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TPS43340-Q1: BUCK3 tracking BUCK1

Markus Mueller12
Prodigy 30 points
Part Number: TPS43340-Q1

I want to use BUCK1 for 5V (from 12V supply) and BUCK3 for 3V3, powered from BUCK1 since 12V supply is not possible.

BUCK2 is an independent rail... 

So far so good, this works well on EVM when using the independent Enables.

Now i'd like the 3V3 track the 5V with the same ramp-up time (and one unique ENABLE)

I tried different configurations on the EVM with a resistor divider from 5V and with/without cap on the SS3 pin.

But the BUCK3 voltage always rises instantly without ramping up!?

Unfortunatelly the documentation won't help me any further, although it mentions in chapter 7.4.1.3:

Alternatively, one can use the soft-start pins as tracking inputs. In this case, connect the pins to the supply to be
tracked via a suitable divider network.

Can you please give some advise on adequate circuitry on SS3 and EN3 pins!?

  • 0
    TI__Genius 12735 points

    Hello,

    The expert for this device is currently out of office and will be returning on Monday. I am looking in to this request but we may have to wait until his response on Monday. I will provide an update tomorrow. 

    Best regards,

    Layne J

  • 0 in reply to Layne J
    TI__Genius 12735 points

    Hello,

    Unfortunately the expert is going to be out of office this week as well. I am going to be looking further in to this request however my experience with this part is very limited. I will provide an update on this thread tomorrow.

    Best regards,

    Layne J

  • 0 in reply to Layne J
    TI__Mastermind 48691 points

    Hello, Markus

      For the tracking purpose, the EN3 pin should be high before 5V power up. The resistor value for the divider from 5V should be high enough with a proper cap value to make the SS3 pin ramp up and then you'll see BUCK3 voltage ramping up in the same way of SS3 ramp up. 

    Thanks!

    Phil 

      

  • 0 in reply to Phil Yi
    Prodigy 30 points

    Hello Phil,

    if i get you right i will have to foresee a resistor divider from 5V (values tbd) PLUS the original cap to GND on SS3.

    Does the resistor divider has to meet the 0.8V reference voltage in any way? The cap value i would assume to be the same like on SS1?

    The EN3 can be hold high, thats doable. Can it alternatively be tied to EN1 (same Enable-signal)?

    Due to Home Office i'm not able to do tests on the EVM atm - this can last another one or two weeks...

    But i'm on a pcb design to adapt the new supply on one of our mainboards for some first tests "in real life" - so actually it would be sufficient to know which parts have to be placed.

    If it is helpful i can provide the schematic!?

  • 0 in reply to Markus Mueller12
    TI__Mastermind 48691 points

    Hello, Markus,

      Read this statement copied from datasheet:  "A 1-μA pullup current is available at the SSx pins, and by choosing a suitable capacitor one can obtain a desired soft-start ramp speed. After start-up, the pullup current ensures that pins SSx are higher than the internal reference of 0.8 V, which then becomes the reference for the buck controllers." 

     The cap value on SS3 pin should be decided by the ramp up timing and resistor divider value you use. EN3 should be hold high instead of tied to EN1. 

    Thanks!
    Phil

      

  • 0 in reply to Phil Yi
    Prodigy 30 points

    Hello Phil,

    thx for your answer - i think the solution is clear now.

    But i have another problem now: i wanted to try the circuit on "real" hardware, but now i cannot even get the IC (like many other market issues)

    Furthermore it seems that is not possible to order or sample anymore on TI site?

    Sorry for the off-topic, but perhaps you forward me to get at least some samples?

    Thank you
    Markus

  • 0 in reply to Markus Mueller12
    TI__Mastermind 48691 points

    Hello, Markus,

    I think you can still request free samples from the link below:

    https://tisamples.ti.com/order/tisamples/en/p/product/?p=TPS43340QPHPRQ1

    Or, buy an EVM board from the link below:

    https://www.mouser.com/Search/Refine?Keyword=TPS43340

    Thanks!

    Phil