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I'm working on a project that requires the following voltages and currents:

  • 3.8V at 2A
  • 3.3V at 900mA
  • 1.8V at 100mA

The input power to the system is a 12V wall wart adapter.

I've decided I'll run the 1.8V from an LDO on the 3.3V supply as it'll keep cost lower than supplying its own SMPS.

Now the question is, should I supply the 3.8V and 3.3V side with 12V OR should I supply the 3.8V with 12V and then feed the 3.3V with the 3.8V whilst upping the current supply for the 3.8 to accommodate for the extra power required.

I'm thinking it'd be most efficient to daisy chain the 3.3V to the 3.8V as it'd lower the switching frequency and thus the switching losses.

Do I have my thinking right, are there any other design concerns I'm missing?

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  • \$\begingroup\$ In general both approaches are valid. In your case though I would do direct connection since the outputs are so close that you may run into dropout issues. Other design concerns: Beat frequencies and high frequency noise from the load of one converter or the converter itself passing straight through the converter out of its input and straight through other converters into their inputs and to their loads. \$\endgroup\$
    – DKNguyen
    Commented Aug 23, 2021 at 20:08
  • \$\begingroup\$ That's 1.5V*100mA or 0.15W in the LDO, you'll need to be sure it's heatsink is big enough, not too bad. Then if you daisy chain, you'll need 3A at 3.8V, not just 2A. It may be easier to run both 3.8 and 3.3V from 12V depending on the price of 2A vs 3A regulators... \$\endgroup\$
    – user16324
    Commented Aug 23, 2021 at 20:20

1 Answer 1

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If you 'daisy chain' devices, it will increase the current requirements of the devices upstream. For example if the 3.3V 900mA regulator sources it's power from 3.8V 2A regulator (And the original requirement for 3.8 is 2A), then the 3.8 line would need an additional 900mA or 2.9A (if it had not been factored in already).

When stepping down voltage, DC DC converter are usually more efficient than LDO's and don't burn up power as heat.

If I were designing this system, I would try and find a 2A DC DC converter to step 12V down to 3.8V and then a separate 1A DC DC converter to go from 12V to 3.3V. Since going from 3.3V to 1.8V at 100mA isn't that much power being dissipated (only 150mW) I'd probably just attach that to the 3.3V DC DC converter.

I might also consider getting a 5V or 6V AC to DC converter for the main power supply.

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