SG3525A

All three are voltage-mode PWM controllers, but the SG3525A is the most capable. The SG3524 has two outputs that each switch at the full oscillator frequency with a fixed dead time and no soft-start pin, so it suits single-ended or forward converters. The TL494 runs from 7 V to 40 V, has two 200 mA outputs with selectable push-pull or single-ended operation and an internal dead-time control, but its 5 V reference is only ±5% and its error amplifiers need more care. The SG3525A adds a ±1% 5.1 V reference, programmable dead time, soft-start, cycle-by-cycle shutdown, undervoltage lockout, oscillator sync and 400 mA peak outputs — the standard choice for bridge and push-pull supplies.

Frequency comes from RT (pin 6) to ground and CT (pin 5) to ground using the datasheet's design equations and curves: a 3.6 kΩ RT with a 0.01 µF CT gives about 40 kHz, and 2.0 kΩ with 1 nF reaches the 400 kHz maximum. Dead time is set by a resistor between CT and Discharge (pin 7); the discharge current mirror and the internal 0.5 µs typical clock width define the floor, and increasing the resistor extends dead time to protect against cross-conduction. Remember that in a push-pull or bridge converter each output runs at half the oscillator frequency, so set the oscillator to twice your intended transformer frequency.

The 5.1 V reference is rated for up to 50 mA and has a short-circuit current of 80 mA to 100 mA, so it is robust, but its load regulation is 20 mV typical and 50 mV maximum from 0 mA to 20 mA — if your divider and error-amplifier network draw only microamps, the reference voltage can sit slightly high and shift your feedback threshold. A 1 kΩ to 2 kΩ preload from VREF to ground keeps the reference inside its specified load range and improves long-term stability. Similarly, the totem-pole outputs need a gate resistor and, for large MOSFET banks, an external driver rather than direct connection.

For small converters, yes: each output sources or sinks 400 mA peak with 100 ns rise and 50 ns fall into 1 nF, which suits a pair of moderate-Qg MOSFETs at up to about 100 kHz. Beyond that, or with parallel FETs, add a gate driver such as the TC4420 (6 A), UCC27211 or IR2110 for high-side drive in a half-bridge. Use a separate VC collector supply if you need a gate-drive voltage different from VCC, and keep the 0.1 µF decoupling capacitors on both VCC and VC right at the pins — the output edges will ring badly otherwise.

Soft-start is an external capacitor on pin 8 charged by a 25 µA to 80 µA internal current source, so the ramp time is set by capacitor value: 5 µF gives roughly 100 ms to 200 ms. The shutdown pin (pin 10) is dual-function: a pulse of a few hundred nanoseconds triggers cycle-by-cycle shutdown through the PWM latch with a 0.2 µs typical delay, while a sustained low also discharges the soft-start capacitor toward 0.4 V to 0.6 V so the converter ramps again on release. An MCU open-collector output can drive it directly, which gives firmware-controlled power sequencing and a hardware fault input without extra components.

The onsemi SG3525A is specified over 0 °C to +70 °C ambient, with electrical characteristics guaranteed across that range; the SG2525A is -25 °C to +85 °C and the SG1525A is -55 °C to +125 °C. Inside a power supply the local ambient can easily exceed 70 °C, so for industrial, outdoor or automotive-adjacent products specify SG2525A or the ST/TI industrial variants rather than assuming the commercial grade derates gracefully. Junction maximum is 150 °C in all cases.

Test the reference and the oscillator. A genuine part holds 5.00 V to 5.20 V at 25 °C under a 20 mA load with under 50 mV of load regulation, and the oscillator frequency matches the RT/CT curves within ±6%; remarked or lower-grade die typically sag several hundred millivolts on the reference or drift badly with supply voltage. Physically check for laser-etched onsemi or ST marking with a consistent font and traceable date code, matte leads and clean moulding. We test samples from every rail or reel, including reference load regulation and oscillator accuracy, and supply the results with a certificate of conformance.

MOQ is one rail (25 pcs) of SG3525AN in PDIP-16 for prototyping, 47 pcs per rail for SO-16L, or 1,000 pcs on tape-and-reel (SG3525ADWR2) for volume. Shenzhen stock ships in one to two business days; authorised onsemi channel replenishment is eight to fourteen weeks, and ST SG2525A/SG3525A equivalents are available on similar lead times.