LTC3706 Secondary-Side Synchronous Forward Controller with PolyPhase Capability DESCRIPTION FEATURES n n n n n n n n n n n n Secondary-Side Control for Fast Transient Response Self-Starting Architecture Eliminates Need for Separate Bias Regulator Proprietary Gate Drive Encoding Scheme Reduces System Complexity PolyPhase® Operation Reduces CIN Requirements Current Mode Control Ensures Current Sharing PLL Fixed Frequency: 100kHz to 500kHz ±1% Output Voltage Accuracy True Remote Sense Differential Amplifier Power Good Output Voltage Monitor High Voltage Linear Regulator Controller Wide Supply Range: 5V to 30V Available in a Narrow 24-Lead SSOP Package The LTC®3706 is a PolyPhase capable secondary-side controller for synchronous forward converters. When used in conjunction with the LTC3705 gate driver and primaryside controller, the part creates a complete isolated power supply that combines the power of PolyPhase operation with the speed of secondary-side control. The LTC3706 has been designed to simplify the design of highly efficient, secondary-side forward converters. Working in concert with the LTC3705, the LTC3706 forms a robust, self-starting converter that eliminates the need for the separate bias regulator that is commonly used in secondary-side control applications. In addition, a proprietary scheme is used to multiplex gate drive signals and DC bias power across the isolation barrier through a single, tiny pulse transformer. APPLICATIONS n n n n The LTC3706 provides remote sensing, accurate power good and overvoltage monitoring circuits to support precision, high current applications. A linear regulator controller with thermal protection is also provided to simplify the generation of secondary-side bias voltage. Isolated 48V Telecommunication Systems Internet Servers and Routers Distributed Power Step-Down Converters Automotive and Heavy Equipment L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. PolyPhase is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 6144194, other patents pending. The LTC3706 is available in a 24-lead SSOP package. TYPICAL APPLICATION 36V-72V to 3.3V/20A Isolated Forward Converter VIN+ T1 • Si7852DP 1.2Ω MURS120 1μF 100V s3 VOUT+ L1 1.2μH • Si7336ADP s2 Si7852DP Si7336ADP 330μF 6.3V s3 CMPSH1-4 MURS120 2mΩ 2W 30mΩ 1W VIN– 10μF 25V VOUT– CZT3019 100k FQT7N10 365k 1% NDRV BOOST TG 2.2μF 25V 15k 1% 33nF 1μF TS BG IS FB/IN+ UVLO VCC 2.2μF 16V L1: COILCRAFT SER2010-122 T1: PULSE PA0807 T2: PULSE PA0297 BAS21 0.22μF T2 • 162k SW SG VIN NDRV VCC FS/SYNC FB LTC3706 ITH PT– RUN/SS GND PGND PHASE SLP MODE REGSD FS/IN– GND PGND VSLMT FG IS+ PT+ • LTC3705 SS/FLT IS– 33nF 102k 1% 680pF 20k 22.6k 1% 3706 TA01 3706fb 1 LTC3706 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW VCC ........................................................... –0.3V to 10V VIN ........................................................... –0.3V to 33V SW ............................................................... –5V to 50V NDRV ........................................................ –0.3V to 13V ITH, RUN/SS, VSOUT, VS+, VS–, REGSD ....... –0.3V to 7V All Other Pins ............................................ –0.3V to 10V Operating Temperature Range (Note 2) LTC3706EGN ........................................ –40°C to 85°C LTC3706IGN......................................... –40°C to 85°C Junction Temperature (Note 3) ............................ 125°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C SG 1 24 VCC FG 2 23 PGND PGOOD 3 22 PT+ MODE 4 21 PT – PHASE 5 20 SW FB 6 19 VIN ITH 7 18 NDRV RUN/SS 8 17 REGSD VSOUT 9 16 IS+ VS+ 10 15 IS– VS– 14 SLP 11 GND 12 13 FS/SYNC GN PACKAGE 24-LEAD PLASTIC SSOP TJMAX = 125°C, θJA = 130°C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3706EGN#PBF LTC3706EGN#TRPBF LTC3706EGN 24-Lead Plastic SSOP –40°C to 85°C LTC3706IGN#PBF LTC3706IGN#TRPBF LTC3706IGN 24-Lead Plastic SSOP –40°C to 85°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3706EGN LTC3706EGN#TR LTC3706EGN 24-Lead Plastic SSOP –40°C to 85°C LTC3706IGN LTC3706IGN#TR LTC3706IGN 24-Lead Plastic SSOP –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/ 3706fb 2 LTC3706 ELECTRICAL CHARACTERISTICS The l indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = 7V, VIN = 15V, GND = PGND = 0V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS VFB Regulated Feedback Voltage (Note 4) ITH = 1.2V IFB Feedback Input Current (Note 4) ΔVFB(LINREG) Feedback Voltage Line Regulation VIN = 6V to 30V, ITH = 1.2V MIN TYP MAX UNITS 0.594 0.600 0.606 V 2 100 nA Main Control Loop l 0.001 l %/V ΔVFB(LOADREG) Feedback Voltage Load Regulation Measured in Servo Loop, ITH = 0.5V to 2V VISMAX Maximum Current Sense Threshold RSENSE Mode, 0V < VIS– < 5V VIS– = VCC, 0V < VIS+ < 2V (CT Mode) VISOC Over-Current Shutdown Threshold RSENSE Mode, 0V < VIS– < 5V VIS– = VCC, 0V < VIS+ < 2V (CT Mode) gm Transconductance Amplifier gm IRUN/SS(C) Soft-Start Charge Current IRUN/SS(D) Soft-Start Discharge Current VRUN/SS RUN/SS Pin ON Threshold tON,MIN Minimum ON Time FG, SG RUP FG, SG Driver Pull-Up On Resistance FG, SG Low 1.5 2.3 Ω FG, SG RDOWN FG, SG Driver Pull-Down On Resistance FG, SG High 1.5 2.3 Ω PT+, PT– RUP PT+, PT– RDOWN PT+, PT– Driver Pull-Up Resistance PT+, PT– Low 1.5 2.3 Ω PT+, PT– Driver Pull-Down Resistance PT+, PT– High 1.5 2.3 Ω ΔVFB(OV) Output Overvoltage Threshold VFB Rising 17 19 % 10 V 7.4 V VRUN/SS = 2V –0.01 –0.1 % 68 1.15 78 1.28 88 1.4 mV V 87 1.45 