# Pulling a pin to 12 V using 3.3V

I'm building a boost converter and want to implement a shutdown feature. The particular chip I'm using, LM3478, has a shutdown feature which is activated by a high level on the frequency adjust pin. In this case, I'm assuming a high level would be 12V since the chip is powered by 12V. The control signal I will be using will be from an MCU which has an output level of 3.3V.

I cannot simply use a P-channel MOSFET since 3.3V at the gate is not enough to turn the MOSFET off if the source voltage is 12V. I could use a PNP transistor since it is current controlled but that would require the control signal to be open-collector (tri-stated to turn off, low to pull up).

I'm thinking I could maybe combine a PNP transistor and a small MOSFET to do this. The PNP emitter is connected to 12V and the collector is connected to the pin on the chip. The MOSFET drain is connected to the transistor base through a resistor and the MOSFET source is connected to ground. I would then control the whole thing by a signal on the MOSFET gate.

Is there a better way to do this?

• Are you in frequency adjust mode? – markrages Sep 14 '12 at 16:42

EDIT: Here's a pic of what I'm talking about. Push-pull totem output is internal to the microprocessor. You should reduce $R_{FA}$ by the output resistance of the totem pole, which is in the vicinity of 25 ohms. It's in the micro's datasheet.