TPS552882: Instability issue with TPS552882 RPMR

Hi Nasib,

Thanks for reaching out.

What is the input voltage range? Only 5V? Or 5-20V?

Do you mean when Vin=5V, and Vou=4~7V, this device starts to oscillate?

May I know what is the part number and ESR of output capacitors? We need the effective capacitance and ESR value to check the compensation parameters.

BRs,

Bryce

Thank you Bryce for your fast response.  

Yes Vin is always stable 5v and the instability starts when Vout is around 7V to 4V.

C415 to C418: C2012X5R1V226M125AC

Regards

Hi Nasib,

What is the output current? About the schematic:

1.Add a 0.1uF 0402 ceramic capacitor in Vin and Vout pin and put the as close as to the device.

2.Recommend compensation parameters: R417=1kohm, C419=22nF, C420=22pF.

This looks like loop stability problem. Please modify compensation parameters and see if it gets better.

Regards,

Mulin

i testes it in open circuit ( no load ) and with 400mA and I see the same result 

Hi Nasib,

You got the same result with compensation parameters changed?

Regards,

Mulin

No, i didnt test it yet, i will do it as soon as i get the new components 

The stability issue is now solved with your tip. But I see a clear limitation in the current output, the output voltage decreases when the output current increases, even before of reaching the current limitation value . Why is this happening ? 

Hi Nasib,

What is the output current and Vin/Vout when you see output voltage decrease? Since R409= 40mohm, Iout limit=50m/40m=1.25A. What is your target maximum current? Try to reduce R409.

Regards,

Mulin

My Ilim is 1.25 As you said. But even after 0.4A with Vi=5V and Vo=5V when i pass that current the voltage slightly decreasses ... 
The same happens with any Vo. It can go from Vo = 5V with Io=0.1A to Vo =4.7V with Io=0.6A

On the other hand, i have tested the system with different values of the potentiometer used under R412:

(In Kohm)

And the measured voltages are these:

I dont understand why sometimes the voltage does not change at all. 

Thank you so much 

Hi Nasib,

1. What is the current limit of the power supply? What is the output current every time you did the test?

2.Can you help check FB voltage/waveform?

3.Can you try to directly connect R412 to GND and see if the output is normal?

4.Can you provide inductor current/SW1/SW2/Vout waveforms?

Regards,

Mulin

I will provide you with that Info asap. The strange thing is that for Vo below to 10v everything if fine for every value of the potentiometer.

Hi Nasib,

Please help share the feedback Mulin requested last time. We will share our insights when getting the feedback. Thanks.

BRs,

Bryce