Negative low dropout voltage regulator with -5 to -24 input voltage range

Jan 28, 2009 3 Replies
S

I'm looking for a negative linear voltage regulator with at least a



-5 to -24 input voltage range and about 1 to 1.5 volts maximum dropout at 25 mA. I am aware of the LM2991 but am wondering if there is somethign available in a smaller package. Some are available with



-20V input capability but I need to -24. I've measured more than 2 V input-output difference required for the standard LM337.



One option is to use an LM337 to regulate to say -2.8 V, then use a doubler to change that to -5.6 V then use an LDO to regultate that to the final -4 V. Bit I'd prefer to use 1 device rather than 3.



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M

How about the LT1185 in the DDPAK? It is big but not huge.

We may be able to do it in less parts than your "double it" idea.

I'm thinking of a P channel JFET as the pass element. Finding one with the IDSS high enough will be a bit hard but here's the idea for others to throw stones at:

The reference + comparator LT6700-3 has section B wired as in this.

Ground rail ---------------------+-------+-------+------+------------ ! ! ! ! ! ! / ! \\ ! \\ --- / ! / --- \\ ! ! ! ! /-!------+ ! GND--!!---+-/\\/-+-----< ! ! ! ! \\+!-- \\ ! ! ! ! / ! ! ! [V] \\ ! ! ! ! ! ! ^ ! ! ! ! ----- ! ! ! ! ! ! ! ! ! ! ------------- -----------+---+---+------+------

In this case, the comparator rattles up and down frantically trying to keep the output voltage constant. An extra capacitor in the voltage divider can raise the frequency of this rattling and lower the output ripple.

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N

How about an input of up to 35V absolute max, output up to 500mA, dropout allegedly 1.1V?

Low dropout negative linear regs are a bit scarce but the OnSemi MC79M00 series are pretty good.

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(who I think were originally Motorola back when that company made useful stuff) still do LOTS of useful analog components, but as they aren't leading edge sexy stuff people tend not to talk about them (alas).The package is of sufficient size that it probably won't need heatsinking at 25mA output whatever the input voltage. 500mA regulators are a bit unusual, there's usually a gap between tiny SOT23 devices and 1A monsters, but the mid-range current permits a much smaller package than the heavy duty regulators.

Nemo
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