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TPS54310: Soft start

Oba
Genius 16035 points
Part Number: TPS54310

Hello,

 

My customer has some questions for TPS54310 SS function.

 

They are now using 2 TPS54310s for 1.5V out and 3.3V out.

They have to design to launch 1.5VOUT faster than 3.3VOUT, but want to shorten interval then between 1.5V and 3.3V start-up as much as possible.

 

1)

They put the same 0.022uF on SS/EN pin of both 1.5VOUT configured device and 3.3VOUT configured device.

Then SS pin charge speed of 1.5VOUT configured device is faster than 3.3VOUT configurated device.

They would like to know the reason. In their test, they haven’t test 1.5VOUT and 3.3VOUT configuration in the same device.

So I think the charge speed difference was caused by just a piece-to-piece variation.

Their question is that does VOUT voltage setting affect to SS pin charge speed (= charge current)?

 

2)

The datasheet specifies “charge current, SS/ENA” as 3 ~ 8uA.

Is majority of this variation due to piece-to-piece variation?

They would like to now what parameters like temperature, VOUT, VIN or other, affect to this variation.

 

3)

The datasheet specifies the below formula to calculate the delay and the ramp.

 

Td = C(SS) x 1.2V/5uA

T(SS) = C(SS) x 0.7V/ 5uA

 

Is there any other parameters that could affect to Td and T(SS) ?

 

Regards,

Oba

  • 0
    TI__Genius 15260 points

    Hi Oba,

    1. From the device datasheet description, there is a brief ramp up at the internal soft-start rate, before the externally soft-start rate control. But I don't expect the VOUT to have an influence on the soft-start times. 

    2. The spec of 3 to 8uA includes, unit to unit variation from process, as well as influences from VIN and temperature. 

    3. I don't think anything else affects the ramp time.

    The external capacitor would also be having a certain tolerance. This should also be checked.

    Are both the 1.5V and 3.3V, running off the same VIN rail? Is there anything else other than the cap, connected on the SS/EN pins of both devices?

    How much difference is seen in the ramp times of the 1.5V and 3.3V outputs? If it is like 1ms, I don't expect this to be unit to unit variation.

    Can you attach the customer schematics, plus the startup waveforms of both rails?

    Best regards,

    Varun

  • 0 in reply to Varun John
    TI__Genius 15260 points

    Hi Oba,

    I'm closing this thread as we have continued our conversation via email.

    Best regards,

    Varun