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AP1184

4A ULTRA LOW DROPOUT POSITIVE ADJUSTABLE OR FIXED-MODE REGULATOR

Diodes Incorporated. 

AP1184

4A ULTRA LOW DROPOUT POSITIVE ADJUSTABLE OR

FIXED-MODE REGULATOR

Functional Descriptions ( Continued )

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

Vin

Vctrl

AP1184-ADJ

Vsense

Adj

Vref

R1+

Vout

Iadj=50uA

R2+

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 and 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.

Vin

Vin

Vout

AP1184-ADJ

Vsense

Vctrl

Vctrl

Adj

R1

RL

R2

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.

Thermal Design

The AP1184-XXX incorporates an internal thermal shutdown that protects the device when the junction temperature exceeds the

allowable maximum junction temperature. Although this device can operate with junction temperatures in the range of 150oC, it is

recommended that the selected heat sink be chosen such that during maximum continuous load operation, the junction temperature is

kept below this number. The example below shows the steps in selecting the proper surface mount package.

AP1184 Rev. 1

6 of 12

www.diodes.com

MARCH 2007

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