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TPS7A47: Design review request

Jonathan Geronga
Expert 7930 points
Part Number: TPS7A47

Hi Team,

I am posting this on behalf of our customer.

They are using TPS7A4701 in their new design and facing one issue. They use a Voltage divider 9.76K and 9.76K to the FB to set the output voltage to 2.8V. The output is 2.798 as expected. But when they give GND to 1P6(Pin Number - 8), they are getting 5.98V, but they expect 2.798+1.6 = 4.3V,

Here's the schematic:
8255.SCHEMATIC1 _ PAGE1.pdf

Additional information from the customer:

This is the table which shows the output,
Default 2.8V R13:9.76K,R34:9.76K
Voltage Seen Expected Voltage Difference
0.1 2.982 2.9 0.082
0.2 3.184 3 0.184
0.4 3.5 3.2 0.3
0.8 4.38 3.6 0.78
1.6 5.99 4.4 1.59
3.2 9.2 6 3.2

They are getting double the change.

Please let me know your comment.

Thanks,

Jonathan

  • 0
    TI__Mastermind 23361 points

    Hi Jonathan, 

    The TPS7A47 is recommended to work either with ANY-OUTS or with externally adjustable resistor dividers. You can combine the two and please use the following calculation as an example. 

    For this case, 

    They are using TPS7A4701 in their new design and facing one issue. They use a Voltage divider 9.76K and 9.76K to the FB to set the output voltage to 2.8V. The output is 2.798 as expected. But when they give GND to 1P6(Pin Number - 8), they are getting 5.98V, but they expect 2.798+1.6 = 4.3V,

    Here the ANY-OUT is changing the Vref = 1.4 + 1.6 = 3V and through the resistor diveder, the final output = 3V ( 1 + 9.76/9.76) = 6V. 

    Does it make sense? 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    TI__Expert 7930 points

    Hello Jason Song.

    Thank you for your support in this thread.

    Upon sharing your answer to the customer, here's the follow up question we got from him:

    How did you calculated 1.6V?
    Can you please share the internal block diagram of TPS7A4701?. He thinks in datasheet only the block diagram of TPS7A4700 is only available.

    Regards,

    Jonathan

  • 0 in reply to Jonathan Geronga
    TI__Mastermind 23361 points

    Hi Jonathan, 

    We actually have the block diagram listed in the datasheet, and the important part here is that the ANY-OUTs pins are controlling the gain from the first amp, and the resistor divider is creating gain on the 2nd stage. 

    When using only the adjustable pins, the second stage has a unity gain. Here is the block diagram that is also available on the datasheet. 

    The 1.6V is from the bandgap in the first stage amp. Does it make sense to you? 

    Regards, 
    Jason Song