100 1.65 113 1.85 mV V 2.40 2.75 3.10 mS –4 –5 –6 μA 0.4 0.45 3 VRUN/SS Rising l μA 0.5 200 15 V ns VCC Supply VCCOP Operating Voltage Range 5 VCCREG Regulated Output Voltage 6.6 ICC Supply Current Operating Shutdown fOSC = 200kHz (Note 5) VRUN/SS = GND VUVLO UV Lockout VCC Rising VHYS UV Hysteresis 7.0 4.2 240 l 4.52 4.60 mA μA 4.70 0.4 V V VIN Supply VINOP Operating Voltage Range IIN Supply Current Normal Mode Shutdown fOSC = 200kHz VRUN/SS = GND UV Lockout VIN Rising VREGSD REGSD Shutdown Threshold VREGSD Rising gm,REGSD REGSD Transconductance VINUVLO 5 VINHYS 30 900 460 l 3.90 4.30 V μA μA 4.51 V 0.2 V 4 V 5 μS 3706fb 3 LTC3706 ELECTRICAL CHARACTERISTICS SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Oscillator and Phase-Locked Loop IFS FS/SYNC Pin Sourcing Current 20 μA fLOW Oscillator Low Frequency Set Point VFS/SYNC = GND 170 200 230 kHz fHIGH Oscillator High Frequency Set Point VFS/SYNC = VCC 255 300 345 kHz Δf (RFS) Oscillator Resistor Set Accuracy 75kΩ < RFS/SYNC < 175kΩ –20 20 % fPLL(MAX) Maximum PLL Sync Frequency 500 kHz fPLL(MIN) Minimum PLL Sync Frequency 75 kHz PGOOD Output VFBH/0.6 Power Good Upper Threshold VFB Rising 115 117 119 % VFBL1/0.6 Power Good Lower Threshold VFB Rising 91.5 93 94.5 % VFBL2/0.6 Power Good Lower Threshold VFB Falling 89.5 91 92.5 % VS– = GND, 1V ≤ VS+ ≤ 5V 0.994 1 1.006 V/V Differential Amplifier (VSENSE AMP) ADA Gain CMRR DA Common Mode Rejection Ratio 75 dB RIN Input Resistance 80 kΩ fBW –3dB Bandwidth 3 MHz 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 LTC3706E is guaranteed to meet the performance specifications over the 0°C to 85°C operating temperature range. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3706I is guaranteed and tested over the full –40°C to 85°C operating temperature range. Note 3: Junction temperature TJ (in °C) is calculated from the ambient temperature TA and the average power dissipation PD (in Watts) by the formula: TJ = TA + θJA • PD Refer to the Applications Information section for details. Note 4: The LTC3706 is tested in a feedback loop that servos VFB to a voltage near the internal 0.6V reference voltage to obtain the specified ITH voltage (VITH = 1.2V). Note 5: Operating supply current is measured in test mode. Dynamic supply current is higher due to the internal gate charge being delivered at the switching frequency. See Typical Performance Characteristics. 3706fb 4 LTC3706 TYPICAL PERFORMANCE CHARACTERISTICS VCC Supply Current vs Input Voltage VCC Regulator Output Voltage vs Temperature Maximum Current Sense Threshold vs Duty Cycle 7.06 fOSC = 200kHz ALL GATES: CLOAD = 0 100 7.04 5 4 80 7.00 VIS/VIS,MAX (%) 6 RSLP = 0 7.02 VCC OUTPUT VOLTAGE (V) SUPPLY CURRENT (mA) 7 TA = 25°C, unless otherwise noted. 6.98 6.96 6.94 100kΩ 60 RSLP = 50k 40 6.92 6.90 20 6.88 6.86 –50 3 6 5 7 8 INPUT VOLTAGE (V) 10 9 0 –25 0 25 50 75 TEMPERATURE (°C) 100 VIS/VIS,MAX (%) IS Pins Source Current vs Temperature 40 20 400 265 300 260 IS PIN SOURCE CURRENT (μA) IS PIN SOURCE CURRENT (μA) 100 200 100 CT-MODE: IIS+ 0 –100 –200 –400 0 0 0.5 1.0 1.5 2.0 ITH VOLTAGE (V) 3.0 2.5 –1 0 1 5 2 3 4 IS+, IS– COMMON-MODE VOLTAGE (V) 3706 G04 460 100.0 99.5 6 25 50 75 TEMPERATURE (°C) 240 230 –50 100 125 3706 G07 –25 0 25 50 75 TEMPERATURE (°C) 125 3706 G06 5 450 440 420 –50 100 Oscillator Frequency vs Temperature 430 0 245 PERCENT CHANGE IN FREQUENCY (%) 101.0 VRUN/SS (mV) 470 –25 250 RUN/SS ON Threshold vs Temperature 101.5 99.0 –50 255 3706 G05 Maximum Current Sense Threshold vs Temperature 100.5 RS-MODE: (IIS+ + IIS–) VIS+ = VIS– = 0V 235 RS-MODE: (IIS+ + IIS–) –300 80 60 3706 G03 IS Pins Source Current 60 40 3706 G02 Maximum Current Sense Threshold vs ITH Voltage 80 20 0 DUTY CYCLE (%) 3706 G01 VIS/VIS,MAX (%) 125 –25 0 25 50 75 TEMPERATURE (°C) 100 125 3706 G08 4 3 2 R = 175KΩ fOSC = 500kHz 1 0 –1 –2 –50 R = 75KΩ fOSC = 100kHz –25 0 25 50 75 TEMPERATURE (°C) 100 125 3706 G14 3706fb 5 LTC3706 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. REGSD Shutdown Threshold vs Temperature Oscillator Frequency vs RFS 600 FB Voltage vs Temperature 4.010 601.0 4.005 600.5 300 VFB (mV) 400 VREGSD (V) FREQUENCY (kHz) 500 4.000 600.0 200 3.995 599.5 100 3.990 –50 0 50 75 100 125 150 RFS (kΩ) 200 175 –25 0 25 50 75 TEMPERATURE (°C) 100 3706 G09 599.5 4.60 5 0 10 30 15 20 25 VIN SUPPLY VOLTAGE (V) 4.45 4.40 PULL-DOWN 1.4 PULL-UP VIN RISING 1.3 4.25 –50 1.2 –25 0 25 50 75 TEMPERATURE (°C) 100 125 5 6 7 8 VCC VOLTAGE (V) 3706 G16 9 10 3706 G12 Efficiency (Figure 5) Load Step (Figure 5) 95 VCC = 7V VIN = 36V 2.25 VOUT 100mV/DIV EFFICIENCY (%) 2.00 RDS,ON (Ω) 1.5 4.35 Gate Driver On-Resistance vs Temperature 125 1.6 4.50 3706 G11 1.75 1.50 90 VIN = 72V IOUT 10A/DIV 85 20μs/DIV VIN = 48V VOUT = 3.3V LOAD STEP = 0A TO 20A 1.25 1.00 –50 100 1.7 VCC RISING 4.55 4.30 599.0 25 50 75 TEMPERATURE (°C) 1.8 RDS,ON (Ω) UVLO THRESHOLD VOLTAGE (V) 600.0 0 Gate Driver On-Resistance vs VCC 4.65 600.5 –25 3706 G15 Undervoltage Lockout vs Temperature 601.0 2.50 599.0 –50 3706 G10 FB Voltage Line Regulation VFB (mV) 125 80 –25 0 25 50 75 TEMPERATURE (°C) 100 125 3706 G13 0 5 10 15 LOAD CURRENT (A) 20 3706 G18 25 3706 G17 3706fb 6 LTC3706 PIN FUNCTIONS SG (Pin 1): Gate Drive for the “Synchronous” MOSFET. FG (Pin 2): Gate Drive for the “Forward” MOSFET. SLP (Pin 14): Slope Compensation Input. Place a single resistor to ground to set the desired amount of slope compensation. PGOOD (Pin 3): Open-Drain Power Good Output. The FB pin is