FEATURES
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LTC3785
10V, High Efficiency,
Synchronous, No R
SENSE
Buck-Boost Controller
DESCRIPTIO
TM
Single Inductor Architecture Allows V
IN
Above,
Below or Equal to V
OUT
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
The LTC
®
3785 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 is
well suited for a wide variety of single or dual cell Li-Ion
or multi-cell 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 opera-
tion is user controlled and can be enabled by driving the
MODE pin high.
Protection features include foldback current limit, short-
circuit and overvoltage protection.
, 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.
APPLICATIO S
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Palmtop Computers
Handheld Instruments
Wireless Modems
Cellular Telephones
TYPICAL APPLICATIO
V
OUT
V
IN
V
CC
I
SVIN
V
SENSE
TG1
V
BST1
FB
SW1
I
SSW1
V
DRV
BG1
LTC3785
I
SVOUT
TG2
4.7µF
V
IN
2.7V
TO 10V
22µF
100
Efficiency vs Input Voltage
V
OUT
= 3.3V
F
OSC
= 500kHz
4.7µH
EFFICIENCY (%)
95
I
LOAD
= 2A
I
LOAD
= 1A
90
V
C
RT
MODE
V
OUT
3.3V
5A
V
BST2
RUN/SS
I
LSET
CCM
GND
SW2
I
SSW2
BG2
100µF
85
2.5
4
5.5
V
IN
(V)
3785 TA01b
3785 TA01a
U
U
U
7
8.5
10
3785f
1
LTC3785
ABSOLUTE
(Note 1)
AXI U RATI GS
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
TG1, V
BST1
Voltages................................... –0.3V to 16V
With Respect to SW1 ............................... –0.3V to 6V
TG2, V
BST2
Voltages................................... –0.3V to 16V
With Respect to SW2 ............................... –0.3V to 6V
BG1, BG2 Voltage ........................................ –0.3V to 6V
Peak Driver Output Current < 10µs
(TG1, TG2, BG1, BG2).................................................3A
V
CC
Average Output Current .................................100mA
Operating Temperature Range ................. –40°C to 85°C
Storage Temperature Range................... –65°C to 125°C
V
BST1
I
SVIN
24 23 22 21 20 19
RUN/SS 1
V
C
2
FB 3
V
SENSE
4
I
LSET
5
CCM 6
7
RT
8
MODE
9 10 11 12
I
SVOUT
V
BST2
TG2
NC
25
18 I
SSW1
17 BG1
16 V
DRV
15 BG2
14 I
SSW2
13 SW2
UF PACKAGE
24-LEAD (4mm
×
4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 40°C/W 1 LAYER BOARD,
θ
JA
= 30°C/W 4 LAYER BOARD
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3785EUF#PBF
LEAD BASED FINISH
LTC3785EUF
TAPE AND REEL
LTC3785EUF#TRPBF
TAPE AND REEL
LTC3785EUF#TR
PART MARKING
3785
PART MARKING
3785
PACKAGE DESCRIPTION
24-Lead (4mm
×
4mm) Plastic QFN
PACKAGE DESCRIPTION
24-Lead (4mm
×
4mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 85°C
TEMPERATURE RANGE
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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
Overvoltage Threshold
(Note 5)
(Note 5)
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= V
OUT
= V
DRV
= V
BST1
= V
BST2
= 3.6V, R
T
= 49.9k, R
ILSET
= 59k.
CONDITIONS
●
MIN
2.7
SW1
TG1
V
CC
V
IN
V
C
= 0V, MODE = 3.6V (Note 4)
RUN/SS = 0V, V
OUT
= 0V
MODE = 0V (Note 4)
●
V
SENSE
Pin. % Above FB
2
U
W W
W
PIN CONFIGURATION
TOP VIEW
TYP
MAX
10
UNITS
V
µA
µA
mA
V
nA
µA
µA
dB
86
15
0.8
1.200
1.225
1
–500
900
90
●
200
25
1.5
1.25
500
6
10
14
%
3785f
LTC3785
ELECTRICAL CHARACTERISTICS
PARAMETER
Undervoltage Threshold
V
SENSE
Input Current
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
●
●
●
●
●
●
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= V
OUT
= V
DRV
= V
BST1
= V
BST2
= 3.6V, R
T
= 49.9k, R
ILSET
= 59k.
