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LTC2924: DONE signal does not output Low

Dear ADI engineer.

We are designing a power supply system using the LTC2924.

OUT1-4 are all High, and the four surrounding Power supply ICs are running. A voltage of about 0.9V is input to all IN1-4.

However, the Done signal does not go low. The figure below shows the waveforms of DONE and TMR at that time.

          Fig1. TMR & DONE wave

Wave   CH1:DONE,   CH2:TMR

The circuit conditions are as follows.

- VCC=5V, GND=0V,

- The DONE & FAULT pin is pulled up to 1.8V with 4.7kΩ.

- A 0.015uF capacitor is connected between GND on the TMR pin.

- The HYS/CFG pin is pulled down with 49.9kΩ.

- PGT pin is connected to GND.

When simulating with LTspice, the DONE signal was low.

                               Fig2. TMR & DONE 

  

We are in trouble because we do not know the cause.

Please consider what you can think of.

Thank you.