LTM4620A Output Voltage Change Question

hi team

A circuit diagram was designed using LTspice, as shown in the attached picture.
When the voltage on the TK/SS pin changes, so does the output voltage.
Can you explain what the problem is?
The circuit diagram did not observe the reference,
Is there a problem with the design as shown in the picture?
The customer says it is working well and wants proof of the problem.

Vref (V) Vout (V)
1 1.152
1.5 1.522
1.8 1.8
2 1.8
3 1.8


  • 0
    •  Analog Employees 
    on Feb 28, 2020 5:51 PM

    Hi Damon,

    The issue is due to the voltage divider network on the TK/SS pin. For LTM4620A, when the voltage on TK/SS pin is below 0.6V, the FB pin voltage will be regulated equal to the TK/SS voltage instead of the fixed 0.6V, as it is in soft-start state. After the voltage on TK/SS increases over 0.6V, the FB pin will be regulated at a fixed 0.6V, thus allowing the converter output voltage to be regulated at your desired value. In your setup, there is a 3:1 voltage divider between Vref and TK/SS, thus the voltage on TK/SS will be>= 0.6V only when Vref>=1.8V. This is why your output voltage will change according to Vref voltage (TK/SS voltage) when it is smaller than 1.8V and will be regulated at your desired value when Vref>=1.8V.

    Hope this helps!


  • Hi Liyao

    I was moved by your kind reply!

    Finally, is the TK/SS and FB of LTM4638 in the same method?



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