Voltage Regulator

AS29372

AS29372

Rev. 2.7 01/10

Micross Components reserves the right to change products or specifications without notice.

7

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

where V

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

(a condition often found in CMOS in standby), I

a -2% typical error in V

temperature by trimming R

R

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 capaci-

tor. This is relatively inefficient, as increasing the capacitor

from 1μF to 220μF only decreases the noise from 430μV to

160μV

reduced by a factor of four with the adjustable regulators with a

bypass capacitor across R

gain from 4 to unity. Pick

C

1

2

πR

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

be increased to 10μF to maintain stability. These changes re-

duce the output noise from 430μV to 100μV

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.