LED MOSFET suggestions? - Electronic Circuits Projects Diagrams Free

rackley · 12th September 2007, 07:08 PM

Hello all,

I'm planning on driving an LCD backlight (array of 6 LEDs internally) with a MOSFET and PWM from a PIC microcontroller, so it can be dimmed for day and night operation. PIC sends PWM to the MOSFET's gate, MOSFET rapidly switches either high side or low side to create a dimmer effect controlled by the PIC.

Given the thousands and thousands of MOSFETs out there, can anyone suggest a part number or family (surface mount only please) that might fit this bill, or at least narrow my search?

Thanks,
Ray

mneary · 13th September 2007, 01:50 AM

First of all, you want a "logic" MOSFET. This means that you can switch it directly from the PIC output (usually with a series resistor).

The simplest circuit will switch the low side of the LEDs and use an N-channel device. This is good, because N-channel is cheaper for a given spec.

For a backlight, I'm guessing that you need 100 mA at 5V. If this is a cost sensitive commercial product, I would start with the Digikey selection guides. Be sure to allow plenty of margin.

For hobby, margin is less important. The 2N7002 is available in a lot of places for 5 cents and up. I have excellent results with DPI4Parts on ebay. You'll pay $5.15 for 100 of them. If your backlight needs more than about 75 mA, use two or in parallel.

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12th September 2007, 07:08 PM	(permalink)
rackley	LED MOSFET suggestions? Hello all, I'm planning on driving an LCD backlight (array of 6 LEDs internally) with a MOSFET and PWM from a PIC microcontroller, so it can be dimmed for day and night operation. PIC sends PWM to the MOSFET's gate, MOSFET rapidly switches either high side or low side to create a dimmer effect controlled by the PIC. Given the thousands and thousands of MOSFETs out there, can anyone suggest a part number or family (surface mount only please) that might fit this bill, or at least narrow my search? Thanks, Ray

13th September 2007, 01:50 AM	(permalink)
mneary	First of all, you want a "logic" MOSFET. This means that you can switch it directly from the PIC output (usually with a series resistor). The simplest circuit will switch the low side of the LEDs and use an N-channel device. This is good, because N-channel is cheaper for a given spec. For a backlight, I'm guessing that you need 100 mA at 5V. If this is a cost sensitive commercial product, I would start with the Digikey selection guides. Be sure to allow plenty of margin. For hobby, margin is less important. The 2N7002 is available in a lot of places for 5 cents and up. I have excellent results with DPI4Parts on ebay. You'll pay $5.15 for 100 of them. If your backlight needs more than about 75 mA, use two or in parallel.