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AIC1741CS

LOW DROPOUT ADJUSTABLE REGULATOR

Analog Intergrations 

APPLICATION INFORMATIONS

The AIC1741 has two major applications, one as a

low dropout linear regulator and another an error

amplifier for a switching power supply to provide

precision feedback control.

Use as a Linear Regulator

When AIC1741 is connected as a low dropout linear

regulator, two external divider resistors are needed to

set the output voltage (see Test Circuit). The output

voltage can be calculated from the following formula:

VOUT=VREF(1+R1/R2)+IFBR1

where VREF is the nominal 1.235V reference voltage

and IFB is the feedback pin bias current, nominally

-20nA. For better accuracy, the recommended

maximum value of R2 is 100KΩ.

A 1µF (or greater) capacitor is required between the

AIC1741 output and ground for stability. Without this

capacitor the part will oscillate. Even though most

types of capacitor may work, if Aluminum electrolytic

type is used, the equivalent series resistance (ESR)

should be held to 5Ω. or less. Many Aluminum

electrolytics have electrolytes that freeze at about

-30°C, so solid tantalums are recommended for

operation below -25°C. The value of this capacitor

may be increased without limit.

The output capacitance required for stability varies

with output voltage (smaller for higher output voltage)

and output current (smaller for lower output currents).

At the worst-case situation of a 80mA load at 1.235V

output (output shorted to FB pin) a 3.3 µF or greater

capacitor should be used.

AIC1741

LOW DROPOUT ADJUSTABLE REGULATOR

SPEC NO: DS-1741-00

nearby noise sources can also upset the output. This

may especially be a problem when using high value

external resistors to set the output voltage. Adding a

100pF capacitor between output and feedback and

increasing the output capacitor to at least 3.3µF will

cure this problem.

The AIC1741 can drive low cost NPN bipolar

transistor to deliver more output current. Typical

application circuit is an example of 5V to 3.3V linear

regulator for VGA cards power supply. A 390Ω

resistor and 1µF capacitor are needed between

VOUT pin and ground for stability. The minimum

dropout voltage between input to output is VBEQ1+VDS

of AIC1741 output PMOSFET, typical 1V @ 3A

output current.

Use as an Error Amplifier

Fig. 1 shows a popular application for the AIC1741 in

switching power supply. The output voltage is sensed

between FB pin and GND pin of the AIC1741 through

the divider resistor. The VIN pin servos to control the

duty cycle of the pulse width modulator to stabilize

the output voltage at the set voltage. The FB pin and

VIN pin are in same phase while the FB pin and

VOUT pin are in opposite phases. Connecting a

capacitor from VOUT pin to FB pin provides

frequency compensation for the control loop. The

AIC1741 provides output voltage remote sensing to

improve precision and regulation specifications of the

output of the switching power supply.

A 0.1µF capacitor should be placed from the

AIC1741 input to ground if the lead inductance

between the input and power source exceeds 500nH

(approximately 10 inches of trace). Noise injection

into the feedback terminal of the AIC1741 from

ANALOG INTEGRATIONS CORPORATION

www.analog.com.tw

4F, 9 Industry E. 9th Road, Science-based Industrial Park , Hsinchu, Taiwan, R.O.C. TEL: (8863)577-2500 FAX:(8863)577-2510

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