
10
0.1
1.1
V
V
μA
μA
mV
mV
mV
mV
μA
μA
μA
V
V
μA
V
μA
μs
30
100
155
–170
–35
150
20
5
0.4
CCM Input Threshold (High)
CCM Input Threshold (Low)
CCM Input Current
Burst Mode Operation
Mode Threshold
Mode Input Current
t
ON
Time
Oscillator
Frequency Accuracy
Switching Characteristics
Maximum Duty Cycle
TG1, TG2 Driver Impedance
BG1, BG2 Driver Impedance
TG1, TG2 Rise Time
BG1, BG2 Rise Time
TG1, TG2 Fall Time
BG1, BG2 Fall Time
Buck Driver Nonoverlap Time
Boost Driver Nonoverlap Time
Power Good
Undervoltage Threshold
Undervoltage Hysteresis
Overvoltage Threshold
Overvoltage Hysteresis
V
SENSE
Rising % Above FB Regulation Voltage,
MODE = 0V
V
SENSE
Falling, % Below FB Regulation Voltage
C
LOAD
= 3300pF (Note 3)
C
LOAD
= 3300pF (Note 3)
C
LOAD
= 3300pF (Note 3)
C
LOAD
= 3300pF (Note 3)
TG1 to BG1
TG2 to BG2
Boost (% Switch BG2 On)
Buck (% Switch TG1 On)
CMM = 3.6V
2.2
0.01
1
2.2
1
l
0.8
1.5
0.01
1.4
l
l
370
80
509
90
99
2
2
20
20
20
20
100
100
650
kHz
%
%
Ω
Ω
ns
ns
ns
ns
ns
ns
–5
5
–7.5
1.5
7.5
–2
–10.5
10.5
%
%
%
%
37851fa
3
LTC3785-1
ELECTRICAL CHARACTERISTICS
PARAMETER
PGOOD Output Low
PGOOD Leakage
V
SENSE
Input Current
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= I
SVOUT
= V
DRV
= V
BST1
= V
BST2
= 3.6V, R
T
= 49.9k, R
ILSET
= 59k.
CONDITIONS
I
PGOOD
= 500μA
V
PGOOD
= 5.5V
V
SENSE
= Measured FB Voltage
1
MIN
TYP
200
MAX
500
5
500
UNITS
mV
μA
nA
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 2:
The LTC3785E-1 is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
Specification is guaranteed by design and not 100% tested in production.
Note 4:
Current measurements are performed when the outputs are not switching.
Note 5:
The IC is tested in a feedback loop to make the measurement.
TYPICAL PERFORMANCE CHARACTERISTICS
Li-Ion to 3.3V Efficiency
vs Load Current
100
90
80
EFFICIENCY (%)
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.0001
0.001
V
IN
= 4.2V
V
IN
= 3.6V
V
IN
= 3V
MOSFET Si7940
L = 4.7μH WURTH WE-PD
f
OSC
= 500kHz
0.1
0.01
LOAD CURRENT (A)
1
10
37851 G01
Two Li-Ion to 7V Efficiency
vs Load Current
100
100
Burst Mode
OPERATION
90
80
FIXED
FREQUENCY
EFFICIENCY (%)
70
60
50
40
30
20
10
90
80
70
60
50
40
30
20
10
0
0.0001
0.001
V
IN
= 8.4V
V
IN
= 7.2V
V
IN
= 5.4V
MOSFET Si7940
L = 5.6μH MSS1260
f
OSC
= 430kHz
0.01
0.1
LOAD CURRENT (A)
1
10
37851 G02
Li-Ion/9V to 5V V
OUT
Efficiency
vs Load Current
Burst Mode
OPERATION
Burst Mode
OPERATION
FIXED
FREQUENCY
FIXED
FREQUENCY
V
IN
= 9V
V
IN
= 4.2V
V
IN
= 3.6V
V
IN
= 2.7V
MOSFET Si7940
L = 5.6μH MSS1260
f
OSC
= 430kHz
0.001
0.01
0.1
LOAD CURRENT (A)
1
10
37851 G03
0
0.0001
Burst Mode Ripple
V
OUT
50mV/DIV
AC
COUPLED
INDUCTOR
CURRENT
1A/DIV
V
OUT
= 3.3V
C
OUT
= 100μF
5μs/DIV
37851 G04
Line Transient Response
V
OUT
500mV/DIV
V
OUT
200mV/
DIV
V
OUT
Load Transient
V
IN
3V TO 8.5V
I
LOAD
= 300μA
V
OUT
= 5V
500μs/DIV
37851 G05
I
LOAD
10mA TO 2A
V
IN
= 3.6V
V
OUT
= 3.3V
C
100μF
100μs/DIV
37851 G06
37851fa
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LTC3785-1
TYPICAL PERFORMANCE CHARACTERISTICS
V
FB
vs Temperature
1.2255
1.2250
CHANGE FROM 25°C (%)
1.2245
1.2240
V
FB
(V)
1.2235
1.2230
1.2225
1.2220
1.2215
1.2210
–50
–25
50
25
0
TEMPERATURE (°C)
75
100
37851 G07
Normalized Oscillator Frequency
vs Temperature
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–50
0
–25
25
50
0
TEMPERATURE (°C)
75
100
OSCILLATOR FREQUENCY (kHz)
1000
800
600
400
200
1200
Oscillator Frequency vs RT
20
40
60
RT (kΩ)
80
100
37851 G09
37851 G08
V
IN
Start-Up Voltage
vs Temperature
2.490
100
V
IN
Burst Quiescent Current
vs Temperature
8
6
OV and UV Thresholds
vs Temperature
OV THRESHOLD
V
IN
START-UP VOLTAGE (V)
2.485
V
IN
CURRENT (mA)
95
THRESHOLD (%)
4
2
0
–2
–4
–6
UV THRESHOLD
–25
25
50
0
TEMPERATURE (°C)
75
100
2.480
90
2.475
85
2.470
2.465
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
80
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
–8
–50
37851 G10
37851 G11
37851 G12
PIN FUNCTIONS
RUN/SS (Pin 1):
Run Control and Soft-Start Input. An
internal 1μA charges the soft-start capacitor and will
charge to approximately 2.5V. During a current limit fault,
the soft-start capacitor will incrementally discharge. Once
the pin drops below 1.225V the IC will enter fault mode,
turning off the outputs for 32 times the soft-start time. If
>5μA (at RUN/SS = 1.225V) is applied externally, the part
will latch off after a fault is detected. If >40μA (at RUN/SS
= 1.225V) is applied externally, current limit faults will not
discharge the SS capacitor.
V
C
(Pin 2):
Error Amp Output. A frequency compensation
network is connected from this pin to the FB pin to compen-
sate the loop. See the section Closing the Feedback Loop
in the Applications Information section for guidelines.
FB (Pin 3):
Feedback Pin. Connect resistor divider tap
here. The feedback reference voltage is typically 1.225V
The output voltage can be adjusted from 2.7V to 10V ac-
cording to the following formula:
V
OUT
=
1.225V •
R1+ R2
R2
37851fa
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