TPS74401: Feedback resistor R2 value

Hi Yarn,

See the note on page 20 of the datasheet, under table 3. If voltage is present on BIAS and EN and no voltage on IN, then 50 uA of current will flow out of the OUT pin. This current will flow through R1 and R2, creating a voltage at the OUT pin. To keep this voltage on the OUT pin below 0.5 V, the sum of the feedback divider resistor values is limited to 10k ohms.

This is the primary reason for requiring R2 <= 4.99k ohms.

Hi, Eric:

Thanks a lot for your explanation. Customer connects EN and BIAS to VIN. So the limition will not be vaild?

Thanks
Yarn

Hi Yarn,

The value of R2 and the FB pin current contribute an error term to the output voltage, so this is another reason for keeping R2 small. See equation 20 in app note SNVA112: www.ti.com/.../snva112a.pdf, where IADJ = IFB.

If the output error term from R2 and FB pin current is acceptable, then the customer can use larger value resistors to decrease the feedback divider current.