LMR36510: Switching Frequency LMR36510ADDAR above 250kHz

Hello

Please provide your schematic.

What is your input voltage, output voltage and load during your testing.

Thanks

Good morning,

Input Voltage: 16V up to 60V

Output Voltage: 12V

Load Current: 100mA up to 400mA

Hello

Under what conditions did you take the 'scope shot that you show here ?

Can you take a 'scope shot of the SW pin under the same conditions ?

Can you temporarily increase your output capacitance to make sure the 

design is stable in PWM ?

Or reduced your value of C25 and test again ?

Thanks

Hi, finally I was able to take some scopes:

Uin = 20V / Iout = ? / Uout = 12V / L1 = 47µH

Cout = 22µF (Yellow: Uout AC Coupling) (Blue: SWN DC-Coupling)

fsw = 85kHz

Cout = 4,7µF (Yellow: Uout AC Coupling) (Blue: SWN DC-Coupling)

fsw = 91kHz

Cout = 47µF (Yellow: Uout AC Coupling) (Blue: SWN DC-Coupling)

fsw = 118kHz

Thanks

Hello

Looks like the increased output capacitance helps.

Also, try adjustment of C25.

The load must be light since you are in PFM mode.

There is not much you can do in FPM mode to alter the frequency.

You can experiment with other values of inductance.

At higher loads you should get full frequency operation.

Thanks

Hi,

C25 is my output capacitance and yes, it helped a bit. I will increase the capacitance more and see what will happen.

Here are the experiments with the inductor:

Uin = 20V / Iout = ? / Uout = 12V / L1 = 47µH

L = 47µH, Cout = 22µF (Yellow: Uout AC Coupling):

fsw = 43kHz

L = 220µH, Cout = 22µF (Yellow: Uout AC Coupling):

fsw = 105.7kHz

L = 1mH, Cout = 22µF (Yellow: Uout AC Coupling):

fsw = 53kHz

L = 10µH, Cout = 22µF (Yellow: Uout AC Coupling):

fsw = 39.6kHz

So as you can see, there is no explainable relation by changing the inductance. 

I urgently need to achieve a higher frequency than 250kHz. My question here is, if this is even possible in PFM Mode.

But if you have any more ideas, let me know :).

Thanks and kind regards.

Hello

In the load range you specify, I don’t think you will get a constant frequency in AUTO mode.

You will need to use a device with a FPWM feature. 

Any device with a MODE pin that allows you to set the mode to FPWM, and

meets your other requirements would be OK. 

Also, some of our devices are pre-set at the factory to use FPWM mode at all loads.

Keep in mind that using FPWM at light loads will degrade the efficiency of the application.

Thanks

Hi,

thank you for your response!

I was about to give up but gave it another try:

In my last post, I changed the inductor values. The scope with the 10µH is wrong (I picked the wrong inductor from our laboraty).

Here is an update with the right 10µH:

L = 10µH, Cout = 22µF (Yellow: Uout AC Coupling, Blue: SWN DC Coupling):

fsw = 407.35kHz

It seems like, I can change the frequency by vary the inductance. Now I must proof this for every scenario.

One last question:

I may have to reduce the inductance from 10µH to 5µH. Do you see a problem by using a small inductance? The voltage ripple in the scope above is small pretty (60mV Pk-to-Pk). Is there something else to consider?

Thanks so much and kind regards.

Stefan 

Hello

As you mentioned, the lower inductance will give more ripple voltage on the output for the same

output capacitance.

Also, be sure that the minimum inductance value mentioned in the data sheet is not violated.

The frequency used in that equation is the full switching frequency at full load.  This consideration 

is only important if you plan to run at the full load of the device operating at full frequency.

Thanks

Hi,

the formula in the datasheet is: Lmin = M * Vout /fsw

With

M = 0.625

Vout = 12V

fsw = 250kHz

I need an inductance with at least 30µH. 

But in minimum load I need 5µH to achieve a higher frequency than 250kHz. So I do violate the equation. Would you consider this as critical? In the datasheet it is mentioned, that subharmonic ocillation can appear. I can't tell, if this is something that I have to avoid or not. 

Thanks and kind regards