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LTC3877EUK#PBF Power good signal is not stable

Category: Hardware
Product Number: LTC3877EUK#PBF

Hello Team,

I am using LTC3877EUK#PBF and LTC3874IUFD#PBF in parallel combination.

The Power good of LTC3877EUK#PBF is connected to the Fault 0 and 1 of LTC3874IUFD#PBF as recommended in the application circuit.

But the Power goog signal will stay High for some time and goes low and this sequence is repeating. could you please let me know what could be the reason for this?

Hence the Power Good of the LTC3877 is connected to the Fault 0 and 1 of the LTC3874 the output of the LTC3874 is following the Power good High and low.

When I disable the LTC3874 the Power good of  LTC3874 stays high and the output is also proper.

When I removed the Mosfet (2N7002) that is mentioned in the application circuit the Power Good also stays high and the output also proper.

Could you please let me know what could be the reason for this scenario and how I need to do the debug?

I am also attaching the screenshot of the PG signal and Schematic pdf

Regards,

Tensil Sebastian

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  • Hello Tensil,

    Here are a few things to check:

    1. Check the ITH network. Per the schematic ITH = (3.3nF+10.7K) || 4.7uF (C73)  || 4.7nF (C1818). Values of 4.7uF and 4.7nF for the capacitors from ITH to GND are too large. This could cause stability problems which could in turn could cause PGOOD to trip. Typical values for the capacitor from ITH to GND is 47pF to 330pF total. Try changing C73 and C1818 to ~100pF each.

    2. Measure VOUT, ITH, INTVCC(LTC3877), INTVCC(LTC3874) and PGOOD when the problem occurs.

    3. Make sure the 2N7002 between the LTC3874 FAULT and the LTC3877 PGOOD is stuffed with the correct polarity. The purpose of this MOSFET is to make sure the LTC3877 ramps up at the same time as the LTC3874. If the 2N7002 was stuffed with the drain and source swapped, the LTC3874 FAULT pins would be low until the PGOOD of the LTC3877 goes high.

    Best regards,

    Mike

Reply
  • Hello Tensil,

    Here are a few things to check:

    1. Check the ITH network. Per the schematic ITH = (3.3nF+10.7K) || 4.7uF (C73)  || 4.7nF (C1818). Values of 4.7uF and 4.7nF for the capacitors from ITH to GND are too large. This could cause stability problems which could in turn could cause PGOOD to trip. Typical values for the capacitor from ITH to GND is 47pF to 330pF total. Try changing C73 and C1818 to ~100pF each.

    2. Measure VOUT, ITH, INTVCC(LTC3877), INTVCC(LTC3874) and PGOOD when the problem occurs.

    3. Make sure the 2N7002 between the LTC3874 FAULT and the LTC3877 PGOOD is stuffed with the correct polarity. The purpose of this MOSFET is to make sure the LTC3877 ramps up at the same time as the LTC3874. If the 2N7002 was stuffed with the drain and source swapped, the LTC3874 FAULT pins would be low until the PGOOD of the LTC3877 goes high.

    Best regards,

    Mike

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