Datasheet

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LTC3785

3785fc

Typical applicaTion

DescripTion

10V, High Efﬁciency,

Synchronous, No R

SENSE

Buck-Boost Controller

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 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 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.

L, LT, LTC, LTM, Linear Technology, the Linear logo 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.

Efﬁciency vs Input Voltage

FeaTures

applicaTions

Single Inductor Architecture Allows V

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

Palmtop Computers

Handheld Instruments

Wireless Modems

Cellular Telephones

SVIN

TG1

SENSE

BG1

TG2

BG2

SW2

3785 TA01a

BST1

SW1

SSW1

SVOUT

DRV

SSW2

BST2

CCM

LSET

MODE

RUN/SS

OUT

LTC3785

GND

4.7µF

2.7V

TO 10V

OUT

3.3V

22µF

4.7µH

100µF

0.22µF

270pF

1.3k

205k 121k

12k

42.2k

59k

121k

205k

1nF

(V)

2.5

EFFICIENCY (%)

3785 TA01b

5.5

7 8.5

100

LOAD

= 1A

OUT

= 3.3V

OSC

= 500kHz

LOAD

= 2A

Summary of content (20 pages)

PAGE 1
LTC3785 10V, High Efficiency, Synchronous, No RSENSE Buck-Boost Controller Description Features n n n n n n n n n n n n 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 voltage. 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 multicell alkaline/NiMH applications.
PAGE 2
LTC3785 Absolute Maximum Ratings Pin Configuration (Note 1) SW1 TG1 VBST1 ISVIN VIN VCC TOP VIEW Input Supply Voltage (VIN).......................... –0.3V to 11V ISVOUT, ISVIN ............................................... –0.3V to 11V SW1, SW2, ISSW1, ISSW2 Voltage: DC.............................................................. –1V to 11V Pulsed, <1µs.............................................. –2V to 12V RUN/SS, MODE, CCM, VDRV, VCC Voltages... –0.3V to 6V VBST1 Voltage.....................
PAGE 3
LTC3785 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = ISVOUT = VDRV = VBST1 = VBST2 = 3.6V, RT = 49.9k, RILSET = 59k.
PAGE 4
LTC3785 electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = ISVOUT = VDRV = VBST1 = VBST2 = 3.6V, RT = 49.9k, RILSET = 59k.
PAGE 5
LTC3785 Typical Performance Characteristics Normalized Oscillator Frequency vs Temperature VFB vs Temperature 0.8 1.2250 CHANGE FROM 25°C (%) 1.2245 1.2240 VFB (V) Oscillator Frequency vs RT 1200 1.0 1.2235 1.2230 1.2225 1.2220 1000 OSCILLATOR FREQUENCY (kHz) 1.2255 TA = 25°C, unless otherwise noted. 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 800 600 400 200 –0.8 1.2215 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) –1.
PAGE 6
LTC3785 Pin Functions VSENSE (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 VOUT be applied. The divider can be skewed from the feedback value to achieve the desired UV or OV threshold. ILSET (Pin 5): Current Limit Set.
PAGE 7
LTC3785 Block Diagram VIN 2.7V TO 10V 24 + – – + 1.225V VBE VIN FAULT LOGIC RUN TSD + – UVLO VREF 1.225V 2.4V ILIMIT 1/25k 1 RUN/SS ISVIN + – gm TG1 ADRV ILIM(OUT) ILIM(OUT) 10µA MAX + X10 – + – IMAX VBST1 SW1 V = 90k/RILSET + – CP1 3 2 RT 7 FB OV REVERSE LIMIT VC RT BG1 BBM SW2 DELAY SW2 PULSE + – 1.
PAGE 8
LTC3785 Operation Main Control Loop The LTC3785 is a buck-boost voltage mode controller that provides an output voltage above, equal to or below the input voltage. The LTC proprietary topology and control architecture also employs drain-to-source sensing (No RSENSE) for forward and reverse current limiting. The controller provides all N-channel MOSFET output switch drive, facilitating single package multiple power switch technology along with lower RDS(ON).
PAGE 9
LTC3785 operation the buck-boost range, approximately 0.7V, the AC switch pair completely phase out the BD pair, and the boost phase begins at duty cycle, D4(SW). So the peak current is independent of VIN and inversely proportional to the f • L product optimizing the energy transfer for various applications. The input voltage, VIN, where the four switch region begins is given by: In Burst Mode operation the maximum output current is given by: 1.
PAGE 10
