Current Limiting Indication

[MrHeckles]

Hi guys. I have made this circuit, and it works well. You can vary the duty cycle between 0 and 90% and the current limiting kicks in as the voltage on the C.L (+) sense (Pin 4) starts going above 0.2v.



However I would like some sort of indication as to when the circuit starts current limiting, such as an LED coming on.

I tried connecting the base of a transistor to pin 4, the collector to pin 16 with a resistor, and the emitter to ground. However that triggered the LED as soon as any voltage was generated on pin 4.

So, any ideas?

[Guest Alun]

I don't know, have you checked the datasheet for the SG3524? One idea could be to connect a comarator to the sense resistor to measure the cirrent through the load.

[MrHeckles]

The intention is for the comparator to go high when the + input (same as PIN 4 on the SG3524) goes above about 0.2v. Ignore the value of the resistor infront of the LED.

Would it work?

[Guest Alun]

That should work, but there's no guarantee it'll turn on at 0.2V due do compnant tollerances, for example if the current limits the voltage on the sense resistor to 0.19V and you've got the comparator set for 0.21V it'll never turn on. I think you should look at the tollerance of the current limiting voltage and set the turn on voltage on the comparator to lower than the lower limit on the tollerance, I hope this makes sense.

[audioguru]

Since an ordinary transistor turns on with only "0.2V" across the current-sense resistor, then the voltage must be full of spikes. Threfore the voltage must be filtered (causing delay) to be sensed by a comparator.
The IC already has a comparator with inputs at pins 4 and 5 , a filter for them and its output at pin 9. Pin 9 goes up to at least 2V during current-limiting (but its pull-up current is unknown) so a darlington transistor with a high-value resistor in series with its input can be triggered by pin 9's voltage. ;D

[MrHeckles]

The output at pin 9 is also the output from the error amplifier. It goes above 2v in normal operation when set for a high duty cycle.

If I used pin 9 and a darlington transistor, wouldnt it light up the LED whenever a high duty cycle was requested even in normal operation?(pin 9 will control the duty cycle between 0 and 90% with a voltage range of between about 1v and 3.5v)

This may be a stupid question, but how would I set up a filter for the LED comparator?

And a point to note is that if max duty cycle is requested (via the error amp) and it is current limiting fully, then the voltage on pin 4 will go up to around 0.5v. Though the current limiting does start around 0.2v on pin 4.

[Shahriar]

Dear MrHeckles

One way to do this is to creat an average  DC Voltage on the Vi+ of the Op-Amp. Beacuse there are lots of unwanted Spikes there. These method is used by New SMPS Controllers and Is called Average Current Control Topology.
Anyway, without this, it is possible, but you should add a Low pass filter to The Vi+.
Look at thescematic I have attached.

HTH - Shahriar

UC3843_Design.pdf