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lt8331 Negative voltage converter

Compare the 135v converter shown on page 23 of the datasheet to the -120v converter shown in fig. 2 of Jesus Rosales' Design Note 545:

135v converter and -120v converter

https://ibb.co/dPsgc5n

-120v converter layout

I built both designs.  The 135v converter works, but the -120v converter doesn't run at all.  Of course, I didn't faithfully implement the schematic of fig. 2.  :)

1)  I left the Sync/Mode pin grounded, rather than connecting it to the IntVcc pin.

2)  The SS cap is still 0.2, rather than 0.22

3)  I omitted the 10 ohm resistor

4)  I used standard high voltage diodes, rather than Schottky diodes, thinking they would compensate for the missing 10 ohm resistor.

Which of these changes is keeping the circuit from working?  I'm guessing it's the Sync/Mode pin but that's the hardest one to change.

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  • The negative voltage converter is working.  I am using mini-grabber test leads to connect 12v and ground.  I think the hook on the positive grabber was slightly corroded making for a high resistance in the 12v supply lead.

    1)  The circuit works with pin 12 (Sync/Mode) grounded and also works with pin 12 connected to pin 5 (IntVcc).

    2)  SS cap of 0.1 or 0.2 both work.

    3) The 10 ohm resistor is not present

    4)  Using standard diodes

  • Hi JohnPizzuto, 

    I have a few comments on your changes. 

    1. Grounding pin 12 will put your converter to BURST mode, which reduces fsw at light load to improve efficiency. There are 5 different modes you can choose from using this pin. 

    2. SS cap sets the start-up time of your converter. The larger SS cap is, the longer it takes to start. 

    Do you have any other questions about this design?

    Regards, 

    Peter Pham

Reply
  • Hi JohnPizzuto, 

    I have a few comments on your changes. 

    1. Grounding pin 12 will put your converter to BURST mode, which reduces fsw at light load to improve efficiency. There are 5 different modes you can choose from using this pin. 

    2. SS cap sets the start-up time of your converter. The larger SS cap is, the longer it takes to start. 

    Do you have any other questions about this design?

    Regards, 

    Peter Pham

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