Request for the Passive POE circuit

Hello,

Here are some comments and recommendations for the LTC7800 converter:

1. Per the schematic Vout is 12V but the FB divider is set for 3.3V. If Vout is 12V, correct the feedback divider by using a 499K / 35.7K divider or other divider with the same ratio.
   
   
2. What is Iout(max) of the LTC7800 supply? Per the schematic, it appears to be about 4A:
   Iout(max) = Vsense(max),min / Rsense – Ipp/2
   Vsense(max),min = 43mV with ILIM floating
   Rsense = 10mOhms
   Ipp = 0.2A for original L and Fsw and 12Vout at Vin=48V
   Iout(max) = 43mV / 10mOhms – 0.1
   Iout(max) ~= 4.0Amps
   
   
3. If Vout is 12V, tie Vout to EXTVCC to minimize losses in the controller. An example of this is shown in Figure 15. Keep in mind that the maximum rating of the EXTVCC pin is 14V. Also consider reducing Fsw to 500kHz (RFREQ = 34K) or less to reduce losses in the controller. With a 500kHz Fsw, use a ~15uH inductor, assuming Iout(max) is 4A. As a guideline, the inductor’s ripple current should be around 30% of Iout(max). 
   
   
4. If Vout = 3.3V, set Fsw to 350kHz (RFREQ = 27.4K) to reduce the losses in the controller and to avoid hitting the minimum on-time. With a 350kHz frequency, use a ~7uH inductor, assuming Iout(max) is 4A.
   
   
5. Change R31 (in series with SNS+ pin) from 100 Ohms to 10 Ohms to avoid over-filtering the current sense signal.
   
   
6. Change Q1 to BSC094N06LS5 to match Q2. Both Q1 and Q2 are the top MOSFETs.

Given that passive POE is used, make sure the ethernet cables & 48V rail can supply the necessary current. Also check the power losses and temperature rise of the bridge diodes (U1 & U3) with full load and Vin=36V (min) to make sure they will not get too hot.

Best regards,

Mike

Hi. Mike

Thanks for letting me know what I missed.

Since you need "48V rail can supply the necessary current",

I am going to use a POE injector that supports 45W to conduct a board test.

Seeking advice on how to test.

Hello Jerome,

I apologize for the late reply. Here are a few points to keep in mind for your tests:

1. The LTC7800 is not a compliant Power Device. Therefore, the POE injector will not recognize the LTC7800, if it does interrogate it. As a result, the LTC7800 may not receive power. I do not know what you are using for the POE injector so I can not say for sure. You will need to check the data sheet.
2. The divider on the RUN pin is setup for a Vin UVLO of 13V. If the LTC7800 will be turned on with Vin, then increase the UVLO to a value such as 30V, roughly, by changing R25 to 237K. This will provide a smoother turn-on. This will also reduce the maximum current flowing through the cables if the POE voltage goes low.
3. Even though the POE injector can supply 45W, the maximum power provided by the LTC7800 converter will be less than 45W due to drops across the ethernet cable and the efficiency of the LTC7800 converter itself.

Best regards,

Mike