# LT4380-2 design for 12V/250V Floating Surge Stopper

There is a 48V Floating Surge Stopper design in data sheet(Fig.11). It can not be powered up in 12V power. If I need to use it in 12V or lower voltage application. What's parts need to be changed ?

• In my Circuit, I changed the Zener Diode between VSS to GND from 43Vz to 27Vz. But the clamp volt is about 29V only. Not 55V I wanted. Anyone know how to setup the clamp voltage in the LT4380-2 floating stopper circuit. The following is my circuit :

• The Circuit clamp voltage is about 32V only(28V + VSS). I think putting the LT4380-2 on the circuit is wrong ! If the Stopper IC change to LT4380-4 , will it working ?

• Yes, but the clamp voltage will still not be correct.

As of now, circuit will clamp at 31.5Vgate_clamp - Vfet_threshold = 28.5V (with 3V typical FET threshold), dictated by SEL = 0V on LTC4380-2. If SEL is tied high (to VCC or OUT), circuit will clamp at Vout_clamp = 27Vcc_clamp + 13.5V – Vfet_threshold = 37.5V (with 3V typical FET Vth). Then if a much higher VCC clamp is used (e.g., 50Vz) and SEL is high, circuit will clamp at 50Vgate_clamp – Vfet_threshold = 47V, dictated by SEL = high, which is still less than the desired 55V. LTC4380-1/-2 cannot have a clamp voltage higher than this value. As you mentioned, change to LTC4380-4 which also has auto-retry like LTC4380-2 but no internal GATE-to-GND clamp. To get 55V clamp at output, the Vcc Zener = 55V + Vfet_threshold – 13.5V = 44.5Vz, which is closer to the initially selected 43Vz.

In summary, output clamp voltage will be the lower of that dictated by SEL pin (only for LTC4380-1/-2) or that dictated by Vcc_clamp + 13.5V - Vfet_threshold (on any LTC4380-n).