LM5022: LM5022

Genie Hsu

Part Number: LM5022
Other Parts Discussed in Thread: PMP20497

Dear Sir, PoE Power Schematic(LM5022).pdf

There is a TI reference design, PMP20497 made by LM5022. I have a question here. I calculated the current passing throguh photo diode (U2), and got 103.4mA [=(54-1.17-1.24)/499], 1.17V is forward voltage of photo diode of U2, 1.24V is the minimum voltage (Vcathod-anode) of U3.

By multipling CTR (=200%~400%) of U2, then getting the current sourcing from U1 pin 3 is 206.8mA~413.6mA. I checked LM5022 datasheet, the maximum souring capability of LM5022 pin 3 is 1.5mA, according above calculation, actually demanding current (206.8mA~413.6mA) is far above 1.5mA, why? Do I make any misunderstanding?

over 4 years ago

0 Garrett Roecker over 4 years ago

TI__Mastermind 21720 points

Hi Genie,

Thanks for reaching out with you questions and for considering the LM5022.

Note that when the FB pin is pulled to ground the error amplifier out put (COMP, pin 3) is disabled. The voltage on the COMP pin is just going to the PWM comparator.

During normal operation the voltage across U3 is going to be higher than 1.24V. the 1.24V is just the minimum operating voltage. So the opto-coupler is not going to pull that much current.

Please let me know if you have any questions.

Thanks,

Garrett

0 Genie Hsu over 4 years ago in reply to Garrett Roecker

Intellectual 360 points

Thanks, Garrett.

Power management

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LM5022: LM5022