LM5119: sync frequency value selection for 4 phase design

John Chou62

Intellectual 1960 points

Part Number: LM5119

hi Team,

customer using 2 LM5119 for 4 phase design.

we'd like to check on a synchronization problem.

The LM5119 RT is 26.4K ohm -> The operating frequency will be 190KHz.

If we used 200KHz sync frequency, the operating frequency will become 100kHz.

So, we changed the sync frequency to 400KHz, the operating frequency will be 200KHz.

Is it correct? are there any reference docs for this?

The sync and PWM waveform is shown as below:

Ch1- SYNC signal

Ch2-LM5119 low side PWM1

Ch2-LM5119 low side PWM2

Thanks very much for the help

over 2 years ago

+1 Matt Schurmann over 2 years ago

TI__Genius 11095 points

Hello, can you show how you have the RT pins connected? I am wondering if you have two RT pins in parallel leading to the lower frequency.

0 Matt Schurmann over 2 years ago in reply to Matt Schurmann

TI__Genius 11095 points

Hello,

After researching some more (this device is still new to me), I believe it should be normal that the switching frequency is 1/2 of the sync frequency. LM5119 has an internal oscillator which runs at 2x Fsw. Synchronizing on the RT pin uses a PLL to synchronize the Sync signal to the internal oscillator. So, synchronizing to 500kHz switching frequency, for example, would require a sync frequency of 1000kHz

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LM5119: sync frequency value selection for 4 phase design