Decrease 5volt to 4volt with low power dissipation

Hi,
I have a low-drop voltage 3.3 regulator that for Vin=4 to 4.3 it has low power dissipation((4-3.3)*I)). In addition I have a 5volt source (7805 regulator) before my 3.3 regulator. How can I downgrade 5volt to 4volt with the lowest power dissipation((5-4)*I) (I is about 1A)). I thought I can use a two series diode (.7 silicon +.3 germanium ) but it power dissipation is 1w for (1Vot*1A) that is too high.

Total power dissipation with linear regulators or other linear voltage drop methods (series resistor, diode) will be always 1.7V * 1A = 1.7V. In so far the question is only about sharing the power dissipation between multiple devices versus suitable heat sinking. In my view, a single LDO with heat sink for 1.7 W would be preferred.

The better alternative, beyond the scope of your question, would be a switch mode converter.

baby_2 said:

Hi,
I have a low-drop voltage 3.3 regulator that for Vin=4 to 4.3 it has low power dissipation((4-3.3)*I)). In addition I have a 5volt source (7805 regulator) before my 3.3 regulator. How can I downgrade 5volt to 4volt with the lowest power dissipation((5-4)*I) (I is about 1A)). I thought I can use a two series diode (.7 silicon +.3 germanium ) but it power dissipation is 1w for (1Vot*1A) that is too high.

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Hi,

if i understand it correctly : You said lowest power dissipation ((5-4)*I) (I is about 1A)) which will be 1W! (same as your diode solution) ,

if you want lower power dissipation you have to use a Switching regulator. Anyother linear solution will have same power dissipation.

BTW,if you want to use it at maximum current (1A) Continuously , your 7805 regulator will also have a lot of power dissipation (min input voltage of 7805 is 8V, so if use it at 1 A , so your regulator will dissipate around 3W at least).

i think if you use a switching regulator (if you can) , it's a lot better. for example you could use LM2576 (it has fixed 3.3V,5V,12V,15 +adjustable output versions) and it won't dissipate a lot of power like a linear regulator.

1.7W Power Dissipation is tolerable by 7805.No doubt since the case TO-220 with a reasonable heatsink.
Even D-PAK package that is soldered to GND plane can easily carry 1.7W dissipation.

Many POL regulators out there for 5V in, 3.3V and lower
out. Micromodules even, some "low noise" (certainly low
enough for digital, maybe not for RF).

What the linear regulator can withstand, may not be
the decision-basis. Could be battery life, could be the
simple desire not to cook your man-parts with the
phone in your pocket. 1.7W with ideal heat sink, fine.
1.7W to a board in a still air isolated enclosure, not
fine.