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LTC3789

3789fa

Typical applicaTion

FeaTures

applicaTions

DescripTion

High Efficiency, Synchronous,

4-Switch Buck-Boost Controller

The LTC

3789 is a high performance buck-boost switch-

ing regulator controller that operates from input volt-

ages above, below or equal to the output voltage. The

constant-frequency, current mode architecture allows

a phase-lockable frequency of up to 600kHz, while an

output current feedback loop provides support for battery

charging. With a wide 4V to 38V (40V maximum) input and

output range and seamless, low noise transitions between

operating regions, the LTC3789 is ideal for automotive,

telecom and battery-powered systems.

The operating mode of the controller is determined through

the MODE/PLLIN pin. The MODE/PLLIN pin can select

between pulse-skipping mode and forced continuous

mode operation and allows the IC to be synchronized to an

external clock. Pulse-skipping mode offers high efficiency

and low ripple at light loads, while forced continuous

mode operates at a constant frequency for noise-sensitive

applications.

A PGOOD pin indicates when the output is within 10% of

its designed set point. The LTC3789 is available in low pro-

file 28-pin 4mm × 5mm QFN and narrow SSOP packages.

Efficiency and Power Loss

Single Inductor Architecture Allows V

Above,

Below or Equal to the Regulated V

OUT

Programmable Input or Output Current

Wide V

Range: 4V to 38V

1% Output Voltage Accuracy: 0.8V < V

OUT

< 38V

Synchronous Rectification: Up to 98% Efficiency

Current Mode Control

Phase-Lockable Fixed Frequency: 200kHz to 600kHz

No Reverse Current During Start-Up

Power Good Output Voltage Monitor

Internal 5.5V LDO

Quad N-Channel MOSFET Synchronous Drive

OUT

Disconnected from V

During Shutdown

True Soft-Start and V

OUT

Short Protection, Even in

Boost Mode

Available in 28-Lead QFN (4mm × 5mm) and

28-Lead SSOP Packages

Automotive Systems

Distributed DC Power Systems

High Power Battery-Operated Devices

Industrial Control

INSNS

OUTSNS

LIM

PGOOD

0.1µF

121k

BOOST1

TG1

SW1

BG1

TG2

BOOST2

SW2

BG2

MODE/PLLIN

RUN

0.010Ω

4.7µF

2200pF

1000pF

1µF

CER

10µF

16V

CER

2.2µF

330µF

16V

ON/OFF

0.01µF

4.7µH

LTC3789

INTV

EXTV

SENSE

OSENSE

–

OSENSE

FREQ

SGND

SENSE

–

PGND

7.5k

3789 TA01

105k, 1%

4V TO

38V

OUT

12V

0.010Ω

100Ω

22µF

50V

CER

L, LT, LT C, LTM , Linear Technology, the Linear logo, µModule and Burst Mode are registered

trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks

are the property of their respective owners. Protected by U.S. Patents, including 5408150,

5481178, 5929620, 6580258, 7365525, 7394231.

(V)

EFFICIENCY (%)

POWER LOSS (W)

10 15 25

20 30 35 40

100

3789 TA01b

OUT

= 12V

LOAD

= 5A

Summary of content (30 pages)

