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TPS730: TPS73001 Cff and feedback resistor questions

BX27
Genius 3870 points
Part Number: TPS730

Hi there,

We are using two TPS73001 to generate Vout1=5V and Vout2=2.5V from 5.5V Vin, less than 50mA load for each Vout. Some questions to consult.

1. In datasheet section 8.1.1, the recommended feedback resistor R2 is 30.1K to set divider current 50uA. Could I use other value? 

2. How does equation(3) come for determining CFF value? 

3. The recommended max. Vin is 5.5V in datasheet section 6.3, and our Vin=5.5V, no margin left. Bu I think there should be no problem, right?

4. The PSR curve is shown on datasheet figure7, but its Vin is 3.8V. Any way to get PSR information under our real condition? Unfortunately I found this part is not supported in Webench tool.

5. Datasheet section 8.1.15 mentioned "If extended reverse voltage operation in anticipated, external limiting to 5% of the rated output current is recommended " . If I reversely parallel a Schottky diode for reverse current protection( diode anode connect to IC Vout), should I choose this diode I(F) to be 5% of 50mA ? this seems to be too small.

Thanks very much!

  • 0
    TI__Guru 55940 points

    Hi Yi,

    1. You are able to use a different value; however, as stated in the section 8.1.1 a larger resistor will degrade accuracy while a smaller resistor will increase power consumption.
    2. You can use other value feedforward capacitors; however, we recommend using the equation to calculate your capacitor.  This equation helps to select a capacitor for improved stability by selecting at which frequencies the capacitor will impact the phase margin.
    3. Operation at 5.5V is still within the recommended operating conditions for this device.  You have ample headroom for both rails so if you wanted to save power, you could reduce the input to a lower voltage.
    4. We do not have PSRR curves available at your specific application; however, your conditions will yield similar to the 10mA curve from figure 7 due to your low current and ample headroom.
    5. You will want to size your diode to handle all of the reverse current.  Placing the external diode provides a robust path for reverse current protecting the LDO from damage from the reverse current.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Genius 3870 points

    Hi Ryan,

    Thanks for reply. Some further discussion.

    2. Actually my question is, how did this equation derived?

    5. I mean, since the max. reverse current could not be known before on-board test, how should we choose a proper Schottky diode parameter based on spec 8.1.15 description : "external limiting to 5% of the rated output current is recommended" . Will a general schottky diode with IF(max)>50mA*5% be ok ? Is there other consideration in choosing diode? Of course the VF should be as low as possible.

  • 0 in reply to BX27
    TI__Guru 55940 points
    Hi Yi,

    The equation sizes the capacitor so that the pole of the feedforward capacitor occurs at a specific frequency. The zero caused by the feedforward network occurs before this pole. More information can be found in section for of this application report: www.ti.com/.../sbva042.pdf

    Assuming that you are not going to externally bias the output of the LDO higher than the input, the cause of a reverse current situation would most likely be Vin collapsing faster than the output capacitors can discharge. You will want to size your diode so that it can handle the discharge of your output capacitors.

    Very Respectfully,
    Ryan