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TPS61378-Q1: TPS61378-Q1 not powering up.

Flux
Intellectual 520 points
Part Number: TPS61378-Q1
Other Parts Discussed in Thread: LM7310

Dear Madam, Dear Sir,

we’re using the TPS61378-Q1 in a new design to drive a load of around 300..400uF at around 15Ohms, Vin=5V, Vout=6V, Inominal=500mA.

Unfortunately the load current at startup is slightly higher than the soft-start peak switching current of 1.15A given in the datasheet. Most of the times the load doesn’t power up at all. The TPS61378 infinitely loops in hiccup mode. Please see the attached scope traces for a proper power-up, a partial hiccup loop and the attached schematics.

The load electronics has been developed in parallel and its power requirements have gotten a bit higher than expected. We want to correct that in the next board iteration using a load switch like LM7310 behind the TPS61378-Q1. The TPS61378-Q1 can power up without load then and the LM7310 can ramp up the load at a higher current or ramp up the load slower. Or, we use another boost converter with a similar footprint.

But for now, we must continue our work with the current prototypes. Do you see a chance to somehow slightly increase the peak switching current of the TPS61378-Q1 during startup? Btw, why has TI chosen to limit the switching current at startup, which leads to this kind of startup problems?

Best regards.

 

 

 

  • 0
    Intellectual 520 points

    Sorry, I forgot to mention, we have already reduced R69 to 20k. So, after the 2.5ms softstart the regulator should work with its maximum switching current limit.

  • 0 in reply to Flux
    Intellectual 520 points

    ... but that does not solve the problem DURING startup.

  • 0 in reply to Flux
    TI__Genius 12635 points

    Hi Flux,

    The device has an isolation fet to have load disconnection function.

    And the iso-fet will be like a LDO when Vin is higher than Vout, like starting or weak short.

    So, the device decreases the current limit to 1/4 or smaller of the normal current limit due to the thermal consideration.

    Usually, the device will start fail when heavy load or large capacitors.

    Best Regards

    Fergus

  • 0 in reply to Fergus He
    Intellectual 520 points

    Hi Fergus,

    thanks for the explanation.

    Do you see a chance to tweak the ramp up for the prototypes?

    I'm not sure the combination +5Vin and +6Vout in general for the TPS61378-Q1 is a good one. 5/6 = 0.83, which is close to the 85% threshold of the device's down mode. Can we be sure, the device doesn't enter down mode here and increase the power consumption? The datasheet isn't very clear here...

    Best regards,

    Flux

  • 0 in reply to Flux
    TI__Genius 12635 points

    Hi Flux,

    Right now, there is no prototype for the tweak the ramp up.

    For the down mode, the fact is that the device detects the on time of the lower switch, when it reaches 70ns(0.15/2.2Mhz), then it will enter down mode.

    So, if you want to ensure the device out of down mode at 5Vin and 6Vout, you can decrease the frequency.

    Best Regards,

    Fergus