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LTC3788-1

37881fc

TYPICAL APPLICATION

DESCRIPTION

2-Phase, Dual Output

Synchronous Boost Controller

The LTC

3788-1 is a high performance 2-phase dual

synchronous boost converter controller that drives all

N-channel power MOSFETs. Synchronous rectification

increases efficiency, reduces power losses and eases

thermal requirements, allowing the LTC3788-1 to be used

in high power boost applications.

A constant-frequency current mode architecture allows

a phase-lockable frequency of up to 850kHz. OPTI-LOOP

compensation allows the transient response to be optimized

over a wide range of output capacitance and ESR values.

The LTC3788-1 features a precision 1.2V reference and a

power good output indicator. A 4.5V to 38V input supply

range encompasses a wide range of system architectures

and battery chemistries.

Independent SS pins for each controller ramp the output

voltages during start-up. The PLLIN/MODE pin selects

among Burst Mode operation, pulse-skipping mode or

continuous inductor current mode at light loads.

For a leadless 32-pin QFN package with additional features

of adjustable current limit, clock out, phase modulation

and two PGOOD outputs, see the LTC3788 data sheet.

L, LT, LTC, LTM, Linear Technology, OPTI-LOOP, Burst Mode and the Linear logo are registered

trademarks and No R

SENSE

and ThinSOT are trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners. Protected by U. S. Patents,

including 5408150, 5481178, 5705919, 5929620, 6144194, 6177787, 6580258.

FEATURES

APPLICATIONS

Synchronous Operation for Highest Efficiency and

Reduced Heat Dissipation

Wide Input Range: 4.5V to 38V (40V Abs Max) and

Operates Down to 2.5V After Start-Up

Output Voltages Up to 60V

±1% 1.2V Reference Voltage

SENSE

or Inductor DCR Current Sensing

100% Duty Cycle Capability for Synchronous MOSFET

Low Quiescent Current: 125µA

Phase-Lockable Frequency (75kHz to 850kHz)

Programmable Fixed Frequency (50kHz to 900kHz)

Adjustable Output Voltage Soft-Start

Power Good Output Voltage Monitor

Low Shutdown Current I

: <8A

Internal LDO Powers Gate Drive from VBIAS or EXTV

Available in a Narrow SSOP Package

Industrial

Automotive

Medical

Military

Efficiency and Power Loss

vs Load Current

15nF

4.7µF

2.7k

12.1k

OUT

12V AT 5A

OUT

24V AT 3A

4.5V TO 12V START-UP VOLTAGE

OPERATES THROUGH TRANSIENTS DOWN TO 2.5V

0.1µF

100pF

0.1µF

220µF

TG1

BOOST1

SW1

BG1

FREQ

SS1 SGND SS2

SENSE1

–

VFB1

ITH1

VBIAS

LTC3788-1

INTV

PLLIN/MODE

TG2

BOOST2

SW2

BG2

SENSE2

–

VFB2

ITH2

PGND

0.1µF

15nF

8.66k

220pF

232k

0.1µF

4.7µF

110k

37881 TA01a

220µF

3.3µH

1.25µH

3m 4m

OUTPUT CURRENT (A)

EFFICIENCY (%)

POWER LOSS (mW)

37881 TA01b

100

1000

0.1

10000

0.0001 0.001 0.01 0.1 1 100.00001

= 12V

OUT

= 24V

Burst Mode OPERATION

FIGURE 9 CIRCUIT

Summary of content (30 pages)

