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I'm designing a circuit to provide a negative voltage rail in a battery for an ADC. I've decided to use a particular monolithic BCD Chip that can work as a step up/down/inverting controller and has an integrated switch.

I've designed the circuit inline with the data sheet inverting reference with the exception of using tantalums, a different shottky (30V 1A) and have a higher input voltage. At this point it doesn't appear that the circuit is switching (at all) from what I can tell, there is no oscillation on the timing cap pin or output, and Vout is measuring around +.3V. The chip is commanding 3W of energy which it appears to be dissipating itself.

Have I missed something here. I've substituted all components, so this is likely a design or layout issue I think. enter image description hereenter image description here

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  • \$\begingroup\$ What is driving VCC, ie what is HYST_OUT? Then : if it's dissipating 4* the Abs Max power for the package, it's completely dead, and everything you can measure stems from that. Next time, power it from a supply current-limited to 50 or 100mA... \$\endgroup\$
    – user16324
    Commented May 25, 2021 at 12:43
  • \$\begingroup\$ HYST_OUT is the input - I am using a current limited supply, that was max observed \$\endgroup\$
    – Alex Turner
    Commented May 25, 2021 at 13:03
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    \$\begingroup\$ I'm not sure you could have made that schematic any more confusing! \$\endgroup\$
    – Finbarr
    Commented May 25, 2021 at 13:23

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R5,R6 are connected incorrectly thus feedback voltage does not match 1.25V.

1 cut 2 jumpers needed. It should be obvious.

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    \$\begingroup\$ How so? Are they not identical to that in the reference? Are you seeing that on the schematic or the layout? Edit | I see it, the 12V reference is on the output of the voltage divider. Thanks for that :) \$\endgroup\$
    – Alex Turner
    Commented May 25, 2021 at 13:06

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