LTC3787 PolyPhase Synchronous Boost Controller FEATURES DESCRIPTION n The LTC®3787 is a high performance PolyPhase® single output synchronous boost converter controller that drives two N-channel power MOSFET stages out-of-phase. Multiphase operation reduces input and output capacitor requirements and allows the use of smaller inductors than the single-phase equivalent.
LTC3787 ABSOLUTE MAXIMUM RATINGS (Notes 1, 3) VBIAS ........................................................ –0.3V to 40V BOOST1 and BOOST2 ................................ –0.3V to 76V SW1 and SW2............................................ –0.3V to 70V RUN ............................................................. –0.3V to 8V Maximum Current Sourced into Pin From Source >8V ..............................................100μA PGOOD, PLLIN/MODE ................................. –0.
LTC3787 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified 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 Main Control Loop VBIAS Chip Bias Voltage Operating Range 4.5 VFB Regulated Feedback Voltage ITH = 1.
LTC3787 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified 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 0.5 2 % 5.2 5.4 5.6 V 0.5 2 % 4.5 4.
LTC3787 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency and Power Loss vs Output Current Efficiency and Power Loss vs Output Current 100 10000 100 1000 80 90 EFFICIENCY (%) 50 10 40 30 20 1 VIN = 12V VOUT = 24V FIGURE 10 CIRCUIT 10 0 0.01 60 40 0.1 3787 G01 VIN = 12V 1 VOUT = 24V 10 Burst Mode OPERATION FIGURE 10 CIRCUIT 0 0.1 0.1 1 10 0.00001 0.0001 0.001 0.
LTC3787 TYPICAL PERFORMANCE CHARACTERISTICS Inductor Current at Light Load Soft Start-Up FORCED CONTINUOUS MODE VOUT 5V/DIV Burst Mode OPERATION 5A/DIV PULSE-SKIPPING MODE 0V 5μs/DIV VIN = 12V VOUT = 24V ILOAD = 200μA FIGURE 10 CIRCUIT 3787 G07 20ms/DIV VIN = 12V VOUT = 24V FIGURE 10 CIRCUIT Regulated Feedback Voltage vs Temperature Soft-Start Pull-Up Current vs Temperature 11.0 1.212 1.209 SOFT-START CURRENT (μA) REGULATED FEEDBACK VOLTAGE (V) 3787 G08 1.206 1.203 1.200 1.197 1.194 10.5 10.
LTC3787 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown (RUN) Threshold vs Temperature Quiescent Current vs Temperature 1.40 180 1.35 RUN PIN VOLTAGE (V) QUIESCENT CURRENT (μA) VIN = 12V VFB = 1.25V 170 RUN = GND 160 150 140 130 RUN RISING 1.30 1.25 1.20 RUN FALLING 1.15 120 1.10 –60 –35 –10 15 40 65 90 115 140 TEMPERATURE (°C) 110 –60 –35 –10 15 40 65 90 115 140 TEMPERATURE (°C) 3787 G14 3787 G13 Undervoltage Lockout Threshold vs Temperature INTVCC Line Regulation 4.4 5.5 4.3 5.
LTC3787 TYPICAL PERFORMANCE CHARACTERISTICS Oscillator Frequency vs Temperature Oscillator Frequency vs Input Voltage 360 600 FREQ = INTVCC OSCILLATOR FREQUENCY (kHz) 550 FREQUENCY (kHz) FREQ = GND 358 500 450 400 FREQ = GND 350 356 354 352 350 348 346 344 342 340 300 –60 –35 –10 15 40 65 90 115 140 TEMPERATURE (°C) 5 10 20 25 30 15 INPUT VOLTAGE (V) 35 3787 G19 SENSE Pin Input Current vs Temperature 120 100 PULSE-SKIPPING MODE SENSE CURRENT (μA) MAXIMUM CURRENT SENSE VOLTAGE (mV) Maximum C
LTC3787 TYPICAL PERFORMANCE CHARACTERISTICS Charge Pump Charging Current vs Operating Frequency 120 Charge Pump Charging Current vs Switch Voltage 80 ILIM = INTVCC 100 ILIM = FLOAT 80 60 ILIM = GND 40 20 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 80 T = –60°C 70 T = –45°C 60 50 T = 25°C 40 T = 130°C 30 T = 155°C 20 10 0 50 150 250 350 450 550 650 750 OPERATING FREQUENCY (kHz) 3787 G25 PIN FUNCTIONS CHARGE PUMP CHARGING CURRENT (μA) CHARGE PUMP CHARGING CURRENT (μA) MAXIMUM CUR
LTC3787 PIN FUNCTIONS (QFN/SSOP) SS (Pin 7/Pin 10): Output Soft-Start Input. A capacitor to ground at this pin sets the ramp rate of the output voltage during start-up. PGND (Pin 19/Pin 22): Driver Power Ground. Connects to the sources of bottom (main) N-channel MOSFETs and the (–) terminal(s) of CIN and COUT. SENSE2– , SENSE1– (Pin 8, Pin 28/Pin 11, Pin 3): Negative Current Sense Comparator Input.
