The circuit’s low-power and low-frequency (approximately 1-kHz) drive contributes little noise to the system, especially if the driving circuit uses a generic Darlington transistor producing relatively slow (3- μs) rise and fall times. In addition, little, if any, voltage spiking is observed.

You can implement the circuit with a protected integrated-circuit transistor, like the LM395, to provide overload protection on the heater drive line. However, the circuit is then limited by the integrated circuit’s voltage and current ratings. Additionally, the LM395’s behavior under some overload conditions may not be entirely satisfactory, especially if the “on” time interval becomes dissipative due to the IC’s internal current limiting.

Adding a small handful of parts, though, creates a betterbehaved pulse-by-pulse currentlimited driver (Fig. 1). The enhanced circuit detects the emitter current from the Darlington (X1) and triggers a composite pnp-npn latch should the current exceed the VBE threshold of Q1. When triggered, the latch diverts the Darlington’s base drive coming through the 2.2k resistor (R1). A small capacitor (C1) stabilizes the trip-point of the latch in the presence of noise.

The circuit permits full output at any duty cycle, as long as the resistive heating element’s resistance exceeds a minimum value. Below that value of load resistance, the latch allows only a minuscule spike of current on each “on” transition (Fig. 2). The circuit is quite sensitive to the critical resistance value and is, of course, selfrecovering as long as the PWM input is present. (If the PWM achieves 100% “on” time, the latch won’t reset until the next pulse occurs.) No significant heatsinking is required.

The circuit can be scaled to different voltage and current levels. However, it may not be appropriate for power levels much higher than 25 W, due to the increasing magnitude of the spike current that appears at short circuit. It’s also not a good choice for driving an incandescent lamp. That is, unless the cold resistance of the lamp is high enough to allow the PWM to unlatch and deliver current.

ADD TWO COMPONENTS TO POWER SUPPLY TO ENSURE PROPER VOLTAGE SEQUENCING
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 along with 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 3.3-V 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. By 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 frequently the output can be pulled lower with the trim pin.

The difference voltage is limited when Q1’s base emitter, D1, D2, and D3 all conduct. Q1 then will turn 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.