4A Ultra Low Dropout Positive Adjustable or Fixed-mode Regulator

AP1184

Anachip Corp.

www.anachip.com.tw

Rev.1.1 Sep 7, 2004

5/11

Functional Descriptions(Continued)

Vout = Vref (1+R2/R1) + Iadj * R2 where : Vref = 1.25V & Iadj=50uA Typically

AP1184-ADJ

Vin

Vctrl

Adj

Vout

Vsense

R1+

R2+

Vref

Iadj=50uA

The AP1184-ADJ keeps a constant 1.25V between

the Vsense pin and the Adj pin. By placing a

resistor R1 across these two pins and connecting

the Vsense ans Vout pin together, a constant

current flows through R1, adding to the Iadj current

and into the R2 resistor producing a voltage equal

to the (1.25/R1)*R2 + Iadj*R2. This voltage is then

added to the 1.25V to set the output voltage. This is

summarized in the above equation. Since the

minimum

load

current

requirement

of

the

AP1184-ADJ is 10mA, R1 is typically selected to be

a 121Ω resistor so that it automatically satisfies

this condition. Notice that since the Iadj is typically

in the range of 50uA it only adds a small error to the

output voltage and should be considered when very

precise output voltage setting is required.

Load Regulation

Since the AP1184 has separate pins for the output

(Vout) and the sense (Vsense), it is ideal for

providing true remote sensing of the output voltage

at the load. This means that the voltage drops due

to parasitic resistance such as PCB traces between

the regulator and the load are compensated for

using remote sensing. Figure following shows a

typical application of the AP1184-ADJ with remote

sensing.

AP1184-ADJ

Vin

Vctrl

Vin

Vctrl

Vout

Vsense

Adj

R1

R2

R

L

Stability

The AP1184-XXX requires the use of an output

capacitor as part of the frequency compensation in

order to make the regulator stable. Typical designs

for the microprocessor applications use standard

electrolytic capacitors with typical ESR in the range

of 50 to 100mΩ and an output capacitance of

100uF to 1000uF. Fortunately as the capacitance

increases, the ESR decreases resulting in a fixed

RC

time

constant. The

AP1184-XXX takes

advantage of the phenomena in making the overall

regulator loop stable. For most applications a

minimum of 100uF aluminum electrolytic capacitor

insures both stability and good transient response.