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TPS61085 doesn't work

Tommy Rau
Prodigy 20 points
Other Parts Discussed in Thread: TPS61085

I am design a DC - DC 3V-4V /  2A  (Li-ion battery cell 0.2C discharge current)  to 14V 250mA convertor to charge 12V Pb battery , working with a TPS61085. I follow the design document to connect the wire and check every connection is correct then power on it with no load condition. But unfortunaltly,  it doesn't work. I measure all the pin reading with FLUKE multi meter,  Pin 2 (FB) alwayse a 4.85V out (Pin 6 Vcc = 5V) . At no output condition, it should be 0V. Original design is 156K serial 18K between Vout and Ground. Center pin of two resistor connect to Pin 2(FB).  I try to connect 1K ohm to ground, still the same as 4.85V. Even I short Pn 2 to Ground, TPS61085 work just once at the spot when touch ground.

Then I try to switch this FB pin to ground on and off  quickly, output pin seems get 14V voltage but suddently it burn out.

Can some on tell me where I might be wrong? Or similar experience?

I bought 4 units from DIGI-key last week.  Feel sad at this test result.

Thanks

Tommy

 

 

  • 0
    TI__Guru 65170 points

    Hello Tommy,

    If you have 4.85V on the feedback pin, the IC will not switch, because this voltage shows the IC that the output voltage is a lot too high. The voltage on this pin has to be 1.238V then it is in regulation.

    Please recheck your feedback resistors, because 4.85V on the FB pin with 5V input voltage looks like a short of the top feedback resistor (5V minus the forward drop of the diode or so).

    In addition, it is not recommended to connect the FB pin to GND.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 20 points

    Hi Brigitte:

    Thanks for your reply.

    I double check the connection twice (open for no wire connection and short for which should be conneced)  . 

    I had try to replace a new one, but still not function as it should be.  It seems the SMD chip was damage after soldering. I doubt that maybe the ESD or temperature when soldering.

    Anyway, thanks again for your assistance. I just try another Dip package DC to DC convertor and work normally. After the schedule been cover, I will try this SMD chip again.

    Thanks and Best Regards

    Tommy