I want to drive some LED's using constant current, but also want to control overall LED power output using PWM of the current. As example, a CC supply feeds a string of LED's, the cathode end goes to Drain of nFet, Source of Fet to gnd, gate driven by a PWM signal.

How do CC PSU's react when the the apparent load disappears for a brief period (high %PWM), or if load appears for just a brief period (low %PWM), like if the PWM is 5-10kHz?

An alternative to that is, use PWM signal to change the current setting of the CC PSU, but I was reading about some dwell time for a CC PSU to change quickly, like if you wanted to pulse the LED's for a very small period.

 

Most commercial constant current drivers have a PWM input and they handle it correctly.

A random constant current driver with a long settling time would indeed have a problem. You just have to make sure the settling time is much less than the PWM period.

 

Or, using a PWM circuit, close the loop around the current sense resistor in series with the LEDs. That would work if your desire is to have the benefits of switching power supply but don't mind if the LEDs see DC or nearly DC.


The circuit above is a switch-mode current source. The current is set by the 0.2 ohm resistor and the MC34063's current limit circuit.

 

Ifyou have a clue about the full brightness voltage the string uses, it could work to use a constant VOLTAGE power supply and a low value series resistor No need to drop a lot of voltage across the resistor. Then you can do the PWM and not confuse the power supply.

 

Thanks for the suggestions. I am going route of using opamp with feedback from sense resistor, which drives IGBT. The REF voltage on amp will come from another IC that is a PWN-to-analog item, which I will post in another thread. Thanks.