OPA547: OPA547: ILIMIT NOISE ISSUE DURING THE POSTIVE VOLTAGE CYCLE

Akshay Bhounsley

Part Number: OPA547
Other Parts Discussed in Thread: OPA170

Hi,

We are using OPA547 in one of our programable power supply design where the voltage can be varied from -5V to 5V & Ilimit current is set to 200mA. In this section when ever the voltage is set in positive range (0.25-5V) & current is limited to 200mA. We are observing some current swing (it is not constant @ 200mA). but when the similar settings are done on the Negative range (0.25-5V), we are able to limit the current up to 200mA. The waveforms are also captured and attached for your reference.

Fig1: Waveforms captured during positive cycle (5V & I limit set to 0.2A)----------->Issue

As per the above figure, the current and voltage waveforms are unstable (Current waveform is varying from 0 to 400mA and the average current value is changing continuously from 170mA to 190mA) at +5V output voltage and 200mA current limit set condition.

Fig2: Waveforms captured during positive cycle (-5V & I limit set to 0.2A)------------>Working fine

The schematic is also attached for your reference:

We have also tried by mounting the C358 capacitor and by adding an snubber circuit at the output of the opamp as shown below but the issue is not resolved.

Can you please suggest us with any alternative solution to resolve this issue.

Regards,

Akshay

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

Hi Akshay,

the schematic didn't come through. Can you post again? Click the "Insert File" button (paper clip) in the header.

Kai

0 Akshay Bhounsley over 3 years ago in reply to kai klaas69

Prodigy 120 points

Hi Kai,

Please find the schematic attached for your reference.

Regards,

Akshay

0 kai klaas69 over 3 years ago in reply to Akshay Bhounsley

Guru 140200 points

Hi Akshay,

C356 presents a huge capacitive load to the OPA170 eroding its phase margin and causing instability:

akshay_opa170.TSC

Kai

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

Guru 140200 points

Decrease C356 and / or increase R1047:

R1047 = 270R results in an overshot of about 20% and means a phase margin of about 45° which is considered being the minimum phase margin for stable run :-)

Kai

0 Raymond Zhang1 over 3 years ago in reply to Akshay Bhounsley

TI__Mastermind 48076 points

Hi Akshay,

Kai recommended the fixes in your I_limit circuit. Vout waveform issues are resulted from the instability of buffer circuit that is driving 10nf capacitive load at I_limit pin3. You can remove C356 (10nf) or increase R1047 or place LP RC filter at pin3 of U109, OPA170 op amp.