DESIGN IDEAS Positive-to-Negative Converter Powers by Mitchell Lee –48V Telecom Circuits If you’re designing a system that interfaces to telecom equipment, chances are you’ll need a –48V supply. The circuit in Figure 1 supplies up to 6W at –48V and scales to more than 12W with higher power components. Based on the Cúk topology, the converter exhibits excellent efficiency over a wide range of loading conditions (see Figure 2). The LT1171’s error amplifier is designed for positive-boost applications, and hence its gain and reference are of the wrong phase and polarity for sensing an inverted output. In this application, the error amplifier is sim- ply bypassed and feedback is applied at the compensation (VC) pin. Zener diode D2 senses the output, pulling down on Q1 and the VC pin, in response to small increases in output voltage. Pulling down on the VC pin reduces peak switch current, and constitutes negative feedback. If the output is a little low, the Zener’s diminished feedback signal is overcome by an internal 200µA current source at the VC pin, thereby increasing peak switch current and restoring the output voltage. The combination of the LT1171 and the VP-2 series VERSA-PAC™ CTX02-13836† VIN 12V 1 3 12 2 10 5 11 4 8 6 coil (CTX02-13836) are suited for 120mA output current as shown. For lighter loads of up to 60mA, use the LT1172 and a VP-1-series equivalent to the coil shown. For up to 15W, use the LT1171 and a VP-5 equivalent. High voltage versions of the LT1170 family (-HV) allow inputs of up to 20V without exceeding the peak switchvoltage rating. This converter starts working at 2.7V and will regulate –48V at reduced power. You can add undervoltage lockout by inserting a Zener diode (VZ = VLOCKOUT – 2.7V) between the input supply and the LT1172’s VIN pin. 100 EFFICIENCY (%) 90 80 70 7 9 + + 60 220µF 16V 50 150µF 63V VSW VIN 0 D3** LT1172HV VC 1k 100nF GND 30 90 60 LOAD CURRENT (mA) 120 DI_1171_02. EPS Figure 2. Converter efficiency rises to 80% at only 20mA load. FB VERSA-PAC is a trademark of Coiltronics, Inc. D1* 10nF 1µF GND Q1 2N3904 1k + *D1 = 1N4148 **D3 = MUR120 †COILTRONICS (561) 241-7876 1k D2 1N5261B 47V 220µF 63V VOUT –48V/120mA Figure 1. 12V to –48V features good efficiency over a wide range of loads. 36 For further information on any of the devices mentioned in this issue of Linear Technology, use the reader service card or call the LTC literature service number: 1-800-4-LINEAR Ask for the pertinent data sheets and Application Notes. Linear Technology Magazine • May 1998