LTC3787 BLOCK DIAGRAM PHASMD INTVCC CLKOUT DUPLICATE FOR SECOND CONTROLLER CHANNEL S BOOST DB TG CB Q R SHDN SWITCHING LOGIC AND CHARGE PUMP 20μA FREQ CLK2 VCO 0.425V CLK1 + COUT INTVCC BG SLEEP PGND – PFD – ICMP + – + + IREV – + – VOUT SW L SENSE – 2mV 2.8V 0.7V PLLIN/ MODE SENSE+ SLOPE COMP SYNC DET VIN CIN + 100k SENS LO – 2.5V ILIM VFB CURRENT LIMIT + EA – – VBIAS SHDN EXTVCC EN 5.4V LDO EN – 0.5μA/ 4.5μA + 10μA 3.8V 1.32V 11V – + – 1.
LTC3787 OPERATION INTVCC/EXTVCC Power Power for the top and bottom MOSFET drivers and most other internal circuitry is derived from the INTVCC pin. When the EXTVCC pin is tied to a voltage less than 4.8V, the VBIAS LDO (low dropout linear regulator) supplies 5.4V from VBIAS to INTVCC. If EXTVCC is taken above 4.8V, the VBIAS LDO is turned off and an EXTVCC LDO is turned on. Once enabled, the EXTVCC LDO supplies 5.4V from EXTVCC to INTVCC.
LTC3787 OPERATION In forced continuous operation or when clocked by an external clock source to use the phase-locked loop (see the Frequency Selection and Phase-Locked Loop section), the inductor current is allowed to reverse at light loads or under large transient conditions. The peak inductor current is determined by the voltage on the ITH pin, just as in normal operation. In this mode, the efficiency at light loads is lower than in Burst Mode operation.
LTC3787 OPERATION multiple LTC3787s can be configured for 2-, 3-, 4- , 6- and 12-phase operation. Power Good CLKOUT is disabled when the controller is in shutdown or in sleep mode. The PGOOD pin is connected to an open drain of an internal N-channel MOSFET. The MOSFET turns on and pulls the PGOOD pin low when the VFB pin voltage is not within ±10% of the 1.2V reference voltage. The PGOOD pin is also pulled low when the corresponding RUN pin is low (shut down).
LTC3787 APPLICATIONS INFORMATION The Typical Application on the first page is a basic LTC3787 application circuit. LTC3787 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 tradeoff between cost, power consumption and accuracy.
LTC3787 APPLICATIONS INFORMATION Sense Resistor Current Sensing Inductor DCR Sensing A typical sensing circuit using a discrete resistor is shown in Figure 2a. RSENSE is chosen based on the required output current. For applications requiring the highest possible efficiency at high load currents, the LTC3787 is capable of sensing the voltage drop across the inductor DCR, as shown in Figure 2b. The DCR of the inductor can be less than 1mΩ for high current inductors.
LTC3787 APPLICATIONS INFORMATION To scale the maximum inductor DCR to the desired sense resistor value, use the divider ratio: RD = RSENSE(EQUIV) DCRMAX at TL(MAX) C1 is usually selected to be in the range of 0.1μF to 0.47μF. This forces R1|| R2 to around 2k, reducing error that might have been caused by the SENSE– pin’s ±1μA current.
LTC3787 APPLICATIONS INFORMATION Power MOSFET Selection Two external power MOSFETs must be selected for each controller in the LTC3787: one N-channel MOSFET for the bottom (main) switch, and one N-channel MOSFET for the top (synchronous) switch. The peak-to-peak gate drive levels are set by the INTVCC voltage. This voltage is typically 5.4V during start-up (see EXTVCC pin connection). Consequently, logic-level threshold MOSFETs must be used in most applications.
LTC3787 APPLICATIONS INFORMATION The input ripple current in a boost converter is relatively low (compared with the output ripple current), because this current is continuous. The input capacitor CIN voltage rating should comfortably exceed the maximum input voltage. Although ceramic capacitors can be relatively tolerant of overvoltage conditions, aluminum electrolytic capacitors are not.
