TPS40210 boost converter instability in light load.

[grizedale]

Hello,

We are doing TPS40210 boost converter.

TPS40210 DATASHEET:
https://www.ti.com/lit/ds/symlink/tps40210.pdf

Vin = 12V

Vout = 19V

Iout(max) = 1.62A

F(sw) = 500KHz (Sync'd with diode method)

C(in) = 20uF ceramic

C(out) = 20uF ceramic, then 1.5uH series inductor, then 10uF ceramic capacitor.

Current sense resistor = 10mR

Upper feedback divider resistor = 61.9K

RC series network connected round error amplifier = 4.99K, 22nF

(a 47pF ceramic cap is across this RC series network)

Boost inductor = 10uH.



We are OK in max or high load, but in light load, we are seeing an instability involving a ripple on the Vout of frequency about 5KHz.

-Do you know why this is?

 

[mtwieg]

How light of a load is it? Is it enough to make the converter operate in DCM?

 

[grizedale]

yes, it could go to dcm.

But dcm isnt a problem surely.........any converter compensated for ccm, will always be stable if it goes to dcm

-is it the mimumum on time of 300ns which causes us problems?

 

[mtwieg]

-is it the mimumum on time of 300ns which causes us problems?

That's possible. With your specs, with a ton of 300ns, you will be drawing about 0.325 from the source, and if your load doesn't dissipate that much (minus some for conversion losses), your output voltage will rise until... something happens. I can't tell from the datasheet what happens with your controller when overvoltage occurs. It doesn't explicitly mention any OVP.