PAGE 1
LTC3788-1 2-Phase, Dual Output Synchronous Boost Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n The LTC®3788-1 is a high performance 2-phase dual synchronous boost converter controller that drives all N-channel power MOSFETs. Synchronous rectification increases efficiency, reduces power losses and eases thermal requirements, allowing the LTC3788-1 to be used in high power boost applications. Synchronous Operation for Highest Efficiency and Reduced Heat Dissipation Wide Input Range: 4.
PAGE 2
LTC3788-1 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Notes 1, 3) VBIAS ........................................................ –0.3V to 40V BOOST1, BOOST2 ...................................... –0.3V to 76V SW1, SW2.................................................. –0.3V to 70V RUN1, RUN2 ................................................ –0.3V to 8V Maximum Current Sourced into Pin from Source > 8V .............................................100μA PGOOD1, PLLIN/MODE ............................... –0.
PAGE 3
LTC3788-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C, VBIAS = 12V, unless otherwise noted (Note 2). SYMBOL PARAMETER VLOADREG Output Voltage Load Regulation CONDITIONS TYP MAX UNITS (Note 4) Measured in Servo Loop; ΔITH Voltage = 1.2V to 0.7V l 0.01 0.1 % Measured in Servo Loop; ΔITH Voltage = 1.2V to 2V l –0.01 –0.1 % gm1,2 Error Amplifier Transconductance ITH = 1.
PAGE 4
LTC3788-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C, VBIAS = 12V, unless otherwise noted (Note 2). SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 5.2 5.4 5.6 V 0.
PAGE 5
LTC3788-1 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency and Power Loss vs Output Current Efficiency and Power Loss vs Output Current 100 100 10000 90 80 80 100 50 40 10 30 20 VIN = 12V VOUT = 24V FIGURE 9 CIRCUIT 10 0 0.01 1 0.1 10 0.1 1 OUTPUT CURRENT (A) 1000 70 60 100 50 40 10 30 VIN = 12V 1 VOUT = 24V 10 Burst Mode OPERATION FIGURE 9 CIRCUIT 0 0.1 0.00001 0.0001 0.001 0.01 0.
PAGE 6
LTC3788-1 TYPICAL PERFORMANCE CHARACTERISTICS Inductor Current at Light Load Soft Start-Up FORCED CONTINUOUS MODE Burst Mode OPERATION 5A/DIV VOUT 5V/DIV PULSE-SKIPPING MODE 0V 37881 G07 5μs/DIV VIN = 12V VOUT = 24V ILOAD = 200μA FIGURE 9 CIRCUIT VIN = 12V 20ms/DIV VOUT = 24V FIGURE 9 CIRCUIT Regulated Feedback Voltage vs Temperature Soft-Start Pull-Up Current vs Temperature 1.212 11.0 1.209 SOFT-START CURRENT (μA) REGULATED FEEDBACK VOLTAGE (V) 37881 G08 1.206 1.203 1.200 1.197 1.194 10.
PAGE 7
LTC3788-1 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown (RUN) Threshold vs Temperature Quiescent Current vs Temperature 170 4.4 1.40 VIN = 12V VFB = 1.25V 160 RUN2 = GND 4.3 1.35 150 140 130 120 RUN RISING 1.30 1.25 1.20 RUN FALLING 1.15 100 –45 1.10 –45 –20 4.0 3.9 INTVCC FALLING 3.8 3.7 3.5 –20 5 55 80 30 TEMPERATURE (°C) 105 130 80 55 30 TEMPERATURE (°C) 5 105 3.4 –45 130 EXTVCC Switchover and INTVCC Voltages vs Temperature 5.50 5.4 5.8 5.45 5.1 5.0 4.9 4.8 4.7 INTVCC 5.2 5.
PAGE 8
LTC3788-1 TYPICAL PERFORMANCE CHARACTERISTICS 105 130 260 240 220 200 180 160 140 120 100 80 60 40 20 0 VSENSE = 12V SENSE+ PIN SENSE – PIN 0 0.5 37881 G22 2 1.5 1 ITH VOLTAGE (V) 2.5 3 37881 G23 Maximum Current Sense Threshold vs Duty Cycle 260 240 220 200 180 160 140 120 100 80 60 40 20 0 SENSE+ PIN SENSE – PIN 2.5 7.5 12.5 17.5 22.5 27.5 32.5 37.
PAGE 9
LTC3788-1 PIN FUNCTIONS SENSE1–, SENSE2– (Pin 4, Pin 10): Negative Current Sense Comparator Input. The (–) input to the current comparator is normally connected to the negative terminal of a current sense resistor connected in series with the inductor. The common mode voltage range on these pins is 2.5V to 38V (40V abs max). FREQ (Pin 5): The frequency control pin for the internal VCO. Connecting the pin to GND forces the VCO to a fixed low frequency of 350kHz.
PAGE 10
