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PMP9802: Functionality clarification

Nikos Agianniotis
Prodigy 50 points

Part Number: PMP9802

Hi,

I need to design a circuit that looks similar to the one in PMP9802. I have, hence, been studying the material and how the design works and have a question regarding its functionality. As far as I understand, the whole concept is based on the fact that both the TLV62130 and TPS61089 work at power save mode, therefore their outputs can be combined directly, changing at the same time their output nominal values accordingly.

At some point in the test report (section 2.1) it is mentioned that "When the voltage in their VOUT pin is higher than the setting voltage, the TLV62130 and TPS61089 stop switching. No reverse current is flowing into the converters. Using the principle of PSM, the operation sequence of the TLV62130 and TPS61089 can be implemented by setting a different target output voltage." The sentence highlighted in bold by me puzzles me a little bit. I do understand it for the TPS61089 converter, but looking into the SW pin of TLV62130 we have a path through the body diode of the HS-MOSFET towards pin PVIN, isn't there?

Why then do we say that there is no reverse current flowing? Is it because of the external diodes D1 and D2?

In my case, there will only be VGRID at the input of TLV62130 (and not the battery), so if it is missing, won't there be problems because the VBUS voltage is now fed to the input through the path I described above, causing the TLV62130 to startup and switch? I guess I shouldn't rely only on the fact that the converter is turned-off due to VGRID missing, but due to the above effect I should also make sure its programmed output is lower than TPS61089, so as to ensure it stops operation (as in the reference design).

Moreover, when the TPS61089 converter is not switching, is there any current leaking from the supercap towards the converter and its output? I'm only asking this because in my case, the supercap is in reality a Li-Ion battery and I'm worried that its charger (bq25170) will not be able to terminate the charging cycle within the 10-h window due to current leaking from the battery towards the TPS61089 and VBUS.

  • 0
    TI__Genius 16325 points

    Hi Nikos,

    1. For the TLV6230, when the 12V is lost, its input is NiMH battery and about 6.4V to 4.5V, its output voltage is 4.6V, its body diode is about 0.2V, so there will be few reverse current from its output to its input side.

    2. in your case, if the VGRID missed and the output will charge the input to 4.4V by body diode(D1 is needed to prevent reverse current). the most effective way to stops it from operation is to disable its EN(conncet the EN to VGRID)

    3. when the TPS61089 is not switching, it will not be any current from its input to output unless the output is lower than input(maybe a diode voltage drop must be considered).

    Nikos Agianniotis said:
    not be able to terminate the charging cycle within the 10-h window

    I'm not sure about this, could you have an explanation?

    Regards,

    Fergus

  • 0 in reply to Fergus He
    Prodigy 50 points

    Hi Fergus,

    these are great insights, thank you.

    Just to follow-up on point 2 you made, isn't it the case that if the input is charged by the output to 4.4V, this will also enable TLV6230? I mean, the EN is connected to the input, which is charged by the output. Unless you mean to connect the EN right before the diode; that would make sense.

    What I meant with the 10-h window is that the bq25170 has a fast-charge safety timer (nominal 10 hours) within which the charging algorithm has to be completed. And it's stated in the datasheet that 

    "The system load can be placed in parallel with the battery, as long as the average system load does not prevent the battery from charging fully within the safety timer duration. When the system load is placed in parallel with the battery, the input current is shared between the system and the battery"

    So my concern is that the system load is too high that it prevents charging within this window. But yeah, I think from what you said this is not going to be the case. The output will be higher than the input, so there will be not current flowing that direction.

    Best,
    Nikos

  • 0 in reply to Nikos Agianniotis
    TI__Genius 16325 points

    Hi Nikos,

    Nikos Agianniotis said:
    Just to follow-up on point 2 you made, isn't it the case that if the input is charged by the output to 4.4V, this will also enable TLV6230? I mean, the EN is connected to the input, which is charged by the output. Unless you mean to connect the EN right before the diode; that would make sense.

    Yes, the voltage is enough to enable TLV6230 when EN connected to input. but the device is not able to work as the output is higher than input.

    I will include MV AE Athos Zhao to analysis the working condition of TLV62130.

    For the 10-h window, I understand that it will be ok that TPS61089 does not get any reverse current.

    Regards,

    Fergus

  • 0 in reply to Fergus He
    TI__Genius 13451 points

    Hi Nikos!

    For TLV62130. When your output exists but input voltage is not, the input side when be charge through high-side body diode. If Vout+didoe voltage drop> UVLO and EN pin is enable, the device will switching.

    Shuai

  • 0
    Prodigy 50 points

    That's great, thank you both for the clarifications. I have a much more clear view now of the circuit.