TPS61240-Q1 Output Voltage at light load by Low Temperature

Hello,

I have not come across such issues so far with this device and i do not expect device to degrade significantly at -40C.

* Are you setting the temperature (-40C) of the device alone or the output capacitors and inductor are also at -40C? At cold temp certain capacitors can have very large ESR and cause such issues.
Is this happening on every device or only on few devices?
* Can you check and let me know whether in normal mode (PWM) mode, do you see the same problem?
* Did you check the behavior with 1uH inductor to see whether this issue goes away?
* Very importantly, is your measurement set-up same in room temp and at cold temp? Measurement setup can cause significant difference while doing the ripple measurement. for ripple measurement, you have to measure the voltage directly across Cout with very short connection to oscilloscope probe tip and its GND connection. See the below link on how to measure the ripple.
e2e.ti.com/.../2243.understanding-measuring-and-reducing-output-voltage-ripple
* If you have already checked above things, can you send me your layout diagram and send SW (L pin) waveform, along with Vin and Vout?

Regards,

Murthy

Hi, Muthy

* Are you setting the temperature (-40C) of the device alone or the output capacitors and inductor are also at -40C? At cold temp certain capacitors can have very large ESR and cause such issues.
Is this happening on every device or only on few devices?

Every device.

 * Can you check and let me know whether in normal mode (PWM) mode, do you see the same problem?

In PWM mode,  both of them are so impartial.

 * Did you check the behavior with 1uH inductor to see whether this issue goes away?

Why does inductance value influence?

* Very importantly, is your measurement set-up same in room temp and at cold temp? Measurement setup can cause significant difference while doing the ripple measurement. for ripple measurement, you have to measure the voltage directly across Cout with very short connection to oscilloscope probe tip and its GND connection. See the below link on how to measure the ripple.
e2e.ti.com/.../2243.understanding-measuring-and-reducing-output-voltage-ripple

Both measurement is being drawn out and performed by a lead line.
I find out that it isn't right, but though it's the same condition, it's a problem that the difference goes out.

* If you have already checked above things, can you send me your layout diagram and send SW (L pin) waveform, along with Vin and Vout?

Please wait a moment.

Best Regards,

Hi,

See my comments below:

* 1uH inductor is to just try to see whether your inductor has any issue at cold temp. There is no reason why this device should not work at 2.2uH, but just that device is more optimized for 1uH.

* When I look at the ripple frequency, it is same between room temp and cold temp and hence device switching frequency is not changing and this indicates that device behavior did not change from room temp to cold temp.

* I mostly suspect measurement issue or high ESR of output capacitor at cold temp.

* If you could send me the requested waveforms, that will help to analyze. Also send me the layout diagram and inductor, capacitor datasheets.

If you want, you can email me: kmurthy.hegde@ti.com

Regards,

Murthy