© 2004 California Micro Devices Corp. All rights reserved.

09/14/04

430 N. McCarthy Blvd., Milpitas, CA 95035-5112

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Tel: 408.263.3214

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Fax: 408.263.7846

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www.calmicro.com

11

CM3015

PRELIMINARY

Application Information

Figure 32. Adjustable Regulator Application Example

Output Compensation

Low-dropout regulator topologies use a current-carry-

ing transistor ("pass device") that adds additional gain

and phase shift to the design. It is necessary to com-

pensate for these attributes to prevent the design from

becoming unstable. The simplest way to do this is to

add an output RC filter to roll off the gain. Since all reg-

ulators use an output capacitor anyway, the only com-

ponent needed is a suitable resistance. Fortunately, for

the usual range and types of output capacitors, this

resistance can be supplied by the ESR of the caps.

For many years, capacitors have been the largest com-

ponents in regulator circuits, but newer types are

shrinking in size rapidly. Unfortunately, their ESRs

have shrunk as well, taking them out of the range

where they can perform the R function described in

existing LDOs. The CM3015 is a new breed of LDO

regulator that can work with these tiny low-ESR capac-

itors and thus permit designs that take a minimum of

space.

An attribute of the LDO topology is that the pass device

gain generally increases with increasing output current.

This may affect output stability: a design that didn't

need a capacitor now does, or a design that used a

small value now needs a larger one. The CMD advan-

tage is that this component can be selected from any

capacitor family without regard to its ESR. If the circuit

calls for a tiny, low-ESR capacitor, it will work just fine.

GND

EN

SENSE

BYP

1

2

6

5

3

4

4.992V

31.6k

10.0k

1.20V

For the input voltage, Vin minimum will depend on the output current. For

example, if Iout is 100mA, Vin must be at least 4.992V = 1ohm x 100mA =

4.992 + 0.1 = 5.092V.