LM5017: questions about LM5017 circuit

Neo.Wang

Part Number: LM5017

Hi TI experts,

I have questions for below LM5017 circuit.

1. CR28: initially the FB node is biased through CR28 to be higher than Vref, then FB node voltage drops lower than Vref after C22 is charged. At which point the switching starts?

If it's right at FB drops lower than Vref, how the smooth soft start can be achieved?

2. What's the purpose of CR41 in below schematic?

over 1 year ago

0 Timothy Hegarty over 1 year ago

TI__Guru 73052 points

Hi Neo,

My understanding is that these are circuit additions to provide soft-start functionality.

See Fig. 7.3 in the datasheet:

Regards,

Tim

0 Neo.Wang over 1 year ago in reply to Timothy Hegarty

Intellectual 930 points

Hi Tim,

I did not understand how the soft start can be achieved with this circuit.

In my understanding, in figure 7-3, initially D is conducted and FB is higher than Vref. When C1 is charged and node-B voltage drops, thus FB node voltage drops to lower than Vref, then LM5017 turns on. But at this moment the FB voltage is close to Vref, which requires high duty cycle. So, how to do softstart?

Also, CR41 in my schematic is not shown in LM5017 datasheet. What's the purpose of this didoe?

Thanks

Neo

0 Timothy Hegarty over 1 year ago in reply to Neo.Wang

TI__Guru 73052 points

Neo,

FB is always close to VREF, and there is current coupled into the feedback divider such that the voltage across the upper FB resistor gradually increases.

Your schematic also has a similar circuit from ISO_3_3V, which I assume is the output voltage.

Tim

0 Neo.Wang over 1 year ago in reply to Timothy Hegarty

Intellectual 930 points

Hi Tim,

Thanks for clarification on this.

Neo

0 Timothy Hegarty over 1 year ago in reply to Neo.Wang

TI__Guru 73052 points

No problem, thanks Neo.

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LM5017: questions about LM5017 circuit