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I'm designing a board that can take an input DC supply of between +8 and +36V DC (+12V nominally) and needs rails of +5V, +3.3V, +2.5V and +1.8V. My current design uses a step-down regulator (LMR23625) to generate the +5V and I use LDOs to generate the rest. This all works fine. This block diagram illustrates it:

original design

The problem is that I also have an analog section to the board (8x INA141 instrumentation amplifiers and an 8 channel ADC). The INA141s need a negative supply rail of -5V. What is the best way for me to generate the -5V? I was thinking about daisy-chaining the TPS63700 off the LMR23635, e.g.: TPS63700 daisy-chain

Is this a good idea or should I consider something like a dual-supply/split-rail regulator in place of the LMR23625?

I'm not constrained for PCB space. The board is 200x60 mm (4 layer, double sided) and I can dedicate a good 3rd of that to power supplies. The overall system current is maximum 2.5A (nominally lower).

The worst case current requirement of the -5V rail 80 mA, maybe 100 mA to be safe (10mA per INA141 x 8 + some head-room). Nominal value is about 3 mA each.

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    \$\begingroup\$ Sorry, this is just where engineering gets tedious. You need to find all the possible solutions that meet your needs, then figure out which one is lowest cost (or optimizes some other secondary "want" specification). There's no way for anybody else to answer without intimate knowledge of your design and its requirements. \$\endgroup\$
    – The Photon
    Mar 20, 2020 at 18:24
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    \$\begingroup\$ You have considered whether a single-supply op-amp could work instead, right? \$\endgroup\$
    – Justin
    Mar 20, 2020 at 19:11
  • \$\begingroup\$ @Justin Yes, unfortunately I have no choice with the op-amps. I'm stuck having to use the dual-supply op-amps. \$\endgroup\$
    – Zapho
    Mar 20, 2020 at 19:20
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    \$\begingroup\$ How much current do you need on the -5V rail? This is a very easy problem to solve if you don't need much current, more difficult if you need a lot. \$\endgroup\$
    – Hearth
    Mar 20, 2020 at 19:47
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    \$\begingroup\$ @Hearth Good question - I've updated the question with this. I think 100 mA would be safe. I realize that is a lot though. \$\endgroup\$
    – Zapho
    Mar 20, 2020 at 20:38

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Your use of the inverting switcher is a reasonable idea. However I would suggest a couple of things to improve the analog front the performance.

  1. Have the main converter LMR23625 produce 6V instead.
  2. Use an LDO to produce the 5V for your analog.
  3. Have the TPS63700 invert the 6V to -6V instead.
  4. Use a negative LDO to produce the -5V for your analog.

This will give you the quietest voltage rails for analog as long as you work to isolate the GND regions properly for analog as well.

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