AP1186

1.5A ULTRA LOW DROPOUT POSITIVE ADJUSTABLE OR

FIXED-MODE REGULATOR

AP1186 Rev. 1

5 of 9

MARCH 2007

www.diodes.com

Functional Descriptions

Introduction

The AP1186 regulator is a 5-terminal device designed specifically

to provide extremely low dropout voltages comparable to the

PNP type without the disadvantage of the extra power dissipation

due to the base current associated with PNP regulators. This is

done by bringing out the control pin of the regulator that provides

the base current to the power NPN and connecting it to a voltage

that is greater than the voltage present at the Vin pin. This

flexibility makes the AP1186 ideal for applications where dual

inputs are available such as a computer motherboard with an

ATX style power supply that provides 5V and 3.3V to the board.

One such application is the new graphic chip sets that require

anywhere from 2.4V to 2.7V supply. The AP1186-ADJ can easily

be programmed with the addition of two external resistors to any

voltages within the range of 1.25V to 15.5V. Another major

requirement of these graphic chips is the need to switch the load

current from zero to several amps in tens of nanoseconds at the

processor pins, which translates to an approximately 300 to

500ns of current step at the regulator. In addition, the output

voltage tolerances are also extremely tight and they include the

transient response as part of the specification.

The AP1186 is specifically designed to meet the fast current

transient needs as well as providing an accurate initial voltage,

reducing the overall system cost with the need for fewer number

of output capacitors. Another feature of the device is its true

remote sensing capability that allows accurate voltage setting at

the load rather than at the device.

Output Voltage Setting

The AP1186-ADJ can be programmed to any voltages in the

range of 1.25V to 15.5V with the addition of R1 and R2 external

resistors according to the following formula:

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

AP1186-ADJ

Vin

Vctrl

Adj

Vout

Vsense

R1+

R2+

Vref

Iadj=50uA

The AP1186-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 AP1186-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 AP1186 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 AP1186 with remote

sensing.

AP1186-ADJ

Vin

Vctrl

Vin

Vctrl

Vout

Vsense

Adj

R1

R2

R

L