Hello Guys,
Currently I am working on a low voltage system of 3.3V and I cannot go higher than this because i want to minimise the power consumption. I have to drive a RED led and vary its intensity. I was thinking of using DAC of my MCU to vary voltage on the gate of a mosfet and drive it in the ohmic region to vary the amount of current flowing in the LED. I could not find any mosfet that can run with a Vds of 3.3V. The minimum Vds that I could find is 5V.
Has anyone encountered this situation? If someone has used such a mosfet please share with me.If you guys have any other ideas to tackle this problem then again plz share.
Thanks..
Perhaps you are misinterpreting the mosfet data sheets.
I just put together the simplest circuit on my breadboard, using a red LED as high-side load, a IRF3205 mosfet, a 10k gate pulldown resistor and a 10k gate trimpot to vary Vgs. Using 3.3 volts from a regulated supply, the circuit works fine to control the LED brightness from "off" to relatively full brightness, depending on the setting of the trimpot.
This is a horrible way to dim an LED though. Since the mosfet is working in its linear range it will be dissipating lots of power, and the IRF3205 isn't fully turned "on" at a Vgs of 3.3V (hence I needed no current-limiting resistor for the LED because the mosfet's Rds is limiting the current). Perhaps a different (logic level) mosfet will turn on more fully at Vgs 3.3V and so might drive your high-current LED better.
You would be better off using PWM and a low-pass filter.