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TPS7A47-Q1: how to decide the feedback resistance

Masayuki Uchihara
Intellectual 1655 points
Part Number: TPS7A47-Q1
Other Parts Discussed in Thread: TPS7A47, TPS7A4701EVM-094

Hi Team,

I have question about how to decide the feedback resistance when I use the ldo as a programmable type.

First of all, I'd like to know the circuit topology in this table. I can't imagine the concrete circuit because If all of pins are open, the table said output voltage is 1.4V.

This voltage is equal to Vref voltage. That's why I am confused about the output topology. Could you tell me the topology?

  • 0
    TI__Guru 55940 points

    Hi Masayuki,

    The table is correct.  If you float all of the ANYOUT pins, the TPS7A47 will be in unity gain and you will see the reference voltage on the output.  When you short an ANYOUT pin to GND, you are adding a current path from FB to GND.  This means that you are changing "R2" in the typical resistor divider equation Vout = Vref x (1 + R1/R2).  With all of the ANYOUT pins floated, you do not have an R2 in the circuit and are simply connecting FB to OUT through R1.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1655 points

    Hi Ryan,

    Thank you for your quick response.

    I want to make sure my understanding. You mean that the following figure is correct? In this case, R2 is disconnected. R1 is just connected. If my understanding is correct, I have one more question. How do I deiced R1 resistor value? I'd like to make the output value from 5V to 15V. In this case, is it not necessary to add the R1?

    Best Regards

    Masayuki

  • 0 in reply to Masayuki Uchihara
    TI__Guru 55940 points

    Hi Masayuki,

    Are you planning to use the LDO with ANYOUT (internal resistors) or as a traditional adjustable LDO (external resistor divider)?

    If you are planning to use ANYOUT, R1 is internal to the LDO as well so there is nothing for you to select.  To adjust the output voltage in 100 mV increments, you ground or float the ANYOUT pins as shown in table 2.

    If you are planning to use as a traditional adjustable LDO, you will float all of the ANYOUT pins and connect an external resistor divider as shown in Figure 23 of the datasheet.  Per section 7.5.2 of the datasheet we recommend setting R2 to 10 kOhm to maximize the accuracy.  Table 3 of the datasheet shows example resistor dividers for various voltages.  If you need a different voltage, you can calculate your R1 using the following equation:

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1655 points

    Hi Ryan,

    I am planning to use the ANYOUT option. Finally I can understand it. I'll just connect the FB pin to the Output. But I have other question about TPS7A47xxEVM-094 schematic. This is because it looks like ANYOUT topology but including adjustable LDO. In this case,  the output voltage is regulated by changing FB voltage using jumper. Is it correct? 

    Thanks a lot

    Best Regards,

    Masayuki  

  • 0 in reply to Masayuki Uchihara
    TI__Guru 55940 points
    Hi Masayuki,

    The TPS7A4701EVM-094 was developed to evaluate the full functionality of TPS7A47 family. If you are using it to evaluate the device using ANYOUT functionality, R1, C4, and R2 should be removed from the board. J8 should be used to short the FB pin to OUT and J10 should be used to short the ANYOUT pins to GND to select the desired voltage.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1655 points
    Hi Ryan

    That's make sense.
    Thank you so much!!

    Best Regards,
    Masayuki