LTC3785 Operation switch A and D. In most applications a 0.1µF to 0.47µF, X5R or X7R dielectric capacitor is adequate. Run/Soft-Start (RUN/SS) The RUN/SS pin serves as the enable to the LTC3785, soft-start function, and fault programming. A 1µA current source charges the external capacitor. Once the RUN/SS voltage is above a diode drop(~0.7V) the IC is enabled. Once the IC is enabled, the RUN/SS voltage minus a diode drop (RUN/SS – 0.7V) clamps the output of the error amp (VC) to limit duty cycle.
PAGE 11
LTC3785 operation To have the power converter latch off on a fault, a pull-up current between 4µA and 7µA on the RUN/SS pin will allow the RUN/SS capacitor to discharge during an extended fault, but will prevent cycling of the fault which will cause the converter to stay off. One method to implement this is by placing a diode (anode tied to VOUT) and a resistor from VOUT to the RUN/SS pin. The current sourced into RUN/SS will be VOUT – 0.7 divided by the resistor value.
PAGE 12
LTC3785 Applications Information Inductor Selection The high frequency operation of the LTC3785 allows the use of small surface mount inductors. The inductor current ripple is typically set 20% to 40% of the maximum inductor current.
PAGE 13
LTC3785 applications information eters for the power MOSFETs are the breakdown voltage VBR(DSS), threshold voltage VGS(TH), on-resistance RDS(ON), reverse transfer capacitance CRSS and maximum current IDS(MAX). The drive voltage is set by the 4.5V VCC supply. Consequently, logic-level threshold MOSFETs must be used in LTC3785 applications. If the input voltage is expected to drop below 5V, then sub-logic threshold MOSFETs should be considered.
PAGE 14
LTC3785 applications information Closing the Feedback Loop The LTC3785 incorporates voltage mode control. The control to output gain is given by: GBUCK = 1.6 • VIN , Buck Mode The unity gain frequency of the error amplifier with the Type I compensation is given by: 1.6 • VOUT 2 GBOOST = , Boost Mode VIN The output filter exhibits a double-pole response and is given by: 1 fFILTER _ POLE = 2 • π • L • COUT where COUT is the output filter capacitor.
PAGE 15
LTC3785 applications information Efficiency Considerations The percentage 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 circuits produce losses, four main sources account for most of the losses in LTC3785 application circuits: 1. DC I2R losses.
PAGE 16
LTC3785 applications information Determine the Proper Inductor Type Selection The highest inductor current is during boost mode and is given by: VOUT •IOUT VIN • η IL(MAX _ AV) = where η = estimated efficiency in this mode (use 80%). 3.3 • 3 = 4.6A 2.7 • 0.
PAGE 17
LTC3785 Applications Information PC Board Layout Checklist The basic PC board layout requires a dedicated ground plane layer. Also, for high current, a multilayer board provides heat sinking for power components. • The ground plane layer should not have any traces and it should be as close as possible to the layer with power MOSFETs. • Place CIN, switch A, switch B and D1 in one compact area. Place COUT, switch C, switch D and D2 in one compact area.
PAGE 18
LTC3785 Typical Application VIN 2.7V TO 10V 1nF 121k VIN RUN/SS VSENSE R2 121k 12k 1nF SW1 FB MA CMDSH-3 VBST1 1.3k CIN 22µF MA = MB = MC = MD = 1/2 Si7940DY L1 = WÜRTH ELECTRONICS 744311470 D1 = D2 = PMEG2020EJ CA 0.22µF OPTIONAL ISSW1 VDRV BG1 LTC3785 MB D1 L1 4.7µH VC RT ISVOUT RT 59k TG2 MD VBST2 SW2 ILSET CB 0.22µF COUT 100µF ISSW2 CCM GND BG2 VOUT 3.3V 3A OPTIONAL D2 CMDSH-3 MODE RILSET 42.2k Li-Ion 2.7V TO 4.2V VCC TG1 270pF R1 205k + CVCC 4.
PAGE 19
LTC3785 Revision History (Revision history begins at Rev C) REV DATE DESCRIPTION PAGE NUMBER C 3/10 Added LTC3785IUF (I-Grade) Part. Reflected Throughout Data Sheet 1 Through 20 3785fc 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 20
LTC3785 Typical Application Li-Ion/9V Wall Adapter to 5V/2A VIN 2.7V TO 10V 1nF VIN RUN/SS 66.5k VSENSE 66.5k 12k 59k 1nF MA CMDSH-3 VBST1 SW1 1.3k MA = MB = MC = MD = 1/2 Si7940DY L1 = RLF7030T-3R3M4R1 D1 = D2 = PMEG2020EJ OPTIONAL MB D1 L1 3.3µH VC RT ISVOUT TG2 MD MODE VBST2 ILSET SW2 ISSW2 CCM GND CB 0.22µF BG2 VOUT 5V 2A OPTIONAL D2 CMDSH-3 42.2k CIN 22µF CA 0.22µF ISSW1 VDRV BG1 LTC3785 FB Li-Ion 2.7V TO 4.2V VCC TG1 270pF 205k + CVCC 4.