# Square Wave Voltage Level Shifting

I have a square wave signal of 300 KHz which is 0.2 V to 1 V. It is the output of FP5138 Non-Sync PWM Controller IC. I want to drive a PMOS with this square wave but with the levels shifted to (Vin-12)V ... (Vin-5)V to Vin, where Vin is 20 to 32 V.

To explain it better, let me try this way (the Vin above and below are different):

      0    1
Vin : A -> B
Vout: C -> D

A: 0.1 V typ, 0.2 V max
B: 1.0 V typ, 1.2 V max
C: D-12 min, D-5 max
D: 20 V min, 32 V max


I have 12V, 1A source (78L12) and Vin source of the converter which has boundaries of D above.

It is going to be a P-MOSFET driver and I need up to 0.3 or maybe 0.4 A spikes for Cgs. MOSFET is not yet chosen.

I have tried to build this circuit but no hope.

I hope I could explain myself.

You do not need or want the intermediate voltage represented by V3.Trying to create it as an actual volateg will cause you effort which is not required.

Allow the low side driver to drive a high side driver connected to tje V+ rail.

THEN limit the swing of the high side driver OR the swing that the FET gate sees. Zener diodes are your friends in such cases.

Note that you should zener clamp the INPUT of a driver stage, not the output - so that the driver is not always "fighting" the zener. .

• can you give me a schematic or something like, I couldn't understand what you explained. Nov 17 '11 at 9:48

Have you tried completely reading the datasheet and looking at the application notes towards the end? Figure 16 shows a buck regulator -- yours is 20-32V whereas fig. 16 is 3.3V, but the basics are the same. Use an NPN transistor to turn the PFET on.

At >12V, you won't need to worry about the PFET being fully enhanced, but you will need to protect the gate with a zener between gate and source so that it only sees 12V or so, and not the full 20-32V (which can damage a MOSFET).

You will also need to worry about high-speed effects -- 300kHz PWM is fairly fast, and NPN transistors in this sort of topology (emitter grounded, base as input, collector as output) can have serious slowdowns if not driven properly.

• Yeah, I saw that and I thought it would damage the MOSFET with 20-32V Vin. But I didn't know that you can simply place a zener there, I will try that. I am trying to understand Russell's answer now. Nov 16 '11 at 14:01
• Put a zener across the MOSFET gate, but put a series resistor between the gate and the collector, so that it limits current to the gate. There are other circuits you can use, it's really hard to describe them in words and I don't have a good drawing tools I can sketch quickly. In general PFETs are difficult to drive if the voltages make direct drive of the gate problematic; you'd almost be better off with an N-FET and a gate driver chip that handles the bootstrapping. Nov 16 '11 at 19:00