Today's digital ICs typically demand
complex voltage sequencing, a task that
usually requires dedicated ICs or microprocessors. But what if your requirements are more modest? The sequencing scheme presented here requires only
a single optocoupler and a resistor.
Assume that the dc-dc converter's
On/Off pin is pulled low to turn on and
floats to remain off (see the figure).
Also, in this example, the 5 V needs to
turn on before the 3.3 V. The 5-V converter's On/Off pin is tied low. The 3.3V converter's On/Off pin remains floating when U2 is off.
When power is applied, the 5-V converter turns on. As the 5 V rises, the optocoupler eventually will be driven on, turning
on the 3.3-V converter. (For a non-isolated
application, the optocoupler could be
replaced with a transistor.)
Many logic ICs that use multiple voltages limit the allowable difference in
voltage between the 5 V and 3.3 V to
around 2.5 V. Using this scheme, that
limit could be exceeded if U1 reaches 5
V before the 3.3 V starts up, or if the 3.3
V fails. Let's assume the outputs of the
dc-dc converters change in the direction that the converter's trim pin is pulled.
Many converters spec the trim to be
±5%, but often the output can be pulled
lower by the trim pin.
The difference voltage is limited when
Q1's base emitter, D1, D2, and D3 conduct. Q1 then turns on, turning on Q2,
which then pulls the trim pin down. Therefore, the 5-V output will regulate from 2 V
to 2.5 V above the 3.3-V output. The actual regulation voltage will depend on where
the junctions conduct. R2 is determined
based on the chosen converter.
The final requirement is preventing the
5-V output from going more than 0.7 V
below the 3.3-V output, which could happen at turn-off. Avoid this problem by
using a Schottky diode for D4. The use of
D4 isn't new, but is mentioned just for
completeness.
If there's no voltage difference requirement but the 3.3 V should not turn on
until the 5-V output reaches within 10%
to 15% of its output, adding diodes or a
Zener in series with R3 will work.