monitored to ensure that the output is in regulation. When the output is not in regulation, the PGOOD pin is pulled low. IS– (Pin 15): Negative Input to the Current Sense Circuit. When using current sense transformers, this pin may be tied to VCC for single-ended sensing with a 1.28V maximum current trip level. MODE (Pin 4): Tie to either GND or VCC to set the maximum duty cycle at either 50% or 75% respectively. Tie to ground through either a 200k or 100k resistor (50% or 75% maximum duty cycle) to disable pulse encoding. In this mode, normal PWM signals will be generated at the PT+ pin, while a clock signal is generated at the PT– pin. IS+ (Pin 16): Positive Input to the Current Sense Circuit. Connect to the positive end of a current sense resistor or to the output of a current sense transformer. FB (Pin 6): The Inverting Input of the Main Loop Error Amplifier. REGSD (Pin 17): This pin is used to prevent overheating of the external linear regulator pass device that generates the VCC supply voltage from the VIN voltage. A current proportional to the voltage across the external pass device flows out of this pin. The IC shuts down the linear regulator when the voltage on this pin exceeds 4V. Place a resistor (or a resistor and capacitor in parallel) between this pin and GND to limit the temperature rise of the external pass device. ITH (Pin 7): The Output of the Main Loop Error Amplifier. Place compensation components between the ITH pin and GND. NDRV (Pin 18): Drive Output for the External Pass Device of the VCC Linear Regulator. Connect to the base (NPN) or gate (NMOS) of an external N-type device. RUN/SS (Pin 8): Combination Run Control and Soft-Start Inputs. A capacitor to ground sets the ramp time of the output voltage. Holding this pin below 0.4V causes the IC to shut down all internal circuitry. VIN (Pin 19): Connect to a higher voltage bias supply, typically the output of a peak detected bias winding. When not used, tie together with the VCC and NDRV pins. PHASE (Pin 5): Control Input to the Phase Selector. This pin determines the phasing of the controller CLK relative to the synchronizing signal at the FS/SYNC pin. VSOUT, VS+, VS– (Pins 9, 10, 11): VSOUT is the output of a precision, unity-gain differential amplifier. Tie VS+ and VS– to the output of the main DC/DC converter to achieve true remote differential sensing. This allows DCR error effects to be minimized. GND (Pin 12): Signal Ground. FS/SYNC (Pin 13): Combination Frequency Set and SYNC pin. Tie to GND or VCC to run at 200kHz and 300kHz respectively. Place a single resistor to ground at this pin to set the frequency between 100kHz and 500kHz. To synchronize, drive this pin with a clock signal to achieve PLL synchronization from 75kHz to 500kHz. Sources 20μA of current. SW (Pin 20): Connect to the drain of the “synchronous” MOSFET. This input is used for adaptive shoot-through prevention and leading edge blanking. PT–, PT+ (Pins 21, 22): Pulse Transformer Driver Outputs. For most applications, these connect to a pulse transformer (with a series DC blocking capacitor). The PWM information is multiplexed together with DC power and sent through a single pulse transformer to the primary side. This information may be decoded by the LTC3705 gate driver and primary-side controller. PGND (Pin 23): Gate Driver Ground Pin. VCC (Pin 24): Main VCC Input for all Driver and Control Circuitry. 3706fb 7 LTC3706 BLOCK DIAGRAM IS+ 2× 16 + IS– VCC + 32× C – – 15 2V FG ITRP 2 RESET DOMINANT WAIT OVP PGND R 23 VCC SG Q S + + 0.25V – 6 ZERO CROSSING DETECT BLANK + + gm = 2.8mS 2.5V – 3.2V 14 – OVERCURRENT SLOPE COMP 1 OSC AND PLL 5 MODE 4 VCC PT+ PGND VCC • • PT– 21 PULSE XFMR DRIVE TYPE 13 PHASE 20 22 DRIVER ENCODING AND LOGIC RUN/SS FS/SYNC – OC C EA SLP 0.2V SW – 7 FB + OVP C ITH 0.60V DMAX SKIP C 1 WAIT PWM BLANK DMAX DRIVE/DMAX CONTROL 5VDC TO 30VDC VIN 19 UVLO 4VSB RUN/SS REG SOFTSTART VCCUV R – SHDN 275k S OC OT LATCH OT 5VDC TO 10VDC VIN VIN + gm = 5μS A – 40k 40k 10 VCC 24 4VSB + 9 4V VCC REGSD 17 – PGOOD VCC + VSENSE AMP – VS– 18 Q VSOUT VS+ NDRV A 60k WAIT RESTRT1 1.24V VREF 4VSB SSLOW 4VSB 4VSB + SHDN FB 8 VINUV SHDN UVLO (4.25/4.5) VCCUV FB 0.6V PGOOD/OVP 3 OVP GND 12 3706 BD 40k 40k 11 3706fb 8 LTC3706 OPERATION Main Control Loop The LTC3706 is designed to work in a constant frequency, current mode 2-transistor forward converter. During normal operation, the primary-side MOSFETs (both top and bottom) are “clocked” on together with the forward MOSFET on the secondary side. This applies the reflected input voltage across the inductor on the secondary side. When the current in the inductor has ramped up to the peak value as commanded by the voltage on the ITH pin, the current sense comparator is tripped, turning off the primary-side and forward MOSFETs. To avoid turning on the synchronous MOSFET prematurely and causing shoot-through, the voltage on the SW pin is monitored. This voltage will usually fall below 0V soon after the primary-side MOSFETs have turned completely off. When this condition is detected, the synchronous MOSFET is quickly turned on, causing the inductor current to ramp back downwards. The error amplifier senses the output voltage, and adjusts the ITH voltage to obtain the peak current needed to maintain the desired main-loop output voltage. The LTC3706 always operates in a continuous current, synchronous switching mode. This ensures a rapid transient response as well as a stable bias supply