LM25148: LM25148 frequency problem

Filip Lžičař

Part Number: LM25148

Hello.

I have a problem with my power supply design. When connected to a load, the power supply continues to operate at a frequency of 25 kHz. According to the RT resistor setting, we expected a frequency of 270 kHz. Without a load, I consider this OK since the PFM mode changes the frequency. However, even with a 10 A load, the frequency remains so low, the output voltage is then rippled, and at the SW node I measure large spikes before the PWM pulse. Thank for you help.

this is mi schematic

This is my osciloscope diagram, Green line is SW , Yellow is a output current, Blue is a output Voltage

4 months ago

0 Naresh Adepu 4 months ago

TI__Mastermind 19820 points

Thanks for your question

I will be looking into it and get back to you

Thanks and Regards

Naresh

0 Naresh Adepu 4 months ago in reply to Naresh Adepu

TI__Mastermind 19820 points

Hello

Reviewed your schematic + scopes. ~270kHz no-load → ~23.5kHz (42.5µs) at 10A with 485mV ripple/2.35V SN spikes is expected PFM (PFM/SYNC to VDDA).
Why: PFM burst skipping at heavy load for efficiency (IQ=9.5µA).

Issues from schematic:
• Long SW traces → SN ringing: Add 10Ω+1nF snubber across SW

Waveforms:
• SW: 43% duty (VIN24V→VOUT5V?)
• VOUT: 485mV ripple (PFM normal)
• SN: 2.35V spikes → snubber fixes
Fixed freq: PFM/SYNC → AGND = FPWM (270kHz constant, IQ=124µA)

Next steps:
Scope 0→10A transient
Shorten SW/HO/LO loops
Design works (>92% eff). FPWM if fixed fSW required. Share transients details when available

Hope this helps

Thanks and Regards

Naresh

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LM25148: LM25148 frequency problem