Linear Technology over-voltage protection IC and a neat cutout circuit

Linear Technology just released the LT4356-1 over-voltage protection and inrush limiter IC. This part can take a 4 to 80 volt input and clamp it via a MOPSFET to any level you set with a resistor ratio. This can be very useful for protecting ICs in nasty environments like an automobile. It should be noted that the part is a voltage regulator in a sense—it does not simply shut off the MOSFET when an over-voltage hits, it regulates the output to your set voltage. Of course this implies that the FET will get hot since it is operating as a linear regulator so you have to do the math to insure you have a big enough FET with a biog enough heat sink to keep it form burning up it the over voltage event last a long time. Automotive load dumps—where some yahoo mechanic pulls the battery cable off the battery while the car is running in order to see if the alternator works, will put a hundred volts out for 300 milliseconds, so size your FET accordingly.

When I consulted at HP we were designing automotive diagnostic tools that would help mechanics troubleshoot sick cars. The product had a NiMhd battery and it was a 10.8-volt pack, which might be above or below the car’s battery voltage. I wanted to use an LT1513 SEPIC battery charger but that part was only good for 30 volts absolute maximum and a the Zener TVS clamp I used to protect against load dump clamped at 32 volts worse-case. So crack Linear Tech FAE Jon Dutra (now at National Semiconductor) showed me a great little circuit I could put in that would open a P-channel power FET and disconnect my whole battery charger circuit if the voltage went over 24 volts. That circuit would not continue to power my charger during the over-voltage, but I did not feel that was important. You can set the circuit up to protect against hundreds of volts if you want. The voltage rating of the Q12 FET controls that. The clamp voltage is controlled by zener D4, and remember the tolerance on the zener—a 24 volt zener is pretty much never going to be worse then 10% and even then the 26.4 volts has plenty of margin vis a vis the 30 volt limit of the LT part I was trying to protect. When current flows through Q10s emitter into the zener it will turn on Q10 and crowbar the FET gate voltage to 0.2 volts, the saturation voltage of the Q10. You have to protect the gate of the FET from over-voltage as well, and that is what D6 is doing. The Vgs of the IRFR5305 FET is +/- 20 volts, so a 15-volt zener will make sure you don’t pop the FET gate. If your FET has a lower Vgs max then you have to use a lower zener voltage for D6.

Bear in mind that there is a body diode in the power FET so that current can backflow even when the FET is off. One nice thing about the SEPIC charge is that there is no DC path back to the input and I am pretty sure the LT1513 did not allow any reverse flow as well. The International Rectifier FET is only good for 55 volts but remember I had a TVS clamp that would clamp at 32—I only had to protect the LT part from those 2 little volts above its 30 volt abs max (and yes, they would blow up at 32 volts sometimes—abs max is just that—the absolute maximum you can run the part at even for a nanosecond).