LV2862: with PoE issue

Cheng Andy

Prodigy 130 points

Part Number: LV2862
Other Parts Discussed in Thread: LMR36506

Hi sir,

I use the LV2862 application on the PoE step-down circuit.

Vin = 57V / 0.028A Vout = 3.3V / 0.4A

I use TI Webench to simulate circuit parameters, but the Ethernet does not work properly.

So I changed the capacitance of Cin / Cout, when Cin = 10uF/100V Cout = 22uF*4, the network can operate normally.

I have the following questions to ask,

1. Is this solution possible? Will there be other risks?

2.Why refer to the Webench value, which will cause the network to not operate normally. LV2862.pdf

Cin	L1	Cout	Vout	network
10uF	27uH	22uF	3.3V	No connection
20uF	27uH	22uF	3.3V	No connection
30uf	27uH	22uF	3.3V	No connection
10uF	47uH	22uF	3.3V	Internet intermittent
20uF	47uH	22uF	3.3V	Internet intermittent
30uf	47uH	22uF	3.3V	Internet intermittent
10uF	47uH	22uF*4	3.3V	Network OK

over 4 years ago

0 Andy Chen over 4 years ago

TI__Mastermind 33625 points

Hi Andy,

LV2862 has two freq version 770KHz and 2.1MHz, which one did you use?

B R
Andy

0 Cheng Andy over 4 years ago in reply to Andy Chen

Prodigy 130 points

Hi Andy Chen,

I use LV2862YDDCT (2.1MHz).

Best Regards,

Andy

0 Andy Chen over 4 years ago in reply to Cheng Andy

TI__Mastermind 33625 points

Hi Andy,

For 57V to 3.3V at 2.1MHz, the high-side ON time is only about 30ns which is far below the Ton-min 80ns, so the switching freq may be unstable with load variation.

770KHz version is just OK for this conversion.

B R

Andy

0 Cheng Andy over 4 years ago in reply to Andy Chen

Prodigy 130 points

Hi Andy,

Thanks for your information

How should I design this circuit using LV2862X 770KHz.

How to choose Cin / Cout / inductance.

Andy

0 Andy Chen over 4 years ago in reply to Cheng Andy

TI__Mastermind 33625 points

Hi Andy,

LMR36506 should be better choice with higher input margin and lower Ton-min, pls check if this can meet your application requirement.

0 Cheng Andy over 4 years ago in reply to Andy Chen

Prodigy 130 points

Hi Andy,

Thanks,For the PCB space only LV2862 package can be used.

If I use LV2862X 770KHz ,can I change the Cin capacitance to 10uF/100V?

Cin

Cout

Vout

10uF

47uH

22uF*4

3.3V

Andy

0 Andy Chen over 4 years ago in reply to Cheng Andy

TI__Mastermind 33625 points

Hi Andy,

OK, you can use LV2862X version.

Cin 10uF 100V is OK as the switching duty cycle is very small, normally there should be additional 0.1uF 100V on Cin for high freq bypassing.

B R
Andy

0 Cheng Andy over 4 years ago in reply to Andy Chen

Prodigy 130 points

Hi Andy,

Thanks.

Power management

Power management forum

LV2862: with PoE issue