DC Supply for digital ICs

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engr_joni_ee

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The digital IC usually need 3.3 V or 1.8 V as supply voltage. If the main supply to the PC board is 5V then which power supply option is the better for digital ICs in terms of low ripples, the Linear Regulators or DCDC converters ? Quantitatively how much we can expect Vpp ripples in Linear Regulators and DCDC converters ?
 

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dick_freebird said:
Power supply rejection is had mostly by error amplifier
(inside LDO but including pass transistor and FB pin)
AVOL. As that fades with frequency so must PSRR
(meanwhile in-coupling from VIN to error amp guts
is increasing, with freq).
Click to expand...
This depends on gain, AEA(f) of the error amplifier.
LDOs today have BW>> 10 MHz so PSRR or PSR can be > 40 dB at 1 MHz with FET LDOs. This can also support "Capacitorless" LDOs. See research.

It also depends on the parasitics model of Cout and layout to shift the dominant pole of the EA. Your critical choice of Cout depends on ESR and RdsOn of LDO or ripple current of Cap and I out max of LDO.


 

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