[SOLVED] Need help with Overshoot current on Boot converter

[k113]

I'm having issues with current overshoot on the LM3224 DC Converter.

When configured to run at low current (~50mA), I get extreme current overshoot (very bad).

I'm running it using /SHLD from a PWM output on a PIC.

On the activation through /SHLD, there is a dramatic overshoot.
Over a range of current settings, this peak shoots to 100mA initially.

Here is the circuit

**broken link removed**

and here is the wave form

**broken link removed**

I'm switching the signal on /SHLD at around 120hz and I have the compensation cap as high as I reasonable can put it without impacting that. Lowering Cc (5.6nF) makes the overcurrent worse.

please help.

 

[leo_o2]

This can't be solved in PCB level. It is fixed in IC structure.

 

[mtwieg]

Try putting a small capacitor in parallel with the LED. Should act as a lead compensator and dampen the response. Start small (like maybe 10nF) and increase. Eventually either the waveform will dampen or it will become unstable.

What is the compensation network voltage doing during these overshoots?

 

[dick_freebird]

Use your SS pin, which you have disconnected. The 10uF
will take all the current you can give it until the LED gets
lit (say, 3V). I=CvV/dt says for 10uF, 3V, 10uS (going by
the plot) you'd take 3A. Meaning the error amp is not at
all satisfied, probably winds up and rails because it can't
close the loop in real time.

Soft start lets the load get charged at a rate where the
error amp can stay small-signal. Your comp network has
nothing for the large signal input case.

 

[k113]

Thank you all for your suggestions.

Adding a 10uF cap across the LED terminals cut the current spike back to 50mA which works well, especially since I realized that my current requirements are now at 50mA.

Thank you again!