voltage at light loads. A maximum duty cycle (either 50% or 75%) is internally set via clock dividers to prevent saturation of the main transformer. In the event of an overvoltage on the output, the synchronous MOSFET is quickly turned on to help protect critical loads from damage. Gate Drive Encoding Since the LTC3706 controller resides on the secondary side of an isolation barrier, communication to the primary-side power MOSFETs is generally done through a transformer. Moreover, it is often necessary to generate a low voltage bias supply for the primary-side gate drive circuitry. In order to reduce the number of isolated windings present in the system, the LTC3706 uses a proprietary scheme to encode the PWM gate drive information and multiplex it together with bias power for the primary-side drive and control, using a single pulse transformer. Note that, unlike optoisolators and other modulation techniques, this multiplexing scheme does not introduce a significant time delay into the system. For most forward converter applications, the PT+ and PT– outputs will contain a pulse-encoded PWM signal. These outputs are driven in a complementary fashion with an essentially constant 50% duty cycle. This results in a stable volt-second balance as well as an efficient transfer of bias power across the pulse transformer. As shown in Figure 1, the beginning of the positive half-cycle coincides with the turn-on of the primary-side MOSFETs. Likewise, the beginning of the negative half-cycle coincides with the maximum duty cycle (forced turn-off of primary switches). At the appropriate time during the positive half-cycle, the end of the “on” time (PWM going LOW) is signaled by briefly applying a zero volt differential across the pulse transformer. Figure 1 illustrates the operation of this multiplexing scheme. The LTC3705 primary-side controller and gate driver will decode this PWM information as well as extract the power needed for primary-side gate drive. Self-Starting Architecture When the LTC3706 is used in conjunction with the LTC3705 primary-side controller and gate driver, a complete self-starting isolated supply is formed. When input voltage is first applied in such an application, the LTC3705 will begin switching in an “open-loop” fashion, causing the main output to slowly ramp upwards. This is the primary-side soft-start mode. On the secondary side, the LTC3706 derives its operating bias voltage from a peak-charged capacitor. This peak-charged voltage will rise more rapidly than the main output of the converter, so that the LTC3706 will become operational well before the output voltage has reached its final value. 3706fb 9 LTC3706 OPERATION 150ns 150ns 7V the amount of slope compensation doubles when the duty cycle exceeds 50%. DUTY CYCLE = 0% DUTY CYCLE = 15% 7V Table 1 SLP PIN SLOPE (D < 0.5) SLOPE (D > 0.5) GND 0.05 • ISMAX • fOSC 0.1 • ISMAX • fOSC VCC None None 400kΩ to GND 0.1 • ISMAX • fOSC 0.2 • ISMAX • fOSC Figure 1: Gate Drive Encoding Scheme (VMODE = GND) 200kΩ to GND 0.15 • ISMAX • fOSC 0.3 • ISMAX • fOSC 100kΩ to GND 0.25 • ISMAX • fOSC 0.5 • ISMAX • fOSC When the LTC3706 has adequate operating voltage, it will begin the procedure of assuming control from the primary side. To do this, it first measures the voltage on the power supply’s main output and then automatically advances its own soft-start voltage to correspond to the main output voltage. This ensures that the output voltage increases monotonically as the soft-start control is transferred from primary to secondary. The LTC3706 then begins sending PWM signals to the LTC3705 on the primary side through a pulse transformer. When the LTC3705 has detected a stable signal from the secondary controller, it transfers control of the primary switches over to the LTC3706, beginning the secondary-side soft-start mode. The LTC3706 continues in this mode until the output voltage has ramped up to its final value. If for any reason, the LTC3706 either stops sending (or initially fails to send) PWM information to the LTC3705, the LTC3705 will detect a FAULT and initiate a soft-start retry. (See the LTC3705 data sheet.) 50kΩ to GND 0.5 • ISMAX • fOSC 1.0 • ISMAX • fOSC VPT1+ – VPT1– –7V –7V 1 CLK PER 3706 F01 1 CLK PER Slope Compensation Slope compensation is added at the input of the PWM comparator to improve stability and noise margin of the peak current control loop. The amount of slope compensation can be selected from one of five preprogrammed values using the SLP pin as shown in Table 1. Note that In Table 1 above, ISMAX is the maximum current limit, and fOSC is the switching frequency. Current Sensing and Current Limit For current sensing, the LTC3706 supports either a current sense resistor or a current sense transformer. The current sense resistor may either be placed in series with the inductor (either high side or ground lead sensing), or in the source of the “forward” switch. If a current sense transformer is used, the IS– input should be tied to VCC and the IS+ pin to the output of the current sense transformer. This causes the gain of the internal current sense amplifier to be reduced by a factor of 16×, so that the maximum current sense voltage (current limit) is increased from 78mV to 1.28V. An internal, adaptive leading edge blanking circuit ensures clean operation for “switch” current sensing applications. Current limit is achieved in the LTC3706 by limiting the maximum voltage excursion of the error signal (ITH voltage). Note that if slope compensation is used, the precise value at which current limit occurs will be a function of duty cycle (See Typical Performance Characteristics). If a short circuit is applied, an independent overcurrent comparator may be tripped. In this case, the LTC3706 will enter a “hiccup” mode using the soft-start circuitry. 