CONDITIONS
V
SENSE
Pin. % Below FB
V
SENSE
= Measured FB Voltage
V
IN
= 5V, I
VCC
= –20mA
V
IN
= 3.6V, I
VCC
= –20mA
V
OUT
= V
CC
= 5V
When IC is Enabled
When EA is at Maximum Boost Duty Cycle
RUN/SS = 0V
During Current Limit
20
55
–50
●
●
●
●
MIN
–3.5
TYP
–6.5
1
MAX
–9.5
500
4.55
3.6
UNITS
%
nA
V
V
µA
4.15
3.3
4.35
3.5
800
0.35
0.7
1.9
–1
1
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
5
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
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)
2.2
0.01
1
2.2
1
●
0.8
1.5
0.01
1.4
●
370
80
509
90
99
2
2
20
20
20
20
100
100
650
kHz
%
%
Ω
Ω
ns
ns
ns
ns
ns
ns
●
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 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.
3785f
3
LTC3785
TYPICAL PERFOR A CE CHARACTERISTICS
(T
A
= 25°C unless otherwise noted)
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
3785 G01
Burst Mode
OPERATION
FIXED
FREQUENCY
60
50
40
30
20
10
0
0.0001
0.001
FIXED
FREQUENCY
EFFICIENCY (%)
Burst Mode Ripple
V
OUT
500mV/
DIV
V
IN
3V TO
8.5V
V
OUT
50mV/DIV
AC
COUPLED
INDUCTOR
CURRENT
1A/DIV
V
OUT
= 3.3V
C
OUT
= 100µF
5µs/DIV
3785 G04
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
3785 G07
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–50
OSCILLATOR FREQUENCY (kHz)
4
U W
Two Li-Ion to 7V Efficiency vs
Load Current
100
90
80
70
Burst Mode
OPERATION
100
90
80
70
60
50
40
30
20
10
Li-Ion/9V to 5V V
OUT
Efficiency vs
Load Current
Burst Mode
OPERATION
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
3785 G02
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
3785 G02
0
0.0001
Line Transient Response
V
OUT
200mV/
DIV
V
OUT
Load Transient
I
LOAD
= 300mA
V
OUT
= 5V
C
OUT
= 100µF
500µs/DIV
3785 G05
I
LOAD
10mA TO 2A
V
IN
= 3.6V
V
OUT
= 3.3V
C
OUT
= 100µF
100µs/DIV
3785 G06
Normalized Oscillator Frequency
vs Temperature
1.0
0.8
1000
800
600
400
200
0
–25
25
50
0
TEMPERATURE (°C)
75
100
3785 G08
Oscillator Frequency vs RT
1200
20
40
60
RT (kΩ)
80
100
3785 G09
3785f
LTC3785
TYPICAL PERFOR A CE CHARACTERISTICS
(T
A
= 25°C unless otherwise noted)
V
IN
Start-Up Voltage vs
Temperature
2.490
100
V
IN
START-UP VOLTAGE (V)
2.485
V
IN
CURRENT (µA)
THRESHOLD (V)
2.480
2.475
2.470
2.465
–50
–25
0
25
50
TEMPERATURE (°C)
PI FU CTIO S
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 com-
pensate the loop. See the section “Closing the Feedback
Loop” 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
V
SENSE
(Pin 4):
Overvoltage and Undervoltage Sense.
The overvoltage threshold is internally set 10% above
the regulated FB voltage and the undervoltage threshold
is internally set 6.5% below the FB regulated voltage. This
pin can be tied to FB but to optimize the response time it
is recommended that a voltage divider from V
OUT
be ap-
plied. The divider can be skewed from the feedback value
to achieve the desired UV or OV threshold.
I
LSET
(Pin 5):
Current Limit Set. A resistor from this pin
to ground sets the current limit threshold from the I
SVIN
and I
SSW1
pins.
CCM (Pin 6):
Continuous Conduction Mode Control Pin.
When set low, the inductor current is allowed to go slightly
negative (–15mV referenced to the I
SVOUT
– I
SSW2
pins).
When driven high, the reverse current limit is set to the simi-
lar value of the forward current limit set by the I
LSET
pin.
RT (Pin 7):
Oscillator Programming Pin. A resistor from
this pin to GND sets the free-running frequency of the IC.
f
OSC
≅
2.5e10/RT.
U W
75
3785 G10
V
IN
Burst Quiescent Current vs
Temperature
12
10
95
8
6
4
2
0
–2
–4
–6
100
80
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
3785 G11
OV and UV Thresholds vs
Temperature
OV THRESHOLD
90
85
UV THRESHOLD
–8
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
3785 G12
U
U
U
3785f
5
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