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.
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