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TPS61378-Q1: unstable EN pin and high "no load" current

Buyssens Hubert
Intellectual 685 points
Part Number: TPS61378-Q1


I had 2 problem on my prototype, which is very near TPS61378 EVM design.

 

I’m using the TPS61378 for charging about 4mF capacitor at 17V.

Supply is a high drain Li-ion cells.

 

When I’m driving EN pin with Vbat, with no load at the output (just maintaining the 4mF capacitor at 17V), the current is about 150mA.

I don’t understand why this current is so high?

I see at thermal analysis that inductor is heating at about 50°C.

 

I’ve seen another strange thing :

When I’m touching the wire connected to EN pin through a 100kOhms resistor, the DCDC is starting charging the capacitor.

In this “strange way” current seems normal : high at start of charge and nearly null for maintaining capacitor at 17V.

 

So I’ve tried to had a 100kOhms pull-down resistor to EN pin (should be 800kOhms internal).

The EN pin is more stable.

But there’s still this 150mA extra current that I don’t understand?

 

Question :

Does someone had an explanation or solution for this 150mA extra current?

What is the internal circuitry of the enable pin?

Here's the schematics, before adding the pull down on EN pin:

Circuit TPS61378 - pour forum TI.pdf

  • 0
    TI__Mastermind 34231 points

    Hi Hubert,

    When you are talking about the 150mA current, what's the Vin, Vo voltage?  Do you have the waveform of VIn, Vout, iL when you see this 150mA?

  • 0
    Intellectual 685 points

    Hi Zack,

    150mA is when the 4mF capacitors are at stabilized charge.

    So Vin dc is Li-ion battery. Last test was 3.8Vdc (but made some tests from 2.8 to 4.2Vdc).

    Vo is 17Vdc.

    Seems difficult to mesured iL.

  • +1 in reply to Buyssens Hubert
    TI__Mastermind 34231 points

    Hi Hubert,

    I see the MODE pin is connected to VCC net so TPS61378-Q1 is working in FPWM mode.

    In FPWM mode, the device will keep switching even there is no load and consumes some switching loss. The inductor itself also consumes DCR loss and AC loss. You could connect MODE pin to GND to set it in auto PFM mode to reduce input current at no load.

  • 0
    Intellectual 685 points

    Hi Zack,

    Thank you, I remember now…

    In fact I’ve made a first test session on the EVM and think that we’ve test both MODE. (but’s it was some months ago…)

    PFM consume less, but FPWM charge capacitor faster.

    So we’ve decided to stay in FPWM.

    And for consumption optimisation, we have to drive enable low when the capacitor is charged, and drive enable high only when it’s necessary.