PAGE 1
LTC3789 High Efficiency, Synchronous, 4-Switch Buck-Boost Controller Features n n n n n n n n n n n n n n Description Single Inductor Architecture Allows VIN Above, Below or Equal to the Regulated VOUT Programmable Input or Output Current Wide VIN Range: 4V to 38V 1% Output Voltage Accuracy: 0.
PAGE 2
LTC3789 Absolute Maximum Ratings (Note 1) Input Supply Voltage (VIN).......................... 40V to –0.3V Topside Driver Voltages (BOOST1, BOOST2)................................... 46V to –0.3V Switch Voltage (SW1, SW2)........................... 40V to –5V Current Sense Voltages (IOSENSE+, IOSENSE–).. 40V to –0.3V BOOST1, BOOST2 – SW1, SW2.................... 6V to –0.3V TG1, TG2 – SW1, SW2.................................. 6V to –0.3V EXTVCC Voltage.......................................... 14V to –0.
PAGE 3
LTC3789 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V, unless otherwise noted. SYMBOL PARAMETER VIN Input Supply Voltage CONDITIONS VOUT Output Voltage VFB Regulated Feedback Voltage ITH Voltage = 1.2V (Note 4), TA = –40°C to 85°C ITH = 1.
PAGE 4
LTC3789 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 5.2 5.5 5.8 V 0.2 1.0 % 5.5 5.8 V 0.2 1.0 % INTVCC Linear Regulator VINTVCCVIN Internal VCC Voltage 6.
PAGE 5
LTC3789 Typical Performance Characteristics TA = 25°C unless otherwise noted. Efficiency vs Output Current (Boost Region) 100 100 VIN = 12V VOUT = 12V 90 80 70 70 50 40 30 60 50 40 30 60 50 40 30 20 20 10 0 EFFICIENCY (%) 80 70 60 DCM FCM CIRCUIT OF FIGURE 13 10 99 100 1000 LOAD CURRENT (mA) 20 10 0 10000 CIRCUIT OF FIGURE 13 DCM FCM 100 1000 LOAD CURRENT (mA) 10 3789 G01 10 0 10000 94 FREQUENCY 200kHz 300kHz 400kHz 520kHz 93 92 91 5 0 10 20 VIN (V) 30 4 5.
PAGE 6
LTC3789 Typical Performance Characteristics TA = 25°C unless otherwise noted.
PAGE 7
LTC3789 Typical Performance Characteristics TA = 25°C unless otherwise noted. Maximum Current Sense Threshold vs Duty Factor (Buck) 150 140 140 140 130 130 110 100 90 80 70 120 110 100 90 80 70 60 60 50 50 0 20 60 40 DUTY FACTOR (%) 80 100 0 20 60 40 DUTY FACTOR (%) 80 150 3789 G20 50 0 –50 –100 1.5 VITH (V) 2 80 2.5 3 100 50 0 3789 G22 –100 BOOST 140 –50 –150 BUCK 90 Current Foldback Limit CURRENT LIMIT (mV) CURRENT LIMIT (mV) 100 1 100 160 150 0.
PAGE 8
LTC3789 Typical Performance Characteristics TA = 25°C unless otherwise noted.
PAGE 9
LTC3789 Pin Functions (SSOP/QFN) VFB (Pin 1/Pin 26): Error Amplifier Feedback Pin. Receives the feedback voltage for the controller from an external resistive divider across the output. SS (Pin 2/Pin 27): External Soft-Start Input. The LTC3789 regulates the VFB voltage to the smaller of 0.8V or the voltage on the SS pin. A internal 3µA pull-up current source is connected to this pin. A capacitor to ground at this pin sets the ramp time to final regulated output voltage.
PAGE 10
LTC3789 Pin Functions (SSOP/QFN) INTVCC (Pin 21/Pin 18): Output of the Internal Linear Low Dropout Regulator. The driver and control circuits are powered from this voltage source. Must be bypassed to power ground with a minimum of 4.7µF tantalum, ceramic, or other low ESR capacitor. VIN (Pin 22/Pin 19): Main Supply Pin. A bypass capacitor should be tied between this pin and the signal ground pin. BG1, BG2 (Pins 23, 19/Pins 20, 16): High Current Gate Drives for Bottom (Synchronous) N-Channel MOSFETs.
PAGE 11
LTC3789 Block Diagram INTVCC VIN BOOST1 CHARGE PUMP BOOST1 + VIN CHARGE PUMP BOOST2 TG1 FCB IDREV BUCK LOGIC SW1 INTVCC – SW1 + BG1 PGND IREV RSENSE BG2 – FCB BOOST LOGIC VFLD + INTVCC TG2 ICMP BOOST2 – OV IOSENSE+ + 1.2µA RUN SHDN SLOPE EA FIN 3µA INTVCC RSENSE2 VOUT IOSENSE– – – IOS MODE/ PLLIN SW2 + ILIM – VFB – + + 0.80V SS PHASE DET 220k 10µA ITH FREQ SENSE+ OSCILLATOR SENSE– VIN EXTVCC VIN 4.8V EXTVCC 0.86V + – 5.5V LDO REG 5.
PAGE 12
