Electronic Design

Fast Load Transient Tester Circuit Features Adjustable Slew Rate

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

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