LTC3787 APPLICATIONS INFORMATION Tying the PHASMD pin to INTVCC, SGND or floating generates a phase difference (between PLLIN/MODE and CLKOUT) of 240°, 60° or 90°, respectively, and a phase difference (between CH1 and CH2) of 120°, 180° 0,240 VOUT or 180°. Figure 4 shows the connections necessary for 3-, 4-, 6- or 12-phase operation. A total of 12 phases can be cascaded to run simultaneously out-of-phase with respect to each other.
LTC3787 APPLICATIONS INFORMATION Setting Output Voltage INTVCC Regulators The LTC3787 output voltage is set by an external feedback resistor divider carefully placed across the output, as shown in Figure 5. The regulated output voltage is determined by: The LTC3787 features two separate internal P-channel low dropout linear regulators (LDO) that supply power at the INTVCC pin from either the VBIAS supply pin or the EXTVCC pin depending on the connection of the EXTVCC pin.
LTC3787 APPLICATIONS INFORMATION EXTVCC remains above 4.55V. The EXTVCC LDO attempts to regulate the INTVCC voltage to 5.4V, so while EXTVCC is less than 5.4V, the LDO is in dropout and the INTVCC voltage is approximately equal to EXTVCC. When EXTVCC is greater than 5.4V, up to an absolute maximum of 6V, INTVCC is regulated to 5.4V. Significant thermal gains can be realized by powering INTVCC from an external supply.
LTC3787 APPLICATIONS INFORMATION the entire LTC3787 chip. Once the junction temperature drops back to approximately 155°C, the INTVCC LDO turns back on. Long term overstress (TJ > 125°C) should be avoided as it can degrade the performance or shorten the life of the part. Since the shutdown may occur at full load, beware that the load current will result in high power dissipation in the body diodes of the top MOSFETs. In this case, PGOOD output may be used to turn the system load off.
LTC3787 APPLICATIONS INFORMATION Table 2 summarizes the different states in which the FREQ pin can be used. INTVCC regulator current, 3) I2R losses, 4) bottom MOSFET transition losses, 5) body diode conduction losses. Table 2. 1. The VBIAS current is the DC supply current given in the Electrical Characteristics table, which excludes MOSFET driver and control currents. VBIAS current typically results in a small (<0.1%) loss.
LTC3787 APPLICATIONS INFORMATION Checking Transient Response The regulator loop response can be checked by looking at the load current transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to ΔILOAD(ESR), where ESR is the effective series resistance of COUT .
LTC3787 APPLICATIONS INFORMATION A 6.8μH inductor will produce a 31% ripple current. The peak inductor current will be the maximum DC value plus one half the ripple current, or 9.25A. The RSENSE resistor value can be calculated by using the maximum current sense voltage specification with some accommodation for tolerances: RSENSE ≤ 75mV = 0.008Ω 9.25A Choosing 1% resistors: RA = 5k and RB = 95.3k yields an output voltage of 24.072V.
LTC3787 APPLICATIONS INFORMATION SENSE1– SENSE1+ LTC3787 fIN PHSMD CLKOUT FREQ PLLIN/MODE ILIM PGOOD SW1 TG1 VPULL-UP L1 RSENSE1 CB1 BOOST1 + M1 M2 BG1 VBIAS + GND PGND EXTVCC INTVCC SGND RUN VFB ITH BG2 CB2 M3 VIN + M4 VOUT BOOST2 SS TG2 SW2 SENSE2+ SENSE2– L2 RSENSE2 3787 F08 Figure 8. Recommended Printed Circuit Layout Diagram RSENSE1 L1 SW1 VOUT VIN RIN CIN COUT RSENSE2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH.
LTC3787 APPLICATIONS INFORMATION PC Board Layout Debugging Start with one controller on at a time. It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit. Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage. Check for proper performance over the operating voltage and current range expected in the application.
LTC3787 TYPICAL APPLICATIONS SENSE1– PGOOD SENSE1+ ILIM TG1 PHASMD SW1 CLKOUT PLLIN/MODE BOOST1 SGND BG1 EXTVCC LTC3787 RUN INTVCC MTOP1 SS L1 3.3μH RSENSE1 4mΩ COUTB1 220μF MBOT1 CINT 4.7μF CIN 22μF s2 D2 ITH + D1 PGND RITH, 8.66k COUTA1 22μF s4 CB1, 0.1μF VBIAS INTVCC FREQ CSS, 0.1μF CITH, 15nF 100k BG2 VIN 5V TO 24V VOUT 24V, 10A* MBOT2 CB2, 0.1μF CITHA, 220pF L2 3.3μH BOOST2 RSENSE2 4mΩ SW2 RA, 12.1k RB 232k VFB SENSE2+ SENSE2– TG2 MTOP2 COUTA2 22μF s4 + COUTA1 6.
