The main source of power in a telecommunications system is -48 V. This source’s negative polarity and its large magnitude with respect to ground pose a challenge when designers want to use low-power ICs in the telecom system’s application circuits. Fortunately, the emergence of high-voltage ICs—with operating voltages of 75 V and higher—has enabled the use of simple biasing techniques in designing circuits for -48-V systems.
The technique described here provides a dimming control for an LED. The circuit uses a 65-V hysteretically controlled LED driver (MAX16822A) with its ground pin connected to -48 V and its power input connected to the system ground (Fig. 1). For proper dimming, therefore, the circuit’s logic-level control signal (at Control) must be level-shifted down to -48 V and applied to the DIM input. The high-voltage pnp transistor (CMPT5551) (80 V/500 mA) enables a simple solution to that problem.
The transistor circuit is made to operate at approximate unity gain by setting R1 equal to R2. With the control input at 0 V, the transistor is biased at cut-off, pulling the collector (connected to DIM) down to -48 V. With the control input at 5 V, the collector pulls up to -43 V. Thus, the ground-referenced 0- to 5-V dimming signal at the control input is replicated at DIM, but referenced to the -48-V rail. Figure 2 shows the circuit’s average LED current versus pulse-width modulation (PWM) duty cycle.