3706fb 10 LTC3706 OPERATION Frequency Setting and Synchronization Soft-Start The LTC3706 uses a single pin to set the operating frequency, or to synchronize the internal oscillator to a reference clock with an on-chip phase-locked loop (PLL). The FS pin may be tied to GND, VCC or have a single resistor to GND to set the switching frequency. If a clock signal (>2V) is detected at the FS pin, the LTC3706 will automatically synchronize to the rising edge of the reference clock. Table 2 summarizes the operation of the FS pin. The soft-start circuitry has five functions: 1) to provide a shutdown, 2) to provide a smooth ramp on the output voltage during start-up, 3) to limit the output current in a short-circuit situation by entering a hiccup mode, 4) to limit the maximum power dissipation in the external linear regulator via the REGSD pin, and 5) to communicate fault and shutdown information between multiple LTC3706s in a PolyPhase application. For synchronization between multiple LTC3706s, the PT+ pin of one LTC3706 can be used as a master clock reference and tied to the FS pin of the other LTC3706s. When the RUN/SS pin is pulled to GND, the chip is placed into shutdown mode. If this pin is released, the RUN/SS pin is initially charged with a 50μA current source. After the RUN/SS pin gets above 0.5V, the chip is enabled. At the instant that the LTC3706 is first enabled, the RUN/SS voltage is rapidly preset to a voltage that will correspond to the main output voltage of the DC/DC converter. (See the Self-Starting Architecture section.) After this preset interval has completed, the normal soft-start interval begins and the charging current is reduced to 5μA. The external soft-start voltage is used to internally ramp up the 0.6V reference (positive) input to the error amplifier. When fully charged, the RUN/SS voltage remains at 3V. Table 2 FS PIN SWITCHING FREQUENCY GND 200kHz VCC 300kHz RFS to GND fOSC (Hz) = 4RFS – 200k Reference Clock fOSC = fREF (75kHz to 500kHz) This will cause all LTC3706’s to operate at the same frequency. The phase angle of each LTC3706 that is being synchronized can be set by using the PHASE pin. This pin can be tied to GND, VCC or have a single resistor to VCC to set the phase angle (delay) of the internal oscillator relative to the incoming sync signal on the FS pin. Any one of five preset values can be selected as summarized in Table 3. Table 3 PHASE PIN LTC3706 PHASE DELAY GND 0° VCC 180° 200kΩ to VCC 60° 100kΩ to VCC 90° 50kΩ to VCC 120° In the event that the sensed switch or inductor current exceeds the overcurrent trip threshold, an internal fault latch is tripped. This latch is also tripped when the REGSD voltage exceeds 4V (see the Linear Regulator section). When such a fault is detected, the LTC3706 immediately goes to zero duty cycle and initiates a soft-start retry. Prior to discharging the soft-start capacitor, however, the LTC3706 first puts a voltage pulse on the RUN/SS pin, which trips the fault latch in any other LTC3706 that shares the RUN/SS. This ensures an orderly shutdown of all phases in a PolyPhase application. After the soft-start capacitor is fully discharged, the LTC3706 attempts a restart. If the fault is persistent, the system enters a “hiccup” mode. 3706fb 11 LTC3706 OPERATION Note that in self-starting secondary-side control applications (with the LTC3705), the presence of the LT3706 bias voltage is dependent upon the regular switching of the primary-side MOSFETs. Therefore, depending on the details of the application circuit, the LTC3706 may lose its bias voltage after a fault has been detected and before completing a soft-start retry. In this case, the “hiccupmode” operation is actually governed by the LTC3705 soft-start circuitry. (See the LTC3705 data sheet.) Drive Mode and Maximum Duty Cycle Although the LTC3706 is primarily intended to be used with the LTC3705 in 2-transistor forward applications, the MODE pin provides the flexibility to use the LTC3706 in a wide variety of additional applications. This pin can be used to defeat the gate drive encoding scheme, as well as change the maximum duty cycle from its default value of 50%. The use of the MODE pin is summarized in Table 4. When the gate drive encoding scheme is defeated, a standard PWM-style signal will be present at the PT+ pin and a reference clock (in phase with the PWM signal) will be present at the PT– pin. These outputs can be used in “standalone” applications (without the LTC3705) to drive the gates of MOSFETs in a conventional manner. Table 4 PT+/PT– Mode (MAX DUTY CYCLE) INTENDED APPLICATION GND Encoded PWM (DMAX = 50%) 2-Switch Forward with LTC3705 VCC Encoded PWM (DMAX = 75%) 1-Switch Forward 200kΩ to GND Standard PWM (DMAX = 50%) 2-Switch Forward Standalone 100kΩ to GND Standard PWM (DMAX = 75%) 1-Switch Forward Standalone MODE PIN Power Good/Overvoltage Protection This circuit monitors the voltage on the FB input. The open-drain PGOOD output will be logic high if the voltage on the FB pin is within +17%/–7% of 0.6V. If the voltage on the FB pin exceeds 117% of 0.6V (0.7V), an