LTC3788-1 BLOCK DIAGRAM INTVCC DUPLICATE FOR SECOND CONTROLLER CHANNEL PGOOD1 1.32V + S – R VFB1 Q CB TG SHDN + 1.08V DB BOOST SWITCHING LOGIC AND CHARGE PUMP – 20μA FREQ COUT INTVCC CLK2 VCO VOUT SW BG 0.425V CLK1 + SLEEP PGND – PFD + – + – + + – L – SENSE – 2mV 2.8V 0.7V PLLIN/ MODE SENSE+ SLOPE COMP SYNC DET RSENSE VIN CIN + SENS LO 100k – VFB 2.5V + EA – – 1.2V SS + OV VBIAS 0.5μA/ 4.5μA SHDN EXTVCC – 1.32V ITH CC2 10μA 5.4V LDO EN + 4.8V 5.
PAGE 11
LTC3788-1 OPERATION (Refer to Block Diagram) Main Control Loop The LTC3788-1 uses a constant-frequency, current mode step-up architecture with the two controller channels operating 180 degrees out-of-phase. During normal operation, each external bottom MOSFET is turned on when the clock for that channel sets the RS latch, and is turned off when the main current comparator, ICMP, resets the RS latch.
PAGE 12
LTC3788-1 OPERATION In sleep mode, much of the internal circuitry is turned off, reducing the quiescent current that the LTC3788-1 draws. If one channel is shut down and the other channel is in sleep mode, the LTC3788-1 draws only 125μA of quiescent current. If both channels are in sleep mode, the LTC3788-1 draws only 200μA of quiescent current. In sleep mode, the load current is supplied by the output capacitor. As the output voltage decreases, the EA’s output begins to rise.
PAGE 13
LTC3788-1 OPERATION The typical capture range of the LTC3788-1’s PLL is from approximately 55kHz to 1MHz, and is guaranteed to lock to an external clock source whose frequency is between 75kHz and 850kHz. The typical input clock thresholds on the PLLIN/MODE pin are 1.6V (rising) and 1.2V (falling). Operation When VIN > VOUT When VIN rises above the regulated VOUT voltage, the boost controller can behave differently depending on the mode, inductor current and VIN voltage.
PAGE 14
LTC3788-1 APPLICATIONS INFORMATION The Typical Application on the first page is a basic LTC3788-1 application circuit. LTC3788-1 can be configured to use either inductor DCR (DC resistance) sensing or a discrete sense resistor (RSENSE) for current sensing. The choice between the two current sensing schemes is largely a design trade-off between cost, power consumption and accuracy.
PAGE 15
LTC3788-1 APPLICATIONS INFORMATION Sense Resistor Current Sensing A typical sensing circuit using a discrete resistor is shown in Figure 2a. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold VSENSE(MAX). The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current, IMAX, equal to the peak value less half the peak-to-peak ripple current, ΔIL.
PAGE 16
LTC3788-1 APPLICATIONS INFORMATION due to the extra switching losses incurred through R1. However, DCR sensing eliminates a sense resistor, reduces conduction losses and provides higher efficiency at heavy loads. Peak efficiency is about the same with either method. Inductor Value Calculation The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use of smaller inductor and capacitor values.
PAGE 17
LTC3788-1 APPLICATIONS INFORMATION The MOSFET power dissipations at maximum output current are given by: PMAIN = (VOUT − VIN )VOUT VIN2 •IOUT(MAX)2 • (1+ δ ) • RDS(ON) + k • VOUT3 • IOUT(MAX) VIN • RDR • CMILLER • f PSYNC = VIN 2 •I • (1+ δ ) • RDS(ON) VOUT OUT(MAX) where δ is the temperature dependency of RDS(ON) and RDR (approximately 1Ω) is the effective driver resistance at the MOSFET’s Miller threshold voltage.
PAGE 18
LTC3788-1 IORIPPLE /IOUT APPLICATIONS INFORMATION 3.25 3.00 2.75 2.50 2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0 0.1 Soft-Start (SS Pins) The start-up of each VOUT is controlled by the voltage on the respective SS pins. When the voltage on the SS pin is less than the internal 1.2V reference, the LTC3788-1 regulates the VFB pin voltage to the voltage on the SS pin instead of 1.2V. 1-PHASE 2-PHASE 0.2 0.3 0.4 0.5 0.6 0.7 0.8 DUTY CYCLE OR (1-VIN /VOUT) 0.9 37881 F03 Figure 3.
PAGE 19
LTC3788-1 APPLICATIONS INFORMATION case, power dissipation for the IC is highest and is equal to VIN • IINTVCC. The gate charge current is dependent on operating frequency, as discussed in the Efficiency Considerations section. The junction temperature can be estimated by using the equations given in Note 3 of the Electrical Characteristics.
PAGE 20
