# Voltage drop over MOSFET

I have a question about high-side switching a load using an p-channel MOSFET.

I'm using the following configuration to control the backlight brightness of a LCD panel: Q1 = N-channel logic level MOSFET so it can control the p-channel MOSFET using a microcontroller.

Q2 = SI2319DS p-channel MOSFET

Now because the backlight has a very specific voltage range (between 29,5V to 30,2V) I want to calculate the voltage drop over the source-drain terminals of Q2.

The datasheet of the SI2319DS states that for a Vgs of -10V Rds(on) is about 65 mΩ. This would mean that my voltage drop would be: Vds(drop) = 65 mΩ * 120 mA = 8 mV.

Is this really the case? it seems so low.

• Based on the On-Resistance vs. Drain Current graph on page 3 your calculation is correct. – Transistor Oct 7 '17 at 10:42
• yes, it's amazing – Makoto Oct 7 '17 at 10:56
• MOSFETs usually have fairly low Rds(on) values. Anywhere between a 1mΩ and 1Ω is typical. For high-current applications they can go a fair bit below 1mΩ, e.g. the IRL7472L1 which has an Rds(on) of 340µΩ - critical to the fact that it can handle 375A through the drain. – Polynomial Oct 7 '17 at 14:03
• Your calculations are correct if the current draw really is 120mA. However depending on the load, applying PWM may cause the peak current to be much higher. What type of backlight is it and why does it need exactly 30V? – Bruce Abbott Oct 7 '17 at 17:35