overvoltage (OVP) is detected. For overvoltage protection, the secondary-side synchronous MOSFET is turned on while all other MOSFETs are turned off. This protection mode is not latched, so that the overvoltage detection is cleared if the FB voltage falls below 115% of 0.6V (0.69V). Linear Regulator Operation The LTC3706 provides a linear regulator controller that drives an external N-type pass device. This controller is used to create a 7V DC bias from the peak-charged secondary bias voltage (8V to 30V). Internal divider resistors are used to establish a regulation voltage of 7V at the VCC pin. An auxiliary bias supply with a regulated voltage greater than 7V may be applied to the VCC pin to bypass (bootstrap) the linear regulator. This improves efficiency and also helps to avoid overheating the linear regulator pass device. Thermal protection for the linear regulator pass device is also provided by means of the REGSD pin. A current is sourced from this pin that is proportional to the voltage across the linear regulator pass device (VIN – VCC). Since the VCC load current is essentially constant for a given switching frequency and choice of power MOSFETs, the power dissipated in the external pass device will only vary with the voltage across it. Thus, a single resistor may be placed between the REGSD pin and GND to develop a voltage that is proportional to the power in the external pass device. An additional parallel capacitor can also be used to account for the thermal time constant associated with the external pass device itself. When the voltage on the REGSD pin exceeds 4V, an overtemperature fault occurs and the LTC3706 attempts a soft-start retry. 3706fb 12 LTC3706 OPERATION Slave Mode Operation When two or more LTC3706 devices are used in PolyPhase systems, one device becomes the “master” controller, while the others are used as “slaves.” Slave mode is activated by connecting the FB pin to VCC. In this mode, the ITH pin becomes a high impedance input, allowing it to be driven by the master controller. In this way, equal inductor currents are established in each of the individual phases. Also, in slave mode the soft-start charge/discharge currents are disabled, allowing the master device to control the charging and discharging of the soft-start capacitor. APPLICATIONS INFORMATION Start-Up Considerations In self-starting applications, the LTC3705 will initially begin the soft-start of the converter in an open-loop fashion. After bias is obtained on the secondary side, the LTC3706 assumes control and completes the soft-start interval. In order to ensure that control is properly transferred from the LTC3705 (primary-side) to the LTC3706 (secondary-side), it is necessary to limit the rate of rise on the primary-side soft-start ramp so that the LTC3706 has adequate time to wake up and assume control before the output voltage gets too high. This condition is satisfied for many applications if the following relationship is maintained: CSS,SEC ≤ CSS PRI However, care should be taken to ensure that soft-start transfer from primary-side to secondary-side is completed well before the output voltage reaches its target value. A good design goal is to have the transfer completed when the output voltage is less than one-half of its target value. Note that the fastest output voltage rise time during primary-side soft-start mode occurs with maximum input voltage and minimum load current. The open-loop start-up frequency on the LTC3705 is set by placing a resistor from the FB/IN+ pin to GND. Although the exact start-up frequency on the primary side is not critical, it is generally good practice to set this approximately equal to the operating frequency on the secondary side. The FS/IN– start-up resistor for the LTC3705 may be selected using the following: fPRI(Hz) = 3.2 • 1010 RFS /IN– + 10k In the event that the secondary-side circuitry fails to properly start up and assume control of switching, there are several fail-safe mechanisms to help avoid overvoltage conditions. First, the LTC3705 contains a volt-second clamp that will keep the primary-side duty cycle at a level that cannot produce an overvoltage condition. Second, the LTC3705 contains a time-out feature that will detect a FAULT if the LTC3706 fails to start up and deliver PWM signals to the primary side. Finally, the LTC3706 has an independent overvoltage detection circuit that will crowbar the output of the DC/DC converter using the synchronous MOSFET switch. In the event that a short circuit is applied to the output of the DC/DC converter prior to start-up, the LTC3706 will generally not receive enough bias voltage to operate. In this case, the LTC3705 will detect a FAULT for one of two reasons: 1) the start-up time-out feature will be activated since the LTC3706 never sends signals to the primary side or 2) the primary-side overcurrent circuit will be tripped because of current buildup in the output inductor. In either case, the LTC3705 will initiate a shutdown followed by a soft-start retry. See the LTC3705 data sheet for further details. 