Linear LED driver question

[dareon]

I am working on making a linear constant current LED driver. Attached is a circuit that I have used and works but has some issues(this is a simplified circuit)

The circuit works well and is pretty simple accept. I need to be able to Dim the LEDs by PWMing the input power. When doing this when the square wave goes away. The input voltage drops to low to maintain current regulation so the opamp saturates the FET. When the wave comes back enough voltage stays in the caps to keep the opamp on and the FET staturated. This causes a short(few microsecond) overshoot in the current going through the LEDs before the opamp can correct.

Any ideas on how to fix this. I am open to totally different circuits but I cannot tolerate much dropout voltage. I need to be able to support 1amp or more and burn off atleast a few watts in the FET.

View attachment linear regulation.pdf

Thanks for the time.

 

[keith1200rs]

Have you tried removing C5 and PWM the voltage to the non-inverting input to the opamp rather than the power? You don't say what the opamp is, or the MOSFET.

Keith.

 

[dareon]

Keith,

The opamp opa2171 and the FET is a Fairchild FDD8778

This is for a product and the way the customer is currently doing it to switch the power. I don't believe it is an option to change this.

Thank you for any help

Russell

 

[Ogu Reginald]

Please check whether the components are in goon working condition.