To supply small amounts of 5-V power in voice- and data-communications hardware,
it may be more convenient to derive the voltage from -48 V (if it is available)
than to add a separate supply powered by batteries or the ac line. To accomplish
that, though, the dc-dc converters required for such supplies would involve
expensive modules or discrete components or transformers.
This circuit (see the figure) offers
a better solution to the problem. For most applications that don't require isolation,
this circuit generates 5 V at 150 mA without a transformer. The negative input
voltage can vary from -35 to -75 V.
The switching regulator (IC1) operates in a classic "boost"
configuration, powered by a Zener-regulated 6.2 V (relative to -48 V). Because
its ground terminal connects to -48 V, the regulator behaves as if it was converting
48 V to 53 V. A feedback signal is level-shifted down from 5 V to the IC's feedback
input by the transistor current source Q2. Transistor Q3,
included only to compensate for the temperature variation of VBE
in Q2, can be omitted if approximate -2 mV/°C temperature
drifts are acceptable.
Conversion efficiency ranges from 70 to 76%, depending on the input voltage
and the load. Highest efficiency occurs at full load, when Zener current is
the lowest percentage of the total supply current. The switching FET (Q1)
need not have low on-resistance, because the circuit's supply current is only
about 20 mA at full load and the 1-Ω sense resistor (R2) limits
peak currents to about 200 mA.
The device selected for Q1 (a Siliconix 2N7004 in a 4-pin DIP)
exhibits a typical RDS(on) of 1 Ω when operating with the 6-V
gate drive provided by the circuit. A SOT-89 surface-mounted FET, such as the
IRFS1ZO developed by International Rectifier, also performs well. Rated at 2.4
Ω, the IRFS1ZO reduces efficiency by no more than one or two percentage
points. All components in the circuit are available in surface-mount versions.
