Isolated resistor and feedback resistor value recommendation

Stanley Dai

Part Number: OPA2180

Hi Team,

Please help recommend suitable isolated resistor(R79) and feedback resistor(R47) value regarding to below customer's application. Many thanks.

Best Regards,

Stanley.

over 3 years ago

0 Michael Steffes over 3 years ago

Guru 47535 points

Hey Stanley, putting that R79 inside the loop is a bad idea, just creating another loop pole - putting it outside the loop works a lot better where then R47 can be zero ohm.

The load C looks like 1uF ?

Maybe an R79 outside the feedback loop of 20ohm would be adequate for good phase margin without excessive bandlimiting to the load C.

0 Raymond Zhang1 over 3 years ago in reply to Michael Steffes

TI__Mastermind 48076 points

Hi Stanley,

Here is a loop stability plot Michael suggested:

If the Vref's resistitive load is high, then you will have tighter 1.5Vref. You may have to adjust R4 value accordingly, somewhere around 500Ohm range with 1uf capacitive load. As it is presented, you have approx. 78.8 degree of phase margin. Enclosed is the simulation.

/cfs-file/__key/communityserver-discussions-components-files/14/OPA188-HZ-10192020.TSC

If you have additional questions, please let us know.

Best,

Raymond

0 Stanley Dai over 3 years ago in reply to Raymond Zhang1

TI__Expert 5810 points

Hi Raymond,

Thanks for detailed explanations. Customer wants to further reduce DC error caused by Riso resistor, so they want to adopt below design. Can you please recommend suitable RF and CF value here? Thanks for your help.

Best Regards,

Stanley.

0 kai klaas69 over 3 years ago in reply to Stanley Dai

Guru 140200 points

Hi Stanley,

have you watched this very nice training video?

Kai

0 Raymond Zhang1 over 3 years ago in reply to kai klaas69

TI__Mastermind 48076 points

Hi Stanley,

Q: Can you please recommend suitable RF and CF value here?

With the following configuration, your BW of the circuit is approx 1.25kHz. If you are using the 1.5V as voltage reference, may be it is adequate.

With 1khom load, you will have approx 64 degree of phase margin, see the simulation. If R5 is reduced to 200-300 ohm range, you may increase the loop-gain slightly, but your phase margin will be reduced.