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OM186ST

Adjustable Low Dropout Positive Voltage Regulator

International Rectifier 

OM186ST

Application Notes:

Stability

The OM186ST Series requires the use of an output capacitor as part of the device frequency compensation.

For all operating conditions, the addition of 150µF aluminum electrolytic or a 22µF solid tantalum on the

output will ensure stability. Normally, capacitors much smaller than this can be used. Many different types

of capacitors with widely varying characteristics are available. These capacitors differ in capacitor tolerance

(sometimes ranging up to ±100%), equivalent series resistance, and capacitance temperature coefficient.

The 150µF or 22µF values given will ensure stability.

When the adjustment terminal is bypassed to improve the ripple rejection, the requirement for an output

capacitor increases. The values of 22µF tantalum or 150µF aluminum cover all cases of bypassing the

adjustment terminal. Without bypassing the adjustment terminal, smaller capacitors can be used with

equally good results and the table below shows approximately what size capacitors are needed to ensure

stability.

Normally, capacitor values on the order of 100µF are used in the output of many regulators to ensure good

transient response with heavy load current changes. Output capacitance can be increased without limit and

larger values of output capacitor further improve stability and transient response of the regulators.

Recommended Capacitor Values

Input

Output

Adjustment

10µF

10µF

10µF Tantalum, 50µF Aluminum

22µF Tantalum, 150µF Aluminum

None

20µF

Output Voltage - Adjustable Regulators

The OM186ST devices develop a 1.25V reference voltage between the output and the adjust terminal (see

Fig 4). By placing a resistor, R1 between these two terminals, a constant current is caused to flow through

R1 and down through R2 to set the overall output voltage. Normally this current is the specified minimum

load current of 10mA. Because IADJ is very small and constant when compared with the current through R1,

it represents a small error and can usually be ignored.

VIN

4

OM186ST

IN

OUT

VOUT

ADJ

VADJ

R1

IADJ

50µA

R2

VOUT = VREF

1+

R2

R1

+ IADJ R2

Fig 4. Basic Adjustable Regulator

www.irf.com

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