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LM5170-Q1: A reverse bias circuit for offset of Gm amplifier

ksma
Intellectual 290 points
Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

Hello


About boost converter using LM5170.


A reverse bias circuit has been proposed as a countermeasure for offset of Gm amplifier. Is the attached schematic valid even with ISETD?


Thank you very much.

  • 0
    TI__Guru* 75895 points

    Hi Ksma,

    Thank you for posting.  First, the 33pF should not attached to SW1 or SW2 of the LM5170.  It should use the switch node of the 10V bias supply.  It is because it needs a continuous, uninterrupted switching signal to maintain the negative bias rail, even when the LM5170 stops switching.  

    Then, the answer to your question: yes, it is needed even if you use ISETD.  Note that the IC actually operate with ISETA.  If you give an ISETD, it just converts the pulse to a voltage at ISETA.  The negative bias is used to overcome the offset voltage of the internal error amplifier (gm amplifier), in order to maintain output regulation under light and no load. 

    Thanks,

    Youhao Xi, Applications Engineering