AP1084
5A Low Dropout Positive Adjustable or Fixed-Mode Regulator
Functional Description
Introduction
The AP1084 adjustable Low Dropout (LDO)
regulator is a 3 terminal device that can easily be
programmed with the addition of two external
resistors to any voltages within the range of 1.25 to
5V. The AP1084 only needs 1.4V differential
between Vin and Vout to maintain output regulation.
In addition, the output voltage tolerances are also
extremely tight and they include the transient
response as part of the specification. For example,
Intel VRE specification calls for a total of +/- 100mV
including initial tolerance, load regulation and 0 to
5.0A load step.
The AP1084 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 output capacitors.
Output Voltage Setting
The AP1084 can be programmed to any voltages in
the range of 1.25V to 5V with the addition of R1 and
R2 external resistors according to the following
formula:
Vout = Vref (1+ R2/R1) +Iadj *R2 , where Vref = 1.25 typically, Iadj = 55uA
typically R1&R2 as shown at below
Vin
AP1084
Adj
Vref
Iadj=55uA
R2
R1
Vout
The AP1084 keeps a constant 1.25V between the
output pin and the adjust pin. By placing a resistor
R1 across these two pins 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 which will be 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 AP1084 is 10mA, R1 is
typically selected to be 121Ω resistor so that it
automatically satisfies the minimum current
requirement. Notice that since Iadj is typically in the
range of 55uA it only adds a small error to the
output voltage and should only be considered when
a very precise output voltage setting is required. For
example, in a typical 3.3V application where
R1=121Ω and R2=200Ω the error due to
Iadj
is only
0.3% of the nominal set point.
Load Regulation
Since the AP1084 is only a 3 terminal device, it is
not possible to provide true remote sensing of the
output voltage at the load. The best load regulation
is achieved when the bottom side of R2 is
connected to the load and the top-side of R1
resistor is connected directly to the case or the Vout
pin of the regulator and not to the load. It is
important to note that for high current applications,
this can re-present a significant percentage of
the overall load regulation and one must keep the
path from the regulator to the load as short as
possible to minimize this effect.
Stability
The AP1084 requires the use of an output capacitor
as part of the frequency compensation in order to
make the regulator stable. For most applications a
minimum of 10uF aluminum electrolytic capacitor
insures both stability and good transient response.
Anachip Corp.
www.anachip.com.tw
4/10
Rev.1.0 Aug.27, 2004
ANACHIP [ ANACHIP CORP ]
