Power Supplies for Subscriber Line Interface Circuits Design Note 130 Eddie Beville As the demand for world wide networking grows, so will the need for advanced data transmission products. In particular, ISDN services have become popular because of the recent development of the Internet. ISDN provides higher speed data transmission than standard modems used in PCs. Also, ISDN supports the standard telephone interface (voice and fax), which includes the Subscriber Line Interface Circuit. A Subscriber Line Interface Circuit requires a negative power supply for the interface and the ringer voltages. The power supplies described herein are designed for these applications. Specifically, these designs address the AMD79R79 SLIC device with on-chip ringing. CIRCUIT DESCRIPTIONS LT®1171 Supplies –23.8V at 50mA and –71.5V at 60mA Figure 1 shows a current mode flyback power supply using the LT1171CQ device. This current mode device has a wide input voltage range of 3V to 60V, current limit protection and an on-chip 65V, 0.30Ω bipolar D3 T1 MURS120 33μH 9V TO 18V 7 VIN C1 330μF 35V + VR1 LT1171 VSW VC GND FB 1 3 + :1 5 D1 P6KE-43A D5 1N4687 OR 4 MMSZ4V3T1 1 D2 1N5817 1: 2 D4 MURS120 6 –23.8V 50mA 5 D6 MBRS1100T3 + :1 2 4 R1 1k C2 0.33μF 4 3 R2 71.5k 1% R4 1k R3 1.24k 1% 6 C6 0.01μF 2 3 V+ U1 LT1006 V– 4 Figure 1 06/96/130_conv 7 + C5 0.1μF – PRIMARY INDUCTANCE: 33μH PRIMARY PEAK CURRENT: 2A PRIMARY DCR: 0.1Ω SECONDARY DCR: 0.5Ω C4 68μF 63V –71.5V 60mA :1 9V TO 18V PE-68488 PULSE ENGINEERING C3 120μF 35V switch. The input voltage range for the circuit is 9V to 18V. This circuit is intended for small wall adapters that power ISDN boxes. The output voltages are –23.8V at 50mA and –71.5V at 60mA. The circuit shown in Figure 1 uses the LT1171 in standard flyback topology. The transformer’s turns ratio is 1:1:1:1, where 23.8V appears across each secondary winding and the primary during the switch off time. The remaining secondary windings are stacked in series to develop –47V. The –47V section is then stacked onto the –23.8V section to get –71.5V. This technique provides very good cross regulation, lowers the voltage rating required on the output capacitors and lowers the RMS currents, allowing the use of cheaper output capacitors. Either the –23.8V output or the –71.5V output can be at full load without effecting the other corresponding output. The circuit’s step response is very good; no significant overshoot occurs after either output is shorted and released. Also, the transformer windings are all quadrafilar to lower the leakage inductance and cost. LT1269 Supplies –23.5V at 60mA and –71.5V at 120mA from 5V Input Figure 2 shows a current mode flyback power supply using the LT1269CQ device. This current mode device has a wide input voltage range, current limit protection and an onboard 60V, 0.20Ω bipolar switch. The input voltage range for the circuit is 5V to 18V. This design provides a wider input voltage range and greater output power than that of Figure 1. The output voltages are –23.5V at 60mA and –71.5V at 120mA (8.6W). This circuit is designed to power two SLIC devices. The circuit operation is identical to Figure 1, except for a larger switching regulator device (VR1) and a different transformer (T1). These changes allow for 5V operation