Chip Set Offers Low Cost Alternative to 48V Telecom Modules Design Note 261 Kurk Mathews The demand for high performance 48V input telecom modules has never been higher. The latest generation modules offer high current, high efficiency alternatives to traditional low voltage potted supplies. Although these drop-in solutions are attractive, their higher cost can quickly add up as product volumes increase. Replacing a module with an on-board solution based on the new LT®1681/LTC ®1698 chip set significantly reduces system cost and takes full advantage of board area, minimizing the need to derate output power at elevated ambient temperatures. Isolated 48V to 3.3V Supply The schematic in Figure 3 presents a complete 36VIN to 72VIN to 3.3VOUT/20A power supply. Total component cost is typically below $30 (50k pcs). The 2-transistor forward converter utilizes low voltage primary switches and provides for recovery of the transformer’s mutual and leakage energy. The LTC1698 secondary controller synchronizes with the LT1681 via a small pulse transformer and drives secondary synchronous MOSFETs. Efficiency is shown in Figure 2. The LTC1698 includes an error amplifier and optocoupler drive buffer, eliminating the output feed-forward path and loop compensation issues associated with ‘431 type references. A margin pin allows the output voltage to be adjusted ±5%. Other output voltages can be realized by substituting components into the same basic circuit. Contact the 07/01/261_conv Figure 1. 3.3V/20A Supply in Half-Brick Footprint 95 36VIN 90 EFFICIENCY (%) The new LT1681 controller contains all the necessary functions for a synchronous forward converter and works together with the LTC1698 secondary controller for isolated applications. The LT1681 features include current-mode operation up to 250kHz, 75V high side driver, leading edge blanking, input under/over voltage protection, synchronous gate output and a thermal shutdown pin. The LTC1698 interfaces directly with the LT1681 providing an error amplifier, current limit and output over voltage protection in addition to synchronous gate drivers. 72VIN 85 48VIN 80 75 70 2 4 6 8 10 12 14 CURRENT (A) 16 18 20 dn261f02 Figure 2. 3.3V Efficiency (See Figure 3) factory for more information including half-brick layout information (see Figure 1). Conclusion The LT1681/LTC1698 combine to provide a low cost, discrete alternative to telecom modules. The LT1681/ LTC1698 features reduce circuit complexity. High efficiency synchronous operation combined with the thermal advantages of an on board supply make the LT1681/LTC1698 the ideal choice for high current, low cost isolated converters. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Data Sheet Download www.linear.com Linear Technology Corporation ● C2 1.5μF 100V C3 1.5μF 100V FAX: (408) 434-0507 ● www.linear.com 0.1μF C26 68μF 20V + MMBZ5248LT1 18V 10k MMBZ5245LT1 15V 267k 0.25W 1nF 24k 1.24k 1% 56k 20k 73.2k 1% BAS21 20 17 0.1μF 100V 19 18 16 11 12 15 BAS21 BAS21 BAT54 330pF 10k BAT54 MMBT3906LT1 Q12 ZVN3310F 10Ω 1μF MMBT3906LT1 5VREF 6 82pF 3 4 4700pF 7 8 3k 3300pF 10Ω 10 3 1t 4 t6 1k 5 8 S 2 1 T1 4 2 14 5 10k 15 3 4700pF 1k 220pF S Q1 12 11 16 2 100Ω 2k 0.25W 6 1k 1000pF S 0.1μF VAUX OPTODRV MARGIN OVPIN 3 4 10 PGND GND PWRGD ICOMP LTC1698 1.24k 1% 13 7 9 L2 2.35μH 2.43k 1% dn261f03 1k 0.33μF 1.78k 1% 3.01k Q6, Q15, Q17 FDS6680A w3 MMBZ5240BLT1 10V 0.22μF 50V MBR0540 4.7Ω 1000pF 100V 0.022μF 1000pF 100V 10Ω 0.25W 10Ω 0.25W VDD ISNS ISNSGND FG CG VCOMP 8 SYNC VFB 1 4.7μF 16V Q13 FZT690 Q5, Q14 FDS6680A w2 C1 2200pF 250V 3.3Ω 5 7 VSEC Figrue 3. 36V-72V DC to 3.3V/20A Isolated Synchronous Forward Converter 52.3k 1% 5 0.1μF T2 ISO1 5VREF MOC207 4 3 1 7 6 3.3Ω BAT54 0.025Ω 1/2W Q3 MURS120T3 MMBT3906LT1 MURS120T3 10Ω VCC VBST BLKSENS TG BSTREF BG SENSE IMAX PGND 13 SG 2 LT1681 OVLO 9 1 V SS V F THERM SYNC SGND 5V REF SET C FB SHDN 14 C4 1.5μF 100V L3 1mH C1: MURATA ERIE GHM3045X7R222K-GC C2, C3, C4: VITRAMON VJ1825Y155MXB C5 TO C8: 330μF 10V KEMET T510X337K010AS C26: AVX TPSE686M020R0150 ISO1: QT OPTOELECTRONICS MOC207 L1: COILCRAFT DO1608C-332 L2: PULSE P1977 PLANAR INDUCTOR L3: COILCRAFT DO1608C-105 Q1, Q3: SILICONIX Si4486EY Q5, Q6, Q14,Q15, Q17: FAIRCHILD FDS6680A Q12: ZETEX ZVN3310F Q13: ZETEX FZT690 T1: PULSE P1976 PLANAR TRANSFORMER T2: MIDCOM 31264R VIN – VIN+ t L1 3.3μH t t (408) 432-1900 For applications help, call (408) 432-1900 dn261f_conv LT/TP 0701 305K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 © LINEAR TECHNOLOGY CORPORATION 2001 + TRIM S VOUT– C5 TO C8 330μF 10V w4 VOUT+