3706fb 13 LTC3706 APPLICATIONS INFORMATION Bias Supply Generation Figure 2 shows a commonly used method of developing a VCC bias supply for the LTC3706. During start-up, bias winding 1 uses a peak detection method to rapidly develop a VIN voltage for the LTC3706, which in turn drives the linear regulator that generates the VCC voltage (7V). When the main output of the converter is in regulation, winding 2 (configured as a forward-style output) is designed to produce a regulated auxiliary voltage of approximately 7.5V to 8.5V. Since the auxiliary voltage is greater than that of the linear regulator, the linear regulator will effectively be shut down. Note that the output inductor L1 must be adequately large so that its ripple current is continuous given the amount of VCC load current, thereby providing a stable output voltage. BAS21 • 1mH WINDING 2 NB2 • 1 BAS21 MBRO530 • 4.7Ω WINDING 1 NB1 MAIN TRANSFORMER FMMT491A 1μF 50V 4.7μF 16V VIN LTC3706 REGSD NDRV CREGSD RREGSD VCC 3706 F02 The turns ratio (NB1) of the bias winding 1 should be chosen to ensure that there is adequate voltage to operate the LTC3706 over the entire range for the DC/DC converter’s input bus voltage (VBUS). This may be calculated using: NB1= VCC(MIN) + 1.5V VBUS(MIN) VCC(MIN) can be as low as 5V (if this provides adequate gate drive voltage to maintain acceptable efficiency) or as high as 7V. For VCC(MIN) = 6V and VBUS = 36V to 72V, this would mean a turns ratio of NB1 ≈ 0.21 and a VIN voltage range at the LTC3706 of 7.5V to 15V. Using the bias circuit of Figure 2, the linear regulator would normally operate only for a brief interval during the initial soft-start ramp of the main output voltage. Under some fault conditions (e.g., output overload), the auxiliary voltage produced by bias winding 2 may decrease below 7V, causing the linear regulator to again supply the VCC bias current. Since the amount of power dissipation in the linear regulator pass device may be quite high, it can take considerable board area when the linear regulator pass device is sized to handle this power continuously. As an alternative, the REGSD pin may be used to effectively detect an overtemperature condition on the linear regulator pass device and generate a shut down (soft-start retry) before overheating occurs. This allows for the use of a small (e.g., SOT-23) package for the linear regulator pass device. Figure 2. Typical Bias Supply Configuration 3706fb 14 LTC3706 APPLICATIONS INFORMATION The REGSD resistor should be selected based upon the steady-state (DC) thermal impedance of the linear regulator pass device. RREGSD = 960k θ JA • ICC(MAX ) TRISE(MAX ) where θJA is the DC thermal impedance of the linear regulator pass device and TRISE(MAX) is the maximum junction temperature rise desired for the pass device. The value for ICC(MAX) depends heavily on the particular switching MOSFETs used, as well as on the details of overall system design. Note that it may include the bias current associated with the primary-side gate driver and controller, if the LTC3705 is being used. The value for ICC is best determined experimentally and then guard banded appropriately to establish ICC(MAX). Using the Typical Application circuit on the first page of this data sheet as an example, if a SOT-23 MOSFET is chosen, we might have θJA = 150°C/W, tRISE(MAX) = 50°C and ICC(MAX) = 35mA so that RREGSD ≈ 100kΩ. In this case, the linear regulator can run continuously for any VIN voltage that is less than: 4V = (VIN – VCC)(5μs)(RREGSD) ⎛ 640k ⎞ VIN(MAX ) = ⎜ + 7V ⎝ RREGSD ⎟⎠ or 13.4V. In addition, a capacitor may be added in parallel with the REGSD resistor to delay the thermal shutdown and thereby account for the thermal time constant of the pass device. When using a delay capacitor, care must be taken to ensure that the safe operating area (SOA) of the pass device is not exceeded. The capacitor should be chosen to provide a time constant that is somewhat faster than the thermal time constant of the pass device in the system. This technique will allow for much higher transient power dissipation, which is particularly useful in larger (PolyPhase) systems that have a higher VCC bias current. For the above SOT-23 example, a capacitor CREGSD = 1μF provides a linear regulator shutdown delay given by: ⎛ ⎞ ⎜ ⎟ 1 t SHDN = (CREGSD )(RREGSD ) ln ⎜ ⎟ 640k ⎜ 1– ⎟ ⎜⎝ ( V – 7 )R ⎟ IN REGSD ⎠ or 33ms at VIN = 30V. This delay provides ample time for linear regulator operation during soft-start, while providing protection for the pass device during fault conditions such as input overvoltage or output overcurrent. Current Sensing The LTC3706 provides considerable flexibility in current sensing techniques. It supports two main methods: 1) resistive current sensing and 2) current transformer current sensing. Resistive current sensing is generally simpler, smaller and less expensive, while current transformer sensing is more efficient and generally appropriate for higher (>20A) output currents. For resistive current sensing, the sense resistor may be placed in any one of three different locations: high side inductor, low side inductor or low side switch, as shown in Figure 3. Sensing the 3706fb 15 LTC3706 APPLICATIONS INFORMATION inductor current (high side or low side) is generally less noisy but dissipates more power than sensing the switch current (Figures 3a and 3b). High side inductor current sensing provides a more convenient layout than low side (no split ground plane), but can only be used for output voltages up to 5.5V, due to the common mode limitations of the current sense inputs (IS+ and IS–). For most applications, low side switch current sensing will be a good solution (Figure 3c). • IS+ • LTC3706 For high current applications where efficiency (power dissipation) is very important, a current sense transformer may