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My requirement is 3.3V 1A output from 5V (this 5V is derived from DC-DC regulator 24v to 5V 1.5A)

i have chosen NCP5662DS33 LDO , but which is best one to use for this current requiremnt ? i have already used Dc-Dc for 24V to 5V, if i use one more DC-Dc for 5V to 3.3V the noise emsissions, ripples will effect?

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I'd offer You to go 24V -> 3V3 using DC-DC. The best efficiency will be achieved. 5V -> 3V3 LDO @ 1A will dissipate 1.7W of heat.

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  • \$\begingroup\$ That will need a heatsink. \$\endgroup\$ – Al Kepp Jan 11 '13 at 9:20
  • \$\begingroup\$ Think of its size... \$\endgroup\$ – Socrates Jan 11 '13 at 9:35
  • \$\begingroup\$ Dissipating 1.7 watts will require a heatsink with a thermal coefficient of no more than about 30 degrees per watt to stay at reasonable temperatures - which is not a big heatsink. But it's still a lot of power to be wasting. \$\endgroup\$ – Nick Johnson Jan 11 '13 at 11:02
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An LDO will have to decrease the voltage by \$5V - 3.3V = 1.7V\$, and you have specified the current at \$1A\$. This means the power dissipated by the regulator is \$ 1.7V \cdot 1A = 1.7W \$. You will need a heatsink, and you will be wasting quite a lot of energy. You will have to decide if this is a problem for your application, or not.

The switching regulator will be more efficient, won't require a heatsink, but is more complex. It would probably more efficient to derive your 3.3V from 24V if you go with a switching regulator. There are also some switching designs that can output multiple voltages through multiple taps on the secondary of the transformer which may simplify your design. And yes, the switching regulator will result in more electromagnetic noise than the LDO. Again, that's a design decision you will have to make based on your application.

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  • \$\begingroup\$ I wouldn't exactly say that a multi-tap transformer is going to "simplify" SMPS design versus a straight buck transformer. You should be able to get a flyback transformer with 1:1:1 secondary windings that'll give you 3.3V from the "centre" tap and ~ 5V from the top one. But a flyback is more complex than buck and you may get tracking problems and the like. \$\endgroup\$ – Barleyman Jul 10 '19 at 10:23
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The LDO of specified type the output is 3.3V but input to LDO is 5V remaining excess power get dissipated in the form of heat since LDO type of linear voltage regulator leading to lesser efficiency Since the efficiency of is the major concern my suggest it will better if you proceed with switching regulators such as buck converters.

Currently in market there are buck converters of variable output levels you can refer that.

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