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TPS61288: TPS61288

Chang Cao
Intellectual 520 points
Part Number: TPS61288

Hi, 

I need your help desperately.

With E2E help, I built a prototype to test, see attached schematic. it consists of 2 portions: TPS61288 boost circuit and a Vin clamp/cut-off circuit (D1,Q1,Q2, D3).

Design goal: Battery voltage 8V~15V, boost output 12V 1.2A (max 1.4A).

The boost circuit alone works well. At Vin close to 12V, the ripple does very high(Vpp=1V). I add an 270uf aluminum cap, the ripple drops to 70mV. When Vin >12V, it directly goes to the Vout.

The Vin clamp/cut-off circuit alone works well too. When Vin <7.2V, Vin is cut off. When Vin in range 7.3V~12V, Q3 output 7.3V~12V. when Vin >12V, Q3 output 12V (when R6=11K).

However, when I connect Q3 output to boost input. It has an audible noise , and Vout is only 9V.

  1. I change L1 to 10uH, doesn’t help.
  2. I change zero/pole distribution by change C7 to 30pf, doesn’t help.
  3. I add a 100uf aluminum cap at Q3 output , doesn’t help.

I am really frustrated, seek help from you.

Thanks.TPS61288 schematic.pdf

Chang Cao

  • 0
    Intellectual 520 points

    I think Q3 is not compatible with boost circuit. I disconnect Q2 emitter from Vin line; disconnect Q3 base from the joint of D2 cathode/R5/R6; connect a 1K resistor to Q3 base, so now only Q3 is connected to the boost circuit (1K provides base current). Still have audible noise. very wierd,  (Vin=9V, boost input 6.85V, boost vout=6.52V, voltage not step up.

    Q3 not saturated. for 12V boost output, if boost vin is only 6V, the input current should be twice of the output current which ~2.4A(i use1.2A load). Q3 is ZXT1053AK , whose hfe(min)=300(Ic=1A); hfe(min)=50(Ic=5A); it seems 1K base resistor is ok

  • 0 in reply to Chang Cao
    TI__Expert 6310 points

    Hi Chang Cao,

    Your analysis is right, Q3 might be not saturated.

    It is likely that the sink current is not as large as you calculated.

    You can try to add a voltage source 15V directly series in 1K to base of Q3, this can ensure the saturation and you can see if the system is stable.

    then you can have a test of the base current and to see why it was inadequate.

    Best Regards

    Fergus