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TLV61070A: Current limit behavior

Oba
Genius 17355 points
Part Number: TLV61070A

Tool/software:

Hello,

I'm evaluating TLV61070A EVM for my customer support.
I modified the EVM to monitor the inductor current.
And I changed the inductor to 1uH because the cusotmer is using it now.
VOUT is set to 5V and the load current is 500mA.
When the VIN drops to less than 1.9V,  VOUT start to drop.
Then I checked the inductor current.

When VIN is around 1.93V, the valley current is about 1.94A which is less than the limit value. But the VOUT just start to drop. 

When VIN is from 1.93V to 1.85V, the valley current increased up to 2.4A while VOUT drops(about 500mV)

When VIN is less than 1.85V, the voalley current is alwys 2.4A which means limit value while VOUT continue to drops.

I can understand the behavior of VIN is less than 1.85V, but can't understand the behavior of 1.93V-1.85V. The valley current increases but VOUT drops.

Could you tell me the reason?

Regards,
Oba
 

  • 0
    TI__Genius 13715 points

    Hi Oba,

    Although the inductor current is still far away from current limit, the internal loop is slow. That means the load requires higher IL, but the internal iEA (response ) is far too slow to follow the change in the requirement. Thus the input power is unable to sustain the output. Anyway, the device is too slow to respond to the needed inductor current , resulting from transient in Vin.

  • 0 in reply to Nini Zhong
    TI__Genius 17355 points

    Hello Nini,

    Thanks for your reply.
    Let me expalin what I did more detail.

    VIN is supplied by a stablized power supply equipment.
    I decreased VIN voltage manually littel by little from around 3V by my hand.
    So it should not be related to the loop response speed. VIN is constant at least during a couple of seconds at least.
    The load current is 0.5A which is a electric load with CC mode. The VOUT is set to 5V.
    VOUT keeps 5V untill about VIN=1.93V. The valley current at VIN=1.093V is 1.94A.
    VOUT starts to drop from 5V after VIN is less than 1.93V.
    During VIN=from 1.93 to 1.85V, the valley current still increased while VOUT dorps more. 
    WHen VIN=1.85V, the valley current finally reached to 2.4A.
    After less than 1.85V, the valley current keeps about 2.4A and VOUT drops more and more.

    My question is VIN=from 1.93 to 1.85V.  Although valle current doesn't reached to the limit value, VOUT starts to drop.
    I would like to know the reason.

    Regards,
    Satoshi Obata

  • 0 in reply to Oba
    TI__Genius 13715 points

    Hi Obata，

    VIN=from 1.93 to 1.85V :  because the loop is too  slow to  follow the transients .  Just the input power is not enough to sustain the output. 

  • 0 in reply to Nini Zhong
    TI__Genius 17355 points

    Hello Nini,

    Thanks for your reply.
    Sorry, I can't still understand at all.
    The VIN is well stable.
    What is "transients" in your reply?

    Regards,
    Satoshi Obata

  • 0 in reply to Oba
    TI__Genius 13715 points

    You mean the Vin doesn't change at all?

    Could you help to provide the VIN waveform and inductor current waveform, Vout waveform?

  • 0 in reply to Nini Zhong
    TI__Genius 17355 points

    Hello,

    The below is the waveform.
    And sorry, VIN voltage value in my previous post was not corretct. The VOUT start to drop at VIN=around 1.55V.
    In the previous post, I just read the value which showed at power supply unit.

    Anyway,  VOUT just start to drop at VIN=1.55V. The valley current is about 1.8A


    I slightly decreased the supply voltage. The inductor current increased, but VOUT decreased.

    I slightly decreased the supply voltage more. The inductor current increased more , but VOUT decreased.
    The valley current is about 2.5A. This valley current doesn't increas anymore. 

    Regards,
    Satoshi Obata

  • 0 in reply to Oba
    TI__Genius 13715 points

    could I see to it when back to office?

  • 0 in reply to Nini Zhong
    TI__Genius 17355 points

    Hello,

    Have you seen it?

    Regards,
    Satoshi Obata

  • +1 in reply to Oba
    TI__Genius 13715 points

    Hi Obata, at the edge of the current limit , the loop is not stable, sense ratio may be changed , then you could observe these behavior close to current limit .