LTC3789 Operation Main Control Loop The LTC3789 is a current mode controller that provides an output voltage above, equal to or below the input voltage. The LTC proprietary topology and control architecture employs a current-sensing resistor. The inductor current is controlled by the voltage on the ITH pin, which is the output of the error amplifier EA. The VFB pin receives the voltage feedback signal, which is compared to the internal reference voltage by the EA.
PAGE 13
LTC3789 Operation Power switch Control Figure 1 shows a simplified diagram of how the four power switches are connected to the inductor, VIN, VOUT and GND. Figure 2 shows the regions of operation for the LTC3789 as a function of duty cycle, D. The power switches are properly controlled so the transfer between regions is continuous. Switch D is always on and switch C is always off in this region. At the start of every cycle, synchronous switch B is turned on first.
PAGE 14
LTC3789 OPERATION The duty cycle of switch C decreases until the minimum duty cycle of the converter reaches DMIN_BOOST , given by:  1 DMIN _ BOOST =   • 100% = 8.33%  12  Figure 5 shows typical boost region waveforms. If VIN approaches VOUT, the buck-boost region is reached.
PAGE 15
LTC3789 OPERATION Constant-Current Regulation The LTC3789 provides a constant-current regulation loop for either input or output current. A sensing resistor close to the input or output capacitor will sense the input or output current. When the current exceeds the programmed current limit, the voltage on the ITH pin will be pulled down to maintain the desired maximum input or output current.
PAGE 16
LTC3789 Applications Information The Typical Application on the first page is a basic LTC3789 application circuit. External component selection is driven by the load requirement, and begins with the selection of RSENSE and the inductor value. Next, the power MOSFETs are selected. Finally, CIN and COUT are selected. This circuit can be configured for operation up to an input voltage of 38V. RSENSE Selection and Maximum Output Current RSENSE is chosen based on the required output current.
PAGE 17
LTC3789 APPLICATIONS INFORMATION With the typical 100Ω resistors shown here, the value of capacitor CF should be 1µF to 2.2µF. The current loop’s transfer function should approximate that of the voltage loop. Crossover frequency should be one-tenth the switching frequency, and gain should decrease by 20dB/decade. Similar current and voltage loop transfer functions will ensure overall system stability. When the IOSENSE common mode voltage is above ~3.2V, the IOSENSE– pin sources 10µA.
PAGE 18
LTC3789 APPLICATIONS INFORMATION clock whose frequency is within range of the LTC3789’s internal VCO. This is guaranteed to be between 200kHz and 600kHz. A simplified block diagram is shown in Figure 10. 2.4V RSET L BUCK > FREQ EXTERNAL OSCILLATOR DIGITAL SYNC PHASE/ FREQUENCY DETECTOR L BOOST > 5V 10µA MODE/ PLLIN For a given ripple the inductance terms in continuous mode are as follows: VCO 3789 F10 Figure 10.
PAGE 19
LTC3789 APPLICATIONS INFORMATION In the boost region, the discontinuous current shifts from the input to the output, so COUT must be capable of reducing the output voltage ripple. The effects of ESR (equivalent series resistance) and the bulk capacitance must be considered when choosing the right capacitor for a given output ripple voltage.
PAGE 20
LTC3789 APPLICATIONS INFORMATION where CRSS is usually specified by the MOSFET manufacturers. The constant k, which accounts for the loss caused by reverse recovery current, is inversely proportional to the gate drive current and has an empirical value of 1.7. For switch D, the maximum power dissipation happens in the boost region, when its duty cycle is higher than 50%.
PAGE 21
LTC3789 APPLICATIONS INFORMATION Significant efficiency and thermal gains can be realized by powering EXTVCC from the output, since the VIN current resulting from the driver and control currents will be scaled by a factor of (Duty Cycle)/(Switcher Efficiency). Tying the EXTVCC pin to a 12V output reduces the junction temperature in the previous example from 125°C to 97°C: TJ = 70°C + (28mA)(12V)(80°C/W) = 97°C Powering EXTVCC from the output can also provide enough gate drive when VIN drops below 5V.
