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LT4275 PoE controller PWRGOOD not turning on

Category: Hardware
Product Number: LT4275

Hello,

I am working on a PoE circuit designed around the LT4275 controller and the LT3748 converter. When I don't use the PWRGD pin, but instead use the 2 resistors in the voltage divider at the EN pin of the LT3748, the circuit works as expected (and I read a voltage of around 1,5V on the PWRGD pin) . However, when I just use the PWRGD signal (removed R2 and R3, added R1), the circuit does not work. I see the detection and the handshaking happening, but after around 500 ms the input drops back to 0V and restarts the handshake over and over. HSGATE does charge up to approximately 10V above HSSRC, but the PWRGD pin stays low. I am suspecting a problem when the Maintain Power Signature (MPS) signal gets sent and because PWRGD is low, the LT3748 does not turn on and as such there is no real 'load' to draw enough current for the MSP signal.

Does someone have an idea how to solve this?