and higher output power. This circuit is designed for full load on the –71V or –23.5V output. This accommodates the ringing on two SLICs or off hook on two DN130 F01 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. Figure 1 Bill of Materials REFERENCE DESIGNATOR QUANITITY PART NUMBER C1* 1 ECA-1VFQ331 C2, C5 1 0805 C3 1 UPL1V121MPH C4 1 UPL1J680MPH C6 2 0805 D1 1 P6KE-43A (MOT), TGL41-43A (GI) D2 1 1N5817 D3, D4 2 MURS120 D5 1 1N4687, MMSZ4V3T1 D6 1 MBRS1100T3 R1, R4 2 0805 R2 1 0805 R3 1 0805 T1* 1 PE-68488 U1 1 LT1006S8 VR1* 1 LT1171CQ *Changes and Additions for Figure 2’s Circuit C1 1 205A100M D5, D7 2 1N4001 R5, R6 2 T1 VR1 1 1 HMOO-96553 LT1269CQ DISCRIPTION Capacitor, 330μF, 35V HFQ Capacitor, 0.33μF Ceramic Capacitor, 120μF, 35V Plastic Capacitor, 68μF, 63V Plastic Capacitor, 0.01μF Diode, 0.5W Zener Diode, 1A Schottky Diode, Ultrafast Diode, Zener Diode Resistor, 1k, 5% SMT Resistor, 71.5k, 1% SMT Resistor, 1.24k, 1% SMT Transformer IC IC T1 MURS120 33μH 2 + C1 100μF 20V D1 P6KE-43A VIN D5 D7 1N4001 1N4001 VR1 LT1269 VSW VC GND FB 1 3 10 D2 1N5817 1: 1 + 9 D4 MURS120 3 D6 MBRS1100T3 –23.5V 60mA + :0.85 2 C3 120μF 35V :1.25 C4 –71.5V 68μF 120mA 63V 8 R1 1k C2 0.33μF 4 R2 71.5k 1% :0.85 7 5 R5 50k 1/4W :0.85 HM00-96553 BI TECHNOLOGIES (714) 447-2656 6 5V TO 18V R4 1k R3 1.24k 1% 7 6 C6 0.01μF 2 + C5 0.1μF – PRIMARY INDUCTANCE: 33μH TO 35μH PRIMARY PEAK CURRENT: 3.5A PRIMARY DCR: 0.065Ω SECONDARY WINDINGS ≈30AWG WIRE. TURN RATIOS ARE SO NOTED ON THE TRANSFORMER 3 V+ U1 LT1006 V– 4 R6 50k 1/4W DN130 F02 Figure 2 Data Sheet Download www.linear.com Linear Technology Corporation TELEPHONE Nichicon Nichicon (708) 843-7500 Motorola or Equiv Motorola or Equiv Motorola or Equiv Motorola or Equiv Motorola or Equiv Pulse Eng LTC LTC Capacitor, 100μF, 20V OS-CON Sanyo Diode Resistor, 50k 0.25W SMT or Through Hole Transformer BI Technology IC LTC SLICs. R5 and R6 are preload resistors for maintaining an accurate –23.5V output at full load with the –71V output at minimum load. D3 5V TO 18V VENDOR Panasonic (408) 432-1900 (408) 432-1900 (619) 661-6835 (714) 447-2656 (408) 432-1900 LAYOUT AND THERMAL CONSIDERATIONS Printed circuit board layout is an important consideration in the design of switching regulator circuits. A good ground plane is required for all ground connections. The path from the input capacitor to the primary winding of the transformer is a high current path, and requires a short, wide copper trace (0.080" to 0.1"). The VSW pin connection also needs a short, wide copper trace. R1 and C2 need to be placed close to VR1. The secondary windings can be connected to their associated components with 0.025" to 0.030" traces. The feedback circuitry needs to be placed close to the FB pin of VR1. Place C5 close to U1 to decouple the op amp power supply. The LT1171CQ and LT1269CQ are surface mount devices that require about a 1" copper pad for heat sink mounting. Heat sinking is most critical for the LT1269CQ because of its high output power. Also, vias from the copper pad to the internal ground layers are highly recommended. BILL OF MATERIALS A bill of materials has been provided with each schematic. For applications help, call (408) 432-1900 dn130f_conv LT/GP 0696 185K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1996