LTC3788-1 APPLICATIONS INFORMATION At higher temperatures, or in cases where the internal power dissipation causes excessive self-heating on chip (such as an INTVCC short to ground), the overtemperature shutdown circuitry will shut down the LTC3788-1. When the junction temperature exceeds approximately 170°C, the overtemperature circuitry disables the INTVCC LDO, causing the INTVCC supply to collapse and effectively shut down the entire LTC3788-1 chip.
PAGE 21
LTC3788-1 APPLICATIONS INFORMATION Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time duration that the LTC3788-1 is capable of turning on the bottom MOSFET. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum ontime limit.
PAGE 22
LTC3788-1 APPLICATIONS INFORMATION The ITH series RC-CC filter sets the dominant pole-zero loop compensation. The values can be modified slightly (from 0.5 to 2 times their suggested values) to optimize transient response once the final PC layout is complete and the particular output capacitor type and value have been determined. The output capacitors must be selected because the various types and values determine the loop gain and phase.
PAGE 23
LTC3788-1 APPLICATIONS INFORMATION PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the IC. These items are also illustrated graphically in the layout diagram of Figure 7. Figure 8 illustrates the current waveforms present in the various branches of the 2-phase synchronous regulators operating in the continuous mode. Check the following in your layout: 1.
PAGE 24
LTC3788-1 APPLICATIONS INFORMATION Investigate whether any problems exist only at higher output currents or only at higher input voltages. If problems coincide with high input voltages and low output currents, look for capacitive coupling between the BOOST, SW, TG, and possibly BG connections and the sensitive voltage and current pins. The capacitor placed across the current sensing pins needs to be placed immediately adjacent to the pins of the IC.
PAGE 25
LTC3788-1 APPLICATIONS INFORMATION RSENSE1 L1 SW1 VOUT1 COUT1 RL1 VIN RIN CIN RSENSE2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. L2 SW2 VOUT2 COUT2 RL2 37881 F08 Figure 8.
PAGE 26
LTC3788-1 APPLICATIONS INFORMATION RA1 12.1k RB1 232k SENSE1– SENSE1+ VFB1 CITH1 220pF CITH1 15nF PGOOD2 PGOOD1 TG1 LTC3788-1 SW1 100k INTVCC MTOP1 L1 3.3μH RSENSE1 4mΩ COUTA1 22μF s4 + COUTB1 220μF CB1 0.1μF BOOST1 RITH1 8.66k CSS1 0.1μF BG1 MBOT1 ITH1 VBIAS SS1 SS2 ITH2 VFB2 INTVCC CINT 4.7μF CINA 22μF s4 PGND BG2 PLLIN/MODE BOOST2 SGND SW2 EXTVCC RUN1 TG2 RUN2 FREQ CB1 0.1μF + VIN 5V TO 24V VOUT 24V, 10A* CINB 220μF MBOT2 L2 3.
PAGE 27
LTC3788-1 APPLICATIONS INFORMATION RS1, 53.6k, 1% RS2 26.1k, 1% RB1 232k 1% C1 0.1μF C3 0.1μF RA1 12.1k, 1% SENSE1– SENSE1+ PGOOD1 INTVCC LTC3788-1 VFB1 CITH1, 220pF CITH1, 15nF 100k D3 MTOP1 TG1 + COUTB1 220μF SW1 BOOST1 RITH1 8.87k, 1% CB1, 0.1μF ITH1 MBOT1 BG1 CSS1, 0.01μF D1 SS1 INTVCC PLLIN/MODE SGND VBIAS INTVCC RUN1 RFREQ 41.2k RUN2 CINA 22μF ×4 CINT 4.7μF PGND EXTVCC D2 BG2 CB1, 0.1μF + VIN 5V TO 24V CINB 220μF MBOT2B MBOT2A BOOST2 FREQ L2 16μH CSS2, 0.
PAGE 28
LTC3788-1 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.
PAGE 29
LTC3788-1 REVISION HISTORY REV DATE DESCRIPTION A 3/10 Updates to Typical Application PAGE NUMBER Updates in the Electrical Characteristics Section Updates to PLLIN/MODE in Pin Functions Updates to Application Information B 9/11 C 1/12 1 3, 4 9 15, 19, 22 New Figure 9 Added 26 Updated Note on Typical Application 30 Updated Related Parts Table 30 Updated the Topside MOSFET Driver Supply (CB, DB) section. 19 Updated the Related Parts.
PAGE 30
LTC3788-1 TYPICAL APPLICATION High Efficiency Dual 12V/24V Boost Converter RB1 232k 1% SENSE1– SENSE1+ RA1 12.1k, 1% 100k PGOOD1 TG1 VFB1 CITH1, 220pF L1 MTOP1 3.3μH COUTA1 22μF s4 + COUTB1 220μF SW1 LTC3788-1 BG1 ITH1 CB1, 0.1μF MBOT1 D1 CSS1, 0.1μF VBIAS SS1 INTVCC PGND PLLIN/MODE SGND BG2 EXTVCC RUN1 BOOST2 RUN2 FREQ CINA 22μF s4 CINT 4.7μF SS2 SW2 CB1, 0.1μF ITH2 TG2 RSENSE2 3mΩ MTOP2 RA2 12.1k COUTA2 22μF s4 VFB2 SENSE2+ SENSE2– RB2 110k CINB 220μF MBOT2 L2 1.