Performance under transient loads
has become the most important
specification of voltage regulators
used to power a wide array of products
whose current demands vary
quickly during operation. Testing
transient performance requires a
load that can be programmed to
change at different rates.
Expensive electronic loads are available,
but I found none that could supply
a load that changes faster than 0.4
A/µs, which is too slow to fully test a
voltage regulator’s transient response.
This idea shows how to build an inexpensive
and simple electronic load that can provide slew rates
exceeding 50 A/µs.
The tester employs an N-FET (Q1) as a switch to connect/
disconnect the load resistance (RLOAD) to the regulator’s
output (Fig. 1). A 50-mΩ sense resistor (R11) in series
with the switch makes it possible for the tester to view the
current waveform.
The frequency of U1, which is a free-running oscillator,
is adjusted by R2. And the duration of U2—a one-shot that
generates the load pulse—is adjusted by R6. Thus, both
the frequency and ON time of the current load pulse can be
adjusted. For the values shown, the frequency can vary from
about 130 Hz to 2 kHz, and the pulse width can vary from
about 20 µs to 400 µs. C6 and R9 reduce the ringing across
Q1 when it switches.
U2’s output drives the gate of Q1, so slowing the rise time
of the gate voltage will slow the rise time of the FET current.
D1, D2, R7, R8, and C5 control the gate-voltage rise and fall
times. D1 and D2 ensure that the rise and fall times can be
adjusted independently. C5’s value depends on the rise and
fall times desired.
The table shows the load-pulse slew rates for the transient
tester for a 0- to 3-A load pulse and different values of
C5. Note that the drive to Q1’s gate is not symmetric. The
output of U2 pulls up to about 11 V, but pulls down only to
ground. Therefore, the FET has much stronger gate drive
turning ON than OFF, which makes the rising edge of the
load pulse faster than the falling edge.
To get equal times for the rising and falling
edges, select C5 based on the desired
slew rate of the falling edge. Adjust R7 until
the falling edge is correct and adjust R8 to
get the rising edge to match the falling edge.
R7 and R8 are multi-turn trimpots,
while R2 and R6 are standard linear
potentiometers. Load resistor R10 must be
non-inductive, preferably carbon film or
metal oxide. Sense resistor R11 also must
be non-inductive. Metal film is recommended.
All capacitors are ceramic, except
for C2, which is a tantalum. Q1 is soldered
to a 2-in.2 copper pad for heatsinking.
In Figure 1, the transient tester is connected
to the output of an LP38501-ADJ,
a 3-A linear regulator with fast transient
response. The regulator was tested with
the output set to 1.8 V, and a 10-µF ceramic
capacitor was used for COUT. The rise and
fall rate of the tester’s current pulse was
adjusted to 0.5 A/µs by using a 0.1-µF
capacitor for C5 and then adjusting R7 and
R8 until the 10% to 90% rise and fall times
of the 3-A pulse were each 6 µs long. R10
was 0.6 Ω so that the load current would be
when Q1 was on.
The
test results showed a small voltage
transient on VOUT when the load
changed (Fig. 2). The peak levels of the
transients are about 40 mV, about 2.2%
output’s nominal value, which is
good performance when using such
small output capacitor
