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LT3855 Output switching issue


I am using LTC3855IUJ#PBF as 10V to 6V buck regulator. we are using both the channels for same voltage generation.

I am facing issue with spike in input voltage once board is turn on. below are screenshot are captured.

SS Vs Output Voltage Vs Vfb,  it seems SS pin is not tracking the output voltage.


Input Voltage(10V) vs Enable (Run pin), seems 3.4V spike at input voltage, because of this whole board will shutdown.


  • Hello,

    The 3.4V positive spike on VIN during turn-on of the converter is unusual. If anything, VIN should dip slightly due to the current drawn by the input of the buck converter during turn-on. Please check the VIN supply.

    In addition, VFB does not follow VOUT when the LTC3855 is disabled. Instead, VFB drops sharply to 0 Volts. This indicates that the differential amplifier is possibly being used. Per the datasheet, the VOUT range of the LTC3855 is 0.6V to 3.3V with the differential amplifier and 0.6V to 12.5V without the differential amplifier. The reason for limiting VOUT to 3.3V when using a differential amplifier is that the maximum output of the differential amplifier is limited to INTVCC-1.4V. Given that INTVCC is 5.0V typical, the limit comes to 3.6V. 

    For your circuit, the feedback divider should be directly placed across VOUT. For the differential amplifier, float DIFFOUT and tie the DIFFP and DIFFN pins to GND. An example is shown in Figure 25 of the data sheet.

    Best regards,