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TPS61021A: Regulated Vout shared

Vincent Rizzo
Prodigy 10 points
Part Number: TPS61021A
Other Parts Discussed in Thread: TPS61094, , TPS61023

Tool/software:

Good morning,

I report the last conversation with Antonio Faggio (TI Field Application Engineer), that asked us to open this ticket on TI E2E forum regarding TPS61021 step up application.

On our application we define the following HW configuration, as shown in the following picture:

INTRO:

  • The Step down output voltage is shared with the Step up output voltage (VCC is the net name)
  • The EN Step Up is always enabled (ON) and Backup Battery always connected (to be ready to go in tamper condition). 

FUNCTIONAL DESCRIPTION:

When +30 Main is present (Power on from external) the output voltage is provided by the Step down and the output is fixed to 5V.

When +30 Main is not connected (Tamper condition), the VCC voltage is provided by the Step down and the output is fixed to 4V.

FOCUS QUESTIONS/ ANALYSIS:

  • Vout step up operative max = 4V (Absolute max 4V8).
  • Vbck battery 4V2 (full charge)

 

QUESTIONS/ ANALYSIS:

1. On +30ON condition (VCC provided by step down=5V), could there be problems with the step up functional operation?

On its Vout pin there is a voltage of 5V and on its Vin pin there is a voltage of 3V6 (considering its absolute operative).

2. On tamper condition, with Vbck battery full charge (4V2), we expect (and it's confirmed by test) that on VOUT step up output pin the Backup battery  = 4V2, although it's set to 4V.

(Functional condition confirmed in the datasheet when Vin > Vout).

In Tamper condition OK, no problem!

 

3. On +30ON condition, when the backup battery voltage is > 4V we have an extra current consumption of 2A on +30!

We observed that when the VCC is fixed to 5V by the step down, the Vout step up would like regulate the VCC to 4V (with Vin step up > 4V).

This is the condition that generates the extra current consumption of about 2A on +30.

As final unwanted effect seem that the backup battery is direct fully recharged by the VCC!

 

As HW mitigation, to avoid the extra current consumption described above, the regulation of VOUT step up could be increased to 4V5 (higher than the Vbck battery full charge of 4V2).

In this way, however, the step up would always work out of specification as it has a typical operative voltage of 4V.

Could you please confirm if this could be damage the component? Or would it still be ok, considering the output absolute voltage of 4V8?
  

I hope I was clear enough in explaining the problem.

Best Regards

Vincent

  • 0
    TI__Expert 4395 points

    Hi Vincent,

    The device will enter passthrough when Vin is pushing Vout over 4V. And we don't recommend setting the device Vout over 1.2V. Is there any reason that you have to use TPS61021A? You can choose TPS61023 to output 4.5V. And we also have TPS61094 which integrates back up boost and charger for back up battery.

    Best Regards,

    Travis

  • 0 in reply to Travis Wang
    Prodigy 10 points

    Hi Travis,

    thanks for your fast reply!

    I don't understood why are you recommending to don't set Vout over 1,2V. Do you mean that Vout_max = 4 + 1,2 = 5,2V?

    Our question arises from the fact that the Vout_max (operative) is 4V and the Vout Absolute is 4,8V.

    I ask the last question again: "Could you please confirm if this could be damage the component, if the Vout is set to 4V5? Or would it still be ok, considering the output absolute voltage of 4V8?"

    Thanks for your suggestion to use the other PN: TPS61023, that fit good with our application, but in our PCB design was previously foreseen the TPS61021A.

    We will keep your suggestion for future designs!

    Best Regards

    Vincent

  • 0 in reply to Vincent Rizzo
    TI__Expert 4395 points

    Hi Vincent,

    Our max Vout means your Vout target should not be set over 4V. Vout absolute means the vout pin can withstand 4.8V. The gap between 4V and 4.8V is because there'll always be switching overshoot on low side MOSFET. So if the internal MOS withstand 4.8V, you cannot set Vout at 4.8V.

    This picture is not taken on TPS61021A but it describes the overshoot.

    So if you set Vout at 4.5V. There will be risk that the IC will get damaged. 

    Best Regards,

    Travis