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TPS62840: Expected behavior when Vin come close to Vout under load compared to TPS62740

Thomas LE BIVIC
Intellectual 510 points
Part Number: TPS62840
Other Parts Discussed in Thread: TPS62740,

Hello, 

I've been using TPS62740 from 3.6V Li-SOCL2 to 3.0V for current pulse like that with success:

but I'm out of spec as the maximum current for vin < vout+0.7V is 100mA.

in a previous post (https://e2e.ti.com/support/power-management/f/196/t/789006#pi320995=2) I got this answer:

"The 0.7V limit is a recommendation, based on how we see most customers using this device. Most customers are powering a radio, which is sensitive to low switching frequencies and the associated higher voltage ripple. To best support this, this IC has a special 100% mode circuit which is explained in section 9.4.2. When Vin is too close to Vout and the load is high, the IC might enter dropout before this 100% mode circuit kicks in. This might create higher ripple and interfere with the radio.
If you have not seen issues in your application, this is better proof than anything else."

Which is not fully satisfying for a conscientious designer :p

To stop being "out of spec" I'm considering using TPS62840 in new design. But as I see many similarities I'm just wondering why this component doesn't have such limitation in output current when Vin < Vout + 0.7V ?

Thank you for enlightening me :)

Thomas

P.S. worst case current pulse with better acquisition frequency:

  • 0
    TI__Mastermind 25430 points

    Hi Thomas, 

    I am looking into this request. I will get back to you early next week.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 25430 points

    Hi Thomas, 

    Could you please share a schematic of the system?

    Regards,

    Febin

  • 0 in reply to Febin
    Intellectual 510 points

    Here it is:

    The worst case current pulse (in previous post) is the RF transmission with Sub-1GHz transceiver and power amplifier at max power. Current measurement has been performed with "OTII" Arc in place of buck converter with 3.0V output.

  • 0 in reply to Thomas LE BIVIC
    TI__Mastermind 25430 points

    Hi Thomas, 

    Can I please rephrase your question. 

    You would like to know why TPS62840 does not have the limitation in output current when Vin < Vout + 0.7V ?

    Regards,

    Febin

  • 0 in reply to Febin
    Intellectual 510 points

    From measurement that I've made, it looks that TPS62840 has similar behavior than TPS62740 when Vin comes close to Vout:

    I tested with Vin = 3.6 then 3.35 and then 3.1 V and Iout = about 200mA
    Setup:

    And results:

    TPS62840 exhibits better RDSon that is why we got little more voltage at Power amplifier level (and then little more current drawn).

    From this experiment I don't understand why you put limitation in TPS62740 datasheet but not in TPS62840 so you can take the question the way you prefer:

    "why TPS62840 does not have the limitation in output current when Vin < Vout + 0.7V ?"

    or

    "why TPS62740 have a 100 mA limitation in output current when Vin < Vout + 0.7V ? whereas things look quite good for Vin + 0.35V at 200mA ?"

    Thanks for support,

    Have a good day,

    Thomas

  • 0 in reply to Thomas LE BIVIC
    TI__Mastermind 25430 points

    Hi Thomas,

    For TPS62740, there is an LDO for providing the LOAD pin feature. As you could read from the datasheet the definition of output current of the device is IOUT + ILOAD and hence the limitation. For TPS62840, LDO is not present. As you mentioned, RDSon is low and so output voltage is higher than TPS62740 for the same input conditions.

    I am not sure about what you imply with Vin + 0.35V at 200mA? Could you please clarify?

    Regards,

    Febin

  • 0 in reply to Febin
    Intellectual 510 points

    Hi,

    Ok I understand better now !

    But still: We can draw up to 300mA with TPS62740 without using Load pin (I'm not using it) so in this case there is no such Vin > Vout + 0.7V limitation right ?

    I have good result with Vout = 3V, Vin = 3.35V and 200mA load.

    Now that you told me it's because of the LDO I think I could have seen it by myself but maybe you could make it clearer in the datasheet:

    By the way I will probably go for the TPS62840 as everything looks just better.

    Thanks again for help,

    Thomas

  • 0 in reply to Thomas LE BIVIC
    TI__Mastermind 25430 points

    Hi Thomas,

    The maximum output current for TPS62740 is 300mA under the recommended operating conditions. If the load pin is not used, just leave it open.

    I understand what you said. As the datasheets are written by different people, the format gets slightly different. But for our newer devices, we have standardized this. 

    TPS62840 is one of our best devices. Please ensure to test the application well and use the recommended components.

    I would like to close this post. Please feel free to contact us for any further support.

    Regards,

    Febin

  • 0 in reply to Febin
    Intellectual 510 points

    Hi,

    You can close this thread, thanks for support,

    Have a good day