The accompanying
circuit maintains
power to the load regardless
of momentary shorts
or opens in the supply
voltage. The low-current
overvoltage-protection IC
(MAX6495) also protects
the load against transient
voltages up to 72 V.
The circuit operates
from a nominal 13-V input
voltage. During momentary
power interruptions, the
large capacitance at the input
to the dc-dc converter
(C1) provides ride-through
capability by supplying the
converter for periods up
to 5 msec or so. During a
momentary short circuit of
source voltage, the circuit
again shields the converter
output from interruption by
preventing discharge of C1
via the shorted supply.
When the 13-V input
drops because of a short
to ground, there must be
a way to prevent storage
capacitor C1 from back discharging
through the short.
Transistors Q1 and Q2 do
this: The short on Q1’s gate
turns it on, connecting the ~13 V on
C1 to the gate of Q2, which turns Q2
on. Q2 shorts to ground the internal
charge pump at the Gate pin, which
drives the pass transistors Q3 and
Q4 to cutoff by quickly discharging
their gate capacitance. With Q3 and Q4 turned off, C1 cannot discharge
through the short, and the output
voltage rides through the disturbance
unaffected as illustrated in
the scope display of the input and
output signals.
The total gate charge for transistors
Q3 and Q4 should be low,
to enable fast turn-on and turn-off
times, and VDS(max) should be high
enough for the highest voltage transient
expected. RDS(on) for Q3 and
Q4 should be low to minimize voltage
drop and power dissipation.
The value for C1 depends on the
load power, the maximum tolerable
voltage droop, and the expected duration
for loss of input voltage (ridethrough
time):
Energy stored in the capacitors
is
E = CV2
i.e.,
E = PΔT = C(ΔV )2
Solving for C,
C = (2PΔT )/ΔV2,
where E = stored energy, C = capacitance,
ΔV = maximum tolerable
droop, P = power used by
load, and ΔT = expected duration
of input-voltage loss, sec.
—
Ronald Moradkhan,
MaximIntegratedProductsInc.,
Sunnyvale,
Calif.