LTC3787 TYPICAL APPLICATIONS SENSE1– SENSE1+ ILIM 100k PGOOD TG1 PHASMD SW1 CLKOUT PLLIN/MODE BOOST1 SGND BG1 EXTVCC LTC3787 RUN MTOP1 SS L1 10.2μH RSENSE1 5mΩ COUTB1 220μF MBOT1 CINT 4.7μF CIN 6.8μF ×4 D2 ITH + D1 PGND RITH, 3.57k COUTA1 6.8μF ×4 CB1, 0.1μF VBIAS INTVCC FREQ CSS, 0.1μF CITH, 15nF INTVCC BG2 VIN 5V TO 36V VOUT 36V, 6A MBOT2 CB2, 0.1μF CITHA, 220pF L2 10.2μH BOOST2 RSENSE2 5mΩ SW2 RA, 12.1k RB 348k VFB SENSE2+ SENSE2– TG2 MTOP2 COUTA2 6.8μF ×4 + COUTA1 6.
LTC3787 TYPICAL APPLICATIONS RS1, 53.6, 1% RS2 26.1k 1% C1 0.1μF C3 0.1μF INTVCC RFREQ, 41.2k SENSE1– SENSE1+ ILIM PHASMD SW1 CLKOUT PLLIN/MODE BOOST1 SGND BG1 EXTVCC LTC3787 RUN FREQ CSS, 0.1μF SS CITH, 15nF 100k PGOOD TG1 INTVCC MTOP1 D3 L1 10.2μH COUTA1 6.8μF s4 + COUTB1 220μF CB1, 0.1μF MBOT1 VIN 5V TO 24V D1 VBIAS INTVCC CINT 4.7μF PGND CINA 22μF s4 + COUTA2 6.8μF s4 + VOUT 24V, 8A CINB 220μF D2 RITH, 8.87k, 1% ITH BG2 MBOT2 CB2, 0.1μF CITHA, 220pF L2 10.
LTC3787 TYPICAL APPLICATIONS SENSE1+ SENSE1– 100k PGOOD TG1 INTVCC ILIM SW1 PHASMD PLLIN/MODE BOOST1 SGND BG1 EXTVCC LTC3787 RUN FREQ CSS, 0.1μF SS CITH, 15nF INTVCC MTOP1 L1 3.3μH RSENSE1 4mΩ L2 3.3μH RSENSE2 4mΩ COUTA1 22μF s4 + COUTA2 22μF s4 + CINA 22μF s4 + COUTA3 22μF s4 + COUTA4 22μF s4 + COUTB1 220μF CB1, 0.1μF MBOT1 D1 VBIAS INTVCC CINT1 4.7μF PGND D2 RITH, 8.66k ITH BG2 MBOT2 CB2, 0.1μF CITHA, 220pF BOOST2 SW2 RA, 12.
LTC3787 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 Rev B) .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 w 45° (0.38 ±0.10) .0075 – .0098 (0.
LTC3787 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.
LTC3787 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 12/10 Updated PGND, BG2, BG1, INTVCC and EXTVCC Pin numbers Updated Block Diagram Updated Figures 11, 12, 13 Updated Related Parts B C 9/11 4/12 Updated graphs on TA01b, G02, G09, G10, G11, G13, G14, G15, G18, G19, G22, and G26.
LTC3787 TYPICAL APPLICATION IIN CIN 12V I1 BG1 TG1 PHASMD 0° I1 BOOST: 24V, 5A LTC3787 I2 I2 BG2 TG2 CLKOUT +90° 180° BOOST: 24V, 5A I3 24V, 20A I3 90,270 CLKOUT PHASMD BG1 TG1 I4 90° BOOST: 24V, 5A COUT ICOUT LTC3787 I4 BG2 TG2 I*IN 270° BOOST: 24V, 5A I*COUT REFER TO FIGURE 15 FOR APPLICATION CIRCUITS * RIPPLE CURRENT CANCELLATION INCREASES THE RIPPLE FREQUENCY AND REDUCES THE RMS INPUT/OUTPUT RIPPLE CURRENT, THUS SAVING INPUT/OUTPUT CAPACITORS 3787 F16 Figure 16.
