LM5026: Can't synchronize with external clock

zixuan zhao

Part Number: LM5026

The circuit is designed as the circuit in the datasheet. The part of Q7 is SSM3K02T from Toshiba，C32 is 33pF，R35 is 10k.

The pin 3 CS of LM5026 is connected to GND, pin 13 COMP is floating . As a result, the duty cycle of pin 8 outputA is maximum. Without the external clock, the frequency is about 350kHz.

When apply a signal to the Gate of Q7, the frequency of is not changed. The signal is 5V, 443kHz and provided by a signal source. The waveform is below

CH1 is the external clock, CH2 is the pin 8 OUTA of LM5026, CH3 is the pin 15 SYNC of LM5026. The input external clock is 443kHz, however the ouput frequency is not changed, remaining about 350kHz. It seems the LM5026 is not synchronized with the external clock. From the CH3 waveform of Oscilloscope , the voltage sometimes is below 0V which is confused.

As the clk of LM5026 is unable to be seen, I have no idea about what is happened in the LM5026. Please help me with the problem ,thanks.

over 3 years ago

0 Teng Feng over 3 years ago

TI__Genius 13660 points

Hi Zixuan,

I have no the EVM on the hand.

Can you please help capture the sync pin waveform at the time without sync signal?

and I suggest that the the pulse width of sync signal should be smaller than the off-time. please use the sync signal with the 50ns pulse width.

Regards

Teng

0 zixuan zhao over 3 years ago in reply to Teng Feng

Prodigy 75 points

0 zixuan zhao over 3 years ago in reply to zixuan zhao

Prodigy 75 points

CH3 is the waveform of sync pin without external clock

0 zixuan zhao over 3 years ago in reply to Teng Feng

Prodigy 75 points

I also tried to connect sync pin to gnd and no use to change the frequency

0 zixuan zhao over 3 years ago in reply to Teng Feng

Prodigy 75 points

Thanks for your reply.

Today I tried to change the resistor on the RT pin to change the frequency.

I found that the LM5026 can synchronize a external clock，but the frequency is limited . When the frequency of LM5026 set by the RT pin is about 210kHz，the LM5026 can synchronize with a external clock which the max frequency is about 230kHz. I also tried several frequency of LM5026，from 100k to 210k，and at these frequency LM5026 can synchronize a external clock. But the max frequency of external clock is about 220kHz to 230kHz.

Can I think that LM5026 can synchronize with a external clock，but max external clock frequency is limited to about 230kHz？ Can you also try in your lab to see whether LM5026 can synchronize a external clock above 230kHz.

0 Teng Feng over 3 years ago in reply to zixuan zhao

TI__Genius 13660 points

Hi Zixuan,

The sync frequency shouldn't be limited.

What is pulse width of the sync signal?

did you try my before suggestion that please use the sync signal with the 50ns~100ns pulse width.

Regards,

Teng

0 zixuan zhao over 3 years ago in reply to Teng Feng

Prodigy 75 points

I have tried the sync signal with the 50ns~100ns pulse width，but it is no use. And I have found that with switching rising ，the pulse width should be decrease，otherwise the sync function failing

0 Teng Feng over 3 years ago in reply to zixuan zhao

TI__Genius 13660 points

Hi Zixuan,

Please try the on time of sync signal larger than off-time of PWM output.

and my email is teng-feng@ti.com

I am closing this thread, because it is opened for too long time.

Thanks,

Teng

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LM5026: Can't synchronize with external clock