LM5146: Zero-cross detect understanding

Luffy Wang

Part Number: LM5146

Hi,

I use the LM5146 in DCM mode and have some questions to be clarified. Datasheet shows the diode emulation zero-cross threshold is [-5, 0, 5]mV. I thought the zero-cross threshold should be near to 0mV under room temperature with normal condition. I tested the LM5146EVM with 48V input under no load. The real ZC currnet is near to -500mA, which is out of the range of [-5mV/13mΩ, 5mV/13mΩ]. The same to another evm.

1. Could you please help to explain why this happend? and what's the typical value from mass production for this spec?

2. Also, I want to know what's the means of the other two specs regrading the zero-cross detect.

BRs,

Luffy

over 2 years ago

0 Orlando Murray over 2 years ago

TI__Mastermind 22301 points

Hi Luffy,

Can you probe with SW and LO? It could be that you are seeing the ringing on inductor current.

Also make sure your probe is de-gaussed and zero'ed, and the current loop should be on VOUT side, not SW side.

Diode emulation is automatically enabled for soft-start, even in FPWM mode. For FPWM mode the threshold is gradually ramped up 200mV until disabled.

Hope this helps,

-Orlando

0 Luffy Wang over 2 years ago in reply to Orlando Murray

Prodigy 55 points

Hi Orlando,

Thanks for your support here. Attached the SW and LO waveform as below. It looks like the zero-cross detect threshold is near or even lower than -5mV.

We know the DCM opeartion has higher conversion efficiency under light load than FPWM mode. The switching frequency in DCM would be the same to FPWM. Is there any case that the LM5146 would work with lower switching frequency in DCM? I want an fixed switching frequency running in DCM, that's why I need know the real zero-cross detect threshold range.

+1 Orlando Murray over 2 years ago in reply to Luffy Wang

TI__Mastermind 22301 points

Hi Luffy,

The oscillator frequency will not change in DCM.

There may be cases of skipped pulses in very light load, however I dont think thats the case with this device.

I'll have to double-check this in lab tomorrow.

-Orlando

0 Luffy Wang over 2 years ago in reply to Orlando Murray

Prodigy 55 points

hello, anyupdate here? thanks

+1 Orlando Murray over 2 years ago in reply to Luffy Wang

TI__Mastermind 22301 points

Hi Luffy,

This device will not change switching frequency no.

Hope this helps.

-Orlando

Power management

Power management forum

LM5146: Zero-cross detect understanding