V

V

V

V

Volta

olta

olta

olta

oltaggggge R

e R

e R

e R

e Reeeeegula

gula

gula

gula

gulator

tor

tor

tor

tor

AS29372

AS29372

Rev. 2.6 06/05

Austin Semiconductor, Inc. reserves the right to change products or specifications without notice.

7

Austin Semiconductor, Inc.

APPLICATIONS INFORMATION

External Capacitors

A 10µF (or greater) capacitor is required between the output

and ground to prevent oscillations due to instability. Most

types of tantalum or aluminum electrolytics will be adequate;

film types will work, but are costly and therefore not

recommended.

Many aluminum electrolytics have

electrolytes that freeze at about -30°C, so solid tantalums are

recommended for operation below -25°C. The important

parameters of the capacitor are an effective series resistance

of about 5

Ω or less and a resonant frequency above 500kHz.

The value of this capacitor may be increased without limit.

At lower values of output current, less output capacitance is

required for output stability. The capacitor can be reduced to

0.5µF for current below 10mA or 0.15µF for currents below

1mA. Adjusting the AS29372 to voltages below 5V runs the

error amplifier at lower gains so that more output capacitance

is needed. For the worst-case situation of 750mA load at

1.23V output (Output shorted to Adjust) a 22µF (or greater)

capacitor should be used.

Programming the Output Voltage

The AS29372 may be programmed for any output voltage

between its 1.235V reference and its 26V maximum rating.

An external pair of resistors is required, as show in Figure 1.

The complete equation for the output voltage is

V

OUT

= V

REF

x {1 + R

1

/R

2

} - |I

FB

|R

1

where V

REF

Is the nominal 1.235 reference voltage and I

FB

is

the Adjust pin bias current, nominally 20nA. The minimum

recommended load current of 1µA forces an upper limit of 1.2M

Ω

on the value R

2

, if the regulator must work with no load (a

condition often found in CMOS in standby), I

FB

will produce a

-2% typical error in V

OUT

which may be eliminated at room

temperature by trimming R

1

. For better accuracy, choosing

R

2

= 100k reduces this error to 0.17% while increasing the

resistor program current to 12µA. Since the AS29372 typically

draws 100µA at no load with SHUTDOWN open-circuited, this

is a negligible addition.

Reducing Output Noise

In reference applications it may be advantageous to reduce

the AC noise present at the output. One method is to reduce

the regulator bandwidth by increasing the size of the output

capacitor. This is relatively inefficient, as increasing the

capacitor from 1µF to 220µF only decreases the noise from

430µV to 160µV

RMS

for a 100kHz bandwidth at 5V output.

Noise can be reduced by a factor of four with the adjustable

regulators with a bypass capacitor across R

1

, since it reduces

the high frequency gain from 4 to unity. Pick

C

BYPASS

z

1

2

πR

1

H

200 Hz

or about 0.01µF. When doing this, the output capacitor must

be increased to 10µF to maintain stability. These changes

reduce the output noise from 430µV to 100µV

RMS

for a

100kHz bandwidth at 5V output. With the bypass capacitor

added, noise no longer scales with output voltage so that

improvements are more dramatic at higher output voltages.