LTC1698 Isolated Secondary Synchronous Rectifier Controller FEATURES DESCRIPTIO n High Efficiency Over Wide Load Current Range The LTC 1698 is a precision secondary-side forward n 0.8% Output Voltage Accuracy converter controller that synchronously drives external n Dual N-Channel MOSFET Synchronous Drivers N-channel MOSFETs. It is designed for use with the n Pulse Transformer Synchronization LT 3781 primary-side synchronous forward converter n Optocoupler Feedback Driver controller to create a completely isolated power supply. n Programmable Current Limit Protection The LT3781 synchronizes the LTC1698 through a small n 5% Margin Output Voltage Adjustment pulse transformer and the LTC1698 drives a feedback n Adjustable Overvoltage Fault Protection optocoupler to close the feedback loop. Output accuracy n Power Good Flag of 0.8% and high efficiency over a wide range of load n Auxiliary 3.3V Logic Supply currents are obtained. n Available in 16-Lead SSOP and SO Packages The LTC1698 provides accurate secondary-side current U limit using an external current sense resistor. The input APPLICATIO S voltage at the MARGIN pin provides 5% output voltage adjustment. A power good flag and overvoltage input are n 48V Input Isolated DC/DC Converters provided to ensure proper power supply conditions. An n Isolated Telecommunication Power Systems auxiliary 3.3V logic supply is included that supplies up to n Distributed Power Step-Down Converters 10mA of output current. n Industrial Control Systems , LTC and LT are registered trademarks of Linear Technology Corporation. n Automotive and Heavy Equipment TYPICAL APPLICATIO L1 V IN V OUT 36V to 72V + R2 C OUT Q1 R1 V BIAS DD + D1 110 T1 R R5 V PWRGD C DD 28 Q4 CG V FB C C Q3 16 Q2 D2 FG LTC1698 C FB 6 12 R V PRISEN COMP I SNS R SECSEN C CILM R CILM 11 13 I I SNSGND COMP C C SG SYNC 15 9 TG BG SG SYNC OVPIN R4 T2 R SYNC 7 MARGIN V MARGIN LT3781 R K V AUX 5 14 OPTODRV V 3.3V AUX C 10mA K O.1F REF PGND GND V FB 34 1681 F01 V C R E R F C F ISOLATION PLEASE REFER TO FIGURE 12 IN THE TYPICAL APPLICATIONS BOUNDARY SECTION FOR THE COMPLETE 3.3V/15A APPLICATION SCHEMATIC Figure 1. Simplified 2-Transistor Isolated Forward Converter 1698f 1 + U ULTC1698 ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) V , PWRGD ....................................................... 13.2V TOP VIEW ORDER PART DD Input Voltage NUMBER V 1 16 FG DD MARGIN, V , OVPIN, I , I ... 0.3V to 5.3V CG 2 15 SYNC FB SNSGND SNS LTC1698EGN SYNC ..................................................... 14V to 14V PGND 3 14 V AUX LTC1698ES Output Voltage GND 4 13 I COMP LTC1698IGN OPTODRV 5 12 I V , I (Note 2)......................... 0.3V to 5.3V SNS COMP COMP LTC1698IS V 6 11 I Power Dissipation.............................................. 500mW COMP SNSGND MARGIN 7 10 PWRGD GN PART MARKING Operating Temperature Range V 8 9 OVPIN FB LTC1698E (Note 3) ............................ 40 C to 85 C 1698 GN PACKAGE S PACKAGE LTC1698I........................................... 40 C to 85 C 1698I 16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO Storage Temperature Range ................. 65 C to 150 C T = 125 C, q = 130 C/W (GN) JMAX JA Lead Temperature (Soldering, 10 sec).................. 300 C T = 125 C, q = 110 C/W (SO) JMAX JA Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The l indicates specifications which apply over the full operating temperature range, otherwise specifications are at T = 25 C. V = 8V, unless otherwise noted. (Note 4) A DD SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Supply Voltage l 6 8 12.6 V DD V Undervoltage Lockout 4V UVLO I V Supply Current V , OVPIN, V , V = 0V, VDD DD FB ISNS ISNSGND C = C = 1000pF, C = 0.1m F, FG CG VAUX V = 0V l 1.8 4 mA SYNC f = 100kHz (Note 5) 5.0 mA SYNC MARGIN and Error Amplifier V Feedback Voltage MARGIN = Open, V = 1V (Note 7) 1.223 1.233 1.243 V FB COMP l 1.215 1.233 1.251 V I Feedback Input Current V = 1.233V l 0.05 1 m A VFB FB V MARGIN Voltage MARGIN = Open 1.65 V MARGIN R MARGIN Input Resistance 16.5 kW MARGIN D V Feedback Voltage Adjustment V = 3.3V l 456 % FB MARGIN V = 0V l 6 5 4 % MARGIN G Error Amplifier Open-Loop DC Gain V = 0.8V to 1.2V, Load = 2kW , 100pF l 65 90 dB ERR COMP BW Error Amplifier Unity-Gain Bandwidth No Load (Note 6) 2 MHz ERR V Error Amplifier Output Clamp Voltage V = 0V 2 V CLAMP FB I Error Amplifier Source Current V = 0V l 25 10 mA VCOMP FB Error Amplifier Sink Current V = 5V, V = 1.233V l 37 mA FB COMP OPTODRV G Opto Driver DC Gain OVPIN, V , V = 0V l 4.75 5 5.25 V/V OPTO ISNS ISNSGND BW Opto Driver Unity-Gain Bandwidth No Load (Note 6) 1 MHz OPTO V Opto Driver Output High Voltage V , OVPIN, V = 0V, V = 50mV, OPTOHIGH FB ISNSGND ISNS I = 10mA l 45 V OPTODRV I Opto Driver Output Short-Circuit Current OVPIN, V , V = 0V, V = 1.233V l 50 25 10 mA OPTOSC ISNSGND ISNS FB 1698f 2 U W U U WWW