PAGE 22
LTC3789 APPLICATIONS INFORMATION Soft-Start Function Efficiency Considerations When a capacitor is connected to the SS pin, a soft-start current of 3µA starts to charge the capacitor. A soft-start function is achieved by controlling the output ramp voltage according to the ramp rate on the SS pin. Current foldback is disabled during this phase to ensure smooth soft-start. When the chip is in the shutdown state with its RUN pin voltage below 1.22V, the SS pin is actively pulled to ground.
PAGE 23
LTC3789 APPLICATIONS INFORMATION Design Example Output voltage is 12V. Select R1 as 20k. R2 is: VIN = 5V to 18V VOUT = 12V IOUT(MAX) = 5A f = 400kHz Maximum ambient temperature = 60°C Set the frequency at 400kHz by applying 1.2V on the FREQ pin (see Figure 7). The 10µA current flowing out of the FREQ pin will give 1.2V across a 120k resistor to GND. The inductance value is chosen first based on a 30% ripple current assumption.
PAGE 24
LTC3789 APPLICATIONS INFORMATION The Vishay SiR422DP has a typical RDS(ON) of 0.010Ω at TJ = 125°C and VGS = 4.5V. The maximum dissipation in QB occurs at maximum input voltage when the circuit is operating in the buck region. The dissipation is: P B,BUCK = VIN − VOUT • IOUT(MAX) 2 • ρ τ • RDS(ON) VIN R DS(ON)(125°C) < 1.3W 18V TO −12V 18V 2 = 0.156Ω • (5A) This seems to indicate that a quite small MOSFET can be used for QB if we only look at power loss.
PAGE 25
LTC3789 APPLICATIONS INFORMATION • Use planes for VIN and VOUT to maintain good voltage filtering and to keep power losses low. • Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. Connect the copper areas to any DC net (VIN or GND). When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3789. These items are also illustrated in Figure 13.
PAGE 26
LTC3789 APPLICATIONS INFORMATION R1 20k R2 280k CSS 6.8nF VPULLUP 1 2 CC2 1000pF CC1 3300pF 3 RC 68k 4 5 6 7 121k 8 ON/OFF VIN VOUT 10Ω VOUT 9 10 11 12 13 VFB PGOOD SW1 SS TG1 BOOST1 SENSE+ PGND SENSE– BG1 ITH SGND VIN LTC3789 MODE/PLLIN INTVCC FREQ EXTVCC RUN BG2 VINSNS BOOST2 VOUTSNS 27 CIN 47µF 26 QA SiR422DP 25 24 DA DFLS160 23 VOUT 19 TG2 SW2 TRIM L 6.8µH D1 B240A CVCC 4.7µF 21 INTVCC 20 QB SiR422DP CF 1µF 22 10mΩ DB DFLS160 QC SiR422DP 18 CB 0.
PAGE 27
LTC3789 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. GN Package 28-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .386 – .393* (9.804 – 9.982) .045 .005 28 27 26 25 24 23 22 21 20 19 18 17 1615 .254 MIN .033 (0.838) REF .150 – .165 .229 – .244 (5.817 – 6.198) .0165 .0015 .150 – .157** (3.810 – 3.988) .0250 BSC 1 RECOMMENDED SOLDER PAD LAYOUT .015 .004 ¥ 45∞ (0.38 0.10) .0075 – .0098 (0.19 – 0.
PAGE 28
LTC3789 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UFD Package 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 ±0.05 4.50 ± 0.05 3.10 ± 0.05 2.50 REF 2.65 ± 0.05 3.65 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ± 0.05 5.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 (2 SIDES) 0.75 ± 0.05 R = 0.
PAGE 29
LTC3789 Revision History REV DATE DESCRIPTION A 9/11 Updated Features, Description and Typical Application. PAGE NUMBER 1 Updated Electrical Characteristics section. 3 Updated text in MODE/PLLIN, BOOST1, BOOST2, SW1, SW2 in Pin Functions section. 9, 10 Updated text in Operation section. 12-15 Updated text in Applications Information section. 16-25 Updated Figure 13. 26 Updated Typical Application and Related Parts.
PAGE 30
LTC3789 Typical Application 24V/5A Buck-Boost Regulator VIN R5 10Ω, 0805 VOS+ INTVCC RFB1 232k RFB2 8.06k 1 2 0.01µF R9, 1.24k 3 R10, 1.24k C11, OPT CC2, 0.01µF 4 VIN R21 121k, 1% D8 BZX84C5V1 7 8 9 C7, 0.1µF R31 12.1k 10 C8, 0.1µF 11 12 R25 0Ω R13, 100Ω R14 100Ω VOUT VIN 13 C10, 2.2µF 14 SW1 TG1 SS SENSE1+ BOOST1 SENSE1– PGND1 CC1, 1000pF 5 ITH RC, 15k 6 R30 + 68.