how to switch constant current source?

[kyungmin1125]

Hi all,

I have a constant current out of a switchmode circuit (https://www.onsemi.com/pub_link/Collateral/LM2575-D.PDF). I am getting a 1A DC out of this. The output voltage is variable, depending on the input level.

My goal is to switch this current on and off with a pulse input. I built a switch using 2 NMOS and drove them with 5V pulses that are out of phase. I didn't use an NMOS-PMOS pair because it may not be as symmetrical.



So I have something like the circuit above. You can ignore the IC chip, it just represents a constant current supply out of the node "LEDA". The problem is when I increase the switching frequency from 1kHZ to 5kHz or higher, the AC current through the LEDs die. Why should frequency affect the amount of current through the LEDs? The voltage at the LEDA pin stays constant at different frequencies though, which is why I am confused why frequency matters.

Any help will be appreciated. Thank you!

 

[crutschow]

Why do you want to switch at such a high frequency?

 

[kyungmin1125]

Im using it for visible light communications, so i am trying to maximize the switching frequency.

Never mind that the voltage at the LEDA pin is constant at all frequencies-- I think it changes although i will have to doublecheck

 

[mtwieg]

Without seeing a more detailed circuit it's hard to tell. How are you measuring the AC current in the LEDs and the regulator output?

 

[kyungmin1125]

I went back to the circuit and checked that the voltage at the LEDA pin is constant, and it sources a DC 1A regardless of the input level. This is without the switch.

When I add the switch and increase the frequency to 10 kHz or so, the current from the LEDA pin drops, which I am measuring with an ammeter, and the current through the LEDs are mostly DC currents where they should be AC since I'm switching the current between the two.

I am doublechecking the validity of the ammeter measurements by looking at voltage across the photodiode, which should be a pulse when the LEDs are switching properly. The photodiode has a DC current through it at higher frequencies.

 

[crutschow]

How are you generating the 5V pulses that drive the MOSFETs?

 

[kyungmin1125]

I'm using an arduino as a signal generator. I tried a regular function generator, but it doesn't fix the problem.

 

[crutschow]

Perhaps the switching pulses are affecting the LM2575 control circuit. Try adding a small output capacitor to ground (say 0.1µF) and see if that makes any difference.