LM5141: the output of the lm5141 is abnormal

cickey xu

Part Number: LM5141

Dear TI technical colleague，

my customer use lm5141 to make a buck circuit，it is from 24v to 3.5v/20A.but the output voltage is abnormal。the abnormal case as below：

1.when there is no load,the output voltage is normal,but the vo ripple is very high,about 800mv;

2,when i add a 5A load,the vo decrease to about 2.8v.measure the sw ,the low_side mosfet gate and high_side mosfet gate wavefrom,the wavefrom is very bad.it seems

to be mixed up with other frequency waveform. some test point waveform as below;

the schematic as below:

please help me why it like this.the case is very critical.please help me to resolve it.

thank you

best regards

cickey xu

over 7 years ago

0 Terry Allinder over 7 years ago

TI__Expert 3545 points

Cickey,

Thanks for your inquire about the LM5141. I agree the waveforms you attached look very strange. Based on the schematic the oscillator frequency should be 440kHz, but the switching waveforms look like the LM5141 is running at a much lower frequency. On one oscilloscope plot can you show me the HO and LO waveforms so I can see the timing?

Can you short the RT pin to ground and see if the oscillator runs at 440kHz.

What happens when you increase Vin to 12V, the plots are with Vin 8V. Also can you send me a scope plot with a 5A load.

Terry

0 Ethan.wang over 7 years ago in reply to Terry Allinder

Prodigy 20 points

As cickey said,LM5141 cant work normally.We test some waveforms when Vin=8V and 12V.Waveforms as below.

0 Terry Allinder over 7 years ago in reply to Ethan.wang

TI__Expert 3545 points

Cickey,

Use a dual channel oscilloscope to make the measurements; I need to see the timing of HO and LO on the same graph so I can see the timing. Also I asked you to remove the RT resistor and ground it, then monitor HO and LO to see if the oscillator frequency is 440kHz. From the waveforms it looks like your oscillator frequency is not correct. Please add a 5A load to see if the waveforms change.

Terry