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TPS54332: the sequence between Vin and Enable(1.25V)

Hirotaka Matsumoto
Guru 27565 points
Part Number: TPS54332

Hi all

Would you mind if we ask TPS54332?

<Question1>
Is there the sequence between Vin and Enable(1.25V)?(At the first, Vin needs to be overt UVLO?)
Especially, after Vin is over UVLO threshold level(3.5V), Enable voltage require to be over 1.25V(typ).
Is our recognition correct?

<Question2>
On the datasheet P11, there is the description "TI recommends using an external VIN UVLO to add Hysteresis unless VIN is greater than (VOUT + 2 V)".
So, unless we use external Vin for Enable, we don't need to keep VIN is greater than (VOUT + 2V), do we?
Our customer uses following condition;
It is no problem, right?
-Condition-
Vin=5V, Vout=3.3V/No Load
We confirmed with above condition, as the result, it didn't output normally like as attachment file.
CH1 : Vin, CH2 : Enable pin, CH3 : SW pin

And then, we tried with Vin=5.3V(Vout3.3V+2.0V), SW operated normally.

Furthermore, when we use internal pull-up about Enable pin, it rewquire to keep that VIN is greater than (VOUT + 2V), doesn't it?

Kind regards,

Hirotaka Matsumoto

  • 0
    Guru 27565 points

    Hi team

    We are sorry to rush you, however could you give us the reply?

    Kind regards,

    Hirotaka Matsumoto

  • 0
    TI__Expert 4035 points

    Hi Hirotaka-san,

    A1: yes, your understanding is right.

    A2: If VIN is less than (VOUT+2V), TI recommends using an external VIN UVLO to add Hysteresis. If VIN is higher than (VOUT+2V), Both external and internal VIN UVLO are ok.

    As for your application case, You could see the noise of the 5V power supply, the minimun Vin is less than 3V, the abnormal waveforms might result from the VIN UVLO tripped. That why you can slove this problem by increasing the Vin a little.

    BRs,

    Ryan. 

  • 0 in reply to Ryan Hu (BSR-MV)
    TI__Intellectual 2790 points

    Hi,

    1. There should be no VIN/EN sequence requirement for this device, the device operates when both Pin are above their resptive rising threshold.

    2. The root cause cause of oscillation at Vin=5V, Vout=3.3V/No Load may be that, at light load, the switching frequency will decrease very low, the Boot capacitor can't be charged sufficient to drive the high side FET, so the Vout drops, when the  Vout drops low enough, the Boot cap is charged through BOOT pin to Vout through the inductor, the high side FET is driven on when the Boot cap is charged sufficient, the Vout goes up. The large overshoot (the max value may be 4V or more in your fig) is because the Vcomp is high when the high side FET is drive on, the inductor current is large , and there is no load at the end, so the Vout is charged high. Below description in the datasheet may explain that.