Enquiry related LTC4271/ LTC4290 PSE controller

Hi,

The LTC4271/LTC4290 is an IEEE 802.3at PSE. The max power draw for each port in IEEE 802.3at is 30W, so 30W x 8 is 240W is required to power all the ports. The maximum power may be less if some of the ports only support IEEE 802.3af. IEEE 802.3at also requires 51V_min supply, so that is 4.71A max of supply current minimum.

VEE is treated as a negative supply, but Power over Ethernet requires that this supply is isolated from the chassis or any external conductors. Terms such as positive or negative have less meaning in an isolated system. PoE PSEs switch the negative rail, so the positive rail makes more sense as the reference.

Please use an isolated 51V to 57V supply capable of sourcing 4.73A, with the positive terminal to AGND and the negative to VEE.

The LTC4290 chip does not consume much current. The VEE current consumption of the IC is typically 9mA, and 15mA max.

Best Regards,

Eric

Hi Eric 

Thanks for response.

As per our requirement i have checked the datasheet and found that it is supporting three power grades.

We will use type c grade i.e. 13 W and also IEEE 802.3at Type 1

We are going to attach 5 PoE cameras with that thus total requirement is 13Wx5 is 65W. According to the datasheet for type1 voltage range is 45V to 57V

That is why i am using 48 V supply regulator.As per your reply supply current is our case will be 1.35A

Please suggest whether we go with negative regulator to supply voltage or positive boost?

Thanks & regards

Pallavi 

Hi Pallavi,

The C-grade is an IEEE 802.3af, or IEEE 802.3at Type 1 PSE sources 15.4W per port. So your calculation should be 5 x 15.4W = 77W. If your 48V regulated supply is isolated, as are most AC to DC converters, then you can power the LTC4290 directly with the 48V rail as long as it stays above 45V under all conditions. Positive goes to AGND and negative goes to VEE. If you maintain the isolation barrier between the LTC4271 and LTC4290, then you should use a flyback converter like LT8303 to power the LTC4271's VDD33 rail.

Best Regards,

Eric

Hi Eric,

Thanks for reply.

We are using fly-back controller LT3748 to supply 48 V to LTC4290 as per PoE electrical standards.

But we are not using any flyback converter to supply VDD33 we are using LTC3891 regulator as 3.3 V is required at other places too in our board.

Is there any help we can get for complete designing of this structure from supply to its output?

Hi Eric,

Thanks for reply.

We are using fly-back controller LT3748 to supply 48 V to LTC4290 as per PoE electrical standards.

But we are not using any flyback converter to supply VDD33 we are using LTC3891 regulator as 3.3 V is required at other places too in our board.

Is there any help we can get for complete designing of this structure from supply to its output?

Hi Pallavi,

You can use a non-isolated supply to derive 3.3V from the PoE's 48V rail as long as those circuits do not connect to the chassis ground or any external connectors aside for an Ethernet cable. Usually the LTC4271 and other digital circuits may connect to the chassis, so I would recommend using an isolated supply to convert the PoE's 48V to 3.3V, something like the LT8304.

Best Regards,

Eric

Hi Eric

Thanks for response.

I am working on LT3748 flyback controller to design supply for LTC4290.

I have studied datasheet but haven't found any reference to design PoE supply.

We have input supply range from 6-36V but 12V/24V is fixed input supply and required output is 48V/2A (Exact output load current will be 1.35A or 1.65 A)

Can we design this using LT3748 ? We are going to use 750311591 (wurth electronics) transformer as suggested in datasheet .

Do we get any reference design as per my requirement?

Please reply soon. 

Hi Pallavi,

I do not know if the LT3748 would be well suited for a 48V / 2A design. You may need a forward converter at this power level.

Check out LT3753. Its demoboard DC2306A is close to what you need, or you can read this article which describes the part and a 54V / 3A solution.

https://www.analog.com/en/technical-articles/lt3753-9v-15v-input-to-54v-3a-isolated-forward-converter.html

Best Regards,

Eric