be used. As shown in Figure 3d, the IS– pin should be tied off to VCC when a current sense transformer is used. This causes the IS+ pin to become a single ended (nondifferential) current sense input with a maximum current sense voltage of 1.28V. Figure 3d shows a typical application circuit using a current transformer. • IS+ • LTC3706 78mV MAX 78mV MAX IS– IS– 3706 F03a 3706 F03b Figure 3a. High Side Inductor: Easier Layout, Low Noise, Accurate Figure 3b. Low Side Inductor: Accurate, Low Noise, High VOUT Capable • • • • 1.28V MAX TRIP IS+ LTC3706 • 78mV MAX IS– IS+ LTC3706 • 5W TO 50Ω VCC IS– 3706 F03c 3706 F03d Figure 3c. Switch Current Sensing: Easy Layout, Accurate, Higher Efficiency, High VOUT Capable Figure 3d. Current Transformer: Highest Efficiency, High VOUT Capable Figure 3. Current Sensing Techniques 3706fb 16 LTC3706 APPLICATIONS INFORMATION PolyPhase Applications LTC3705’s are interconnected, a FAULT (overcurrent, etc.) on any one of the phases will perform a shutdown/restart on all phases together. The LTC3705 is put into slave mode by omitting the resistor on FS/IN–. For the LTC3706, the master performs soft-start and voltage-loop regulation by driving all slaves to the same current as the master using the ITH pins. Faults and shutdowns are communicated via the interconnection of the RUN/SS pins. The LTC3706 is put into slave mode by tying the FB pin to VCC. Figure 4 shows the basic connections for using the LTC3705 and LTC3706 in PolyPhase applications. One of the phases is always identified as the “master,” while all other phases are “slaves.” For the LTC3705 (primary side), the master monitors the VIN voltage for undervoltage, performs the open-loop start-up and supplies the initial VCC voltage for the master and all slaves. The LTC3705 slaves simply stand by and wait for PWM signals from their respective pulse transformers. Since the SS/FLT pins of master and slave VIN+ VOUT+ VBIAS VIN NDRV VCC FS/SYNC NDRV UVLO FB/IN+ • • PT+ VCC FB ITH PT– RUN/SS LTC3706 (MASTER) FS/IN– SS/FLT LTC3705 (MASTER) VIN– VIN NDRV VCC RUN/SS FS/SYNC NDRV SS/FLT FB/IN+ VCC UVLO • • FB PT+ ITH FS/IN– LTC3705 (SLAVE) PT– PHASE LTC3706 (SLAVE) 3706 F05 Figure 4. Connections for PolyPhase Operation 3706fb 17 LTC3706 TYPICAL APPLICATION VIN+ L1 1μH MURS120 T1 • Si7852DP 1μF 100V 1μF 100V s3 • 1nF 100V 1.2Ω Si7336ADP s2 9:2 Si7336ADP Si7852DP VOUT+ L2 1.2μH 10Ω 0.25W 1nF 100V 10Ω 0.25W 330mF 6.3V s3 1μF CMPSH1-4 MURS120 30μΩ 1W VIN– 365k 1% BOOST TG TS BG IS UVLO 100Ω L1: VISHAY IHLP-2525CZ-01 L2: COILCRAFT SER2010-122 T1: PULSE PA0807 T2: PULSE PA0297 2.2μF 25V 15k 1% 33nF FS/IN– 470pF 0.1μF • 5k 1:2 GND PGND VSLMT 162k 33nF FG SW SG VIN IS– 1μF NDRV 102k 1% VCC FS/SYNC IS+ PT+ • 2.2μF 16V 100Ω 1nF T2 100Ω LTC3705 VCC SS/FLT VOUT– CZT3019 FB/IN+ 1nF 10μF 25V 680pF BAS21 0.22μF NDRV 2mΩ 2W 100Ω 100k FQT7N10 2.2nF 250V FB LTC3706 ITH PT– RUN/SS GND PGND PHASE SLP MODE REGSD 100k 680pF 20k 22.6k 1% 3706 F05 Figure 5. 36V-72V to 3.3V/20A Isolated Forward Converter (See Typical Performance Characteristics) 3706fb 18 LTC3706 PACKAGE DESCRIPTION GN Package 24-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .337 – .344* (8.560 – 8.738) 24 23 22 21 20 19 18 17 16 15 1413 .033 (0.838) REF .045 p.005 .229 – .244 (5.817 – 6.198) .254 MIN .150 – .157** (3.810 – 3.988) .150 – .165 1 .0165 p.0015 2 3 4 5 6 7 8 9 10 11 12 .0250 BSC RECOMMENDED SOLDER PAD LAYOUT .015 p .004 s 45o (0.38 p 0.10) .0075 – .0098 (0.19 – 0.25) .0532 – .0688 (1.35 – 1.75) .004 – .0098 (0.102 – 0.249) 0o – 8o TYP .016 – .050 (0.406 – 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) .008 – .012 (0.203 – 0.305) TYP .0250 (0.635) BSC GN24 (SSOP) 0204 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 3706fb 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. 19 LTC3706 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1534 Ultralow Noise 2A Switching Regulator Reduces Conducted and Radiated EMI, Low Switching Harmonics, 20kHz to 250kHz Switching Frequency LT1619 Low Voltage Current Mode Controller 1.9V ≤ VIN ≤ 18V, 300kHz Operation, Boost, Flyback, SEPIC LT1681/LT3781 Dual Transistor Synchronous Forward Controller Operation Up to 72V Maximum LT1693 High Speed MOSFET Driver 1.5A Peak Output Current, 16ns Rise/Fall Time at VCC = 12V, CL = 1nF LTC1698 Secondary Synchronous Rectifier Controller Use with the LT1950 or LT1681, Isolated Power Supplies, Contains Voltage Margining, Optocoupler Driver, Synchronization Circuit with the Primary Side LT1725 General Purpose Isolated Flyback Controller No Optoisolator Required, Accurate Regulation Without User Trims, 50kHz to 250kHz Switching Frequency, SSOP-16 Package LTC1871 Wide Input Range, No RSENSE™ Controller Operation as Low as 2.5V Input, Boost, Flyback, SEPIC LT1910 Protected High Side MOSFET Driver 8V to 48V Supply Range, Protected –15V to 60V Supply Transient LTC3440 Micropower Buck-Boost DC/DC Converter Synchronous, Single Inductor, No Schottky Diode Required LTC3704 Positive-to-Negative DC/DC Controller 2.5V ≤ VIN ≤ 36V, No RSENSE Current Mode Operation, Excellent Transient Response LTC3705 Two-Switch Forward Converter Gate Driver and Controller Use with LTC3706, Isolated Power Supplies, High Speed Gate Drivers LTC3722 Full Bridge Controller Synchronous; ZVS Operation; 24-Pin SSOP No RSENSE is a trademark of Linear Technology Corporation. 3706fb 20 Linear Technology Corporation LT 0808 REV B • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005