TPS61021A: EVK Added Unknown Resistor to Schematic Compared to Datasheet

Lim Phang Moh

Part Number: TPS61021A

I notice both datasheet and Webench doesn't have a resistor R3 (49.9 Ohm) connected to the feedback resistor R1. I would like to know what is the purpose of this resistor as it was not discussed in the datasheet but does appear in the EVK schematic and board.

Something to do with the improving the stability together with the feedforward capacitor C7?

Thank you in advance.

over 7 years ago

0 Helen chen over 7 years ago

TI__Mastermind 40080 points

We droop the 30-50mV sine signal across this resistor to test the loop stability.

0 Lim Phang Moh over 7 years ago in reply to Helen chen

Intellectual 270 points

Hi Helen,

Thank you for the prompt response.

It means, we can omit this resistor from the design unless we wanted to test for loop stability?

Am I right?

0 Helen chen over 7 years ago in reply to Lim Phang Moh

TI__Mastermind 40080 points

Yes, you can omit it. But if you need to test loop stability, then you need to add it back.

0 Lim Phang Moh over 7 years ago in reply to Helen chen

Intellectual 270 points

Okay.

On top of that, it is advisable to use 0.47uH instead of 1uH for faster response when use with burst of current even though the output current needed is less than 150mA right? I found that in our initial design using 1uH, we weren't able to go below 1.2V of input voltage although output current needed is only less than 50mA (short burst of current during radio transmission). We do have bulk output capacitor of 47uF on the regulator output plus another 2 on the radio side.

0 Helen chen over 7 years ago in reply to Lim Phang Moh

TI__Mastermind 40080 points

So your total output cap is 141uF, right?

0 Lim Phang Moh over 7 years ago in reply to Helen chen

Intellectual 270 points

Yes. 1x 47uF exactly at the TPS61021A output and another 2x 47uF at the radio's VCC.

0 Helen chen over 7 years ago in reply to Lim Phang Moh

TI__Mastermind 40080 points

Please parallel a 47-68pF cap with the upper side resistor. if the upper side resistor is in the range of 200k-400k

0 Helen chen over 7 years ago in reply to Helen chen

TI__Mastermind 40080 points

what I mean is the upper side dividing resistor.

0 Lim Phang Moh over 7 years ago in reply to Helen chen

Intellectual 270 points

But, isn't the feedforward capacitor value is calculated as:

C = 1/(2*π*fFFZ*R1)
where fFFZ = 5 kHz if output capacitor is > 40 uF. R1 is 316kR in this case. So the C results as 100 pF (quite exactly). Or you have some tweaking based on simulation?