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TPS40210: Cycle by Cycle Current Limit

Markus Noll
Prodigy 50 points
Part Number: TPS40210
Other Parts Discussed in Thread: LMV331, LM5155

Dear all,

I want to design a boost-converter for a 2s-lipo in, boosting up to 24V. My design goal was 3amps load current output with 7.7v in and 24v out.

I downloaded the spice models and have now everything running smoothly with the loop-compensation, over-current protection, load-jump response looks well damped.

However - I figured out, that TPS40210 is not doing any cycle-by-cycle current-limiting as I originally expected it. Instead, one time, the OC trip-point is triggered, it starts discharging the SS-capacitor - so this means the converter disables completely for a while until it is doing a restart.

Instead I would like to have it working similar like a lab-power-supply. The boost should limit the current (just to not driving the inductor into saturation), but try to boost the voltage as high as possible. This was originally not intented in the TPS40210.

However I found a workaround now and would like to have a review from your side. At least in simulation it's working.

My idea - using a small comparator (lmv331) and checking the current-sense voltage against a reference. The reference is set a little bit lower than the OCP threshold of the TPS40210.

The open-collector of the lmv331 is connected to the comparator-pin of the TPS40210.

If the lmv331 is detecting overcurrent, the COMP pin is pulled low and therefore stopping the current PWM-cycle -> the TPS40210 overcurrent-protection is not triggering and therefore LMV331 is limiting cycle-by-cycle the current.

However I have some concerns with this workaround:

The LMV331 is rated up to max. 5.5V on all pins. The COMP pin of TPS40210 can reach up to 7/8 volts. I would just clamp that COMP pin externally with a 3.3V zener. Is it expected to have some internal problems if max. amplitude on COMP pin is only 3.3V? In simulation it works. I could also use a 5V zener clamping.

Do I have to fear some device destruction if I pull the COMP pin externally directly to GND via the open-collector of LMV331?

See some screenshots in attach (sorry for using your competitor's software - but at least I used your spice-models :-) )

- Schematic

- Waveforms of V_Out, I_Shunt, V_COMP.

At 100us, the 3 ohms loads is switched on the output. As starting condition I have the output-cap precharged.

Thanks a lot

  • 0
    TI__Mastermind 30955 points

    Hi Markus,

    Thanks for using the TPS40210. :)

    From my quick look over, it seems that there should be no problem in pulling comp low, since COMP can be pulled down when designing flyback converter circuits using the TPS40210 with an external optocoupler. However, I'm not too sure if you can pull it all the way to ground. To be safe, I would abstain from pulling COMP all the way to ground.

    However, if you are looking for chips with cycle-by-cycle current limiting feature, I would recommend using the LM5155, which has cycle-by-cycle current protection implemented. 

    To speed up your design, I've linked the application note on designing a boost with the LM5155. http://www.ti.com/lit/an/snva824/snva824.pdf

    You can also find SPICE models as needed to simulate the performance.

    Let me know if you have any questions,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 50 points

    Hi Richard,

    thanks a lot.

    Well - I was not aware of LM5155. But I'll have a closer look in there. Seems to me that this is the cleaner option.