Diodes AP1084 - Manual (Page 6)

AP1084
5A LOW DROPOUT POSITIVE ADJUSTABLE OR
FIXED-MODE REGULATOR
AP1084 Rev. 9 6 of 11 JANUARY 2010
DS31070 www.diodes.com © Diodes Incorporated
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 1.25V to Vin-1.4V. The AP1084 only needs 1.4V differential between V
IN and V OUT
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 Vin-1.4V with the addition of R1 and R2 external resistors
according to the following formula:
V
out
= V
ref
(1+ R2/R1) +I
adj
*R2 , where V
ref
= 1.25 typically, I
adj
=
55uA typically R1 & R2 as shown below:
AP1084
V
in
V
out
V
ref
Adj
R1
R2
I
adj
= 55uA
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 121
Ω 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=121
Ω 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 V
OUT
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.
Thermal Design
The AP1084 incorporates an internal thermal shutdown that protects the device when the junction temperature exceeds the
maximum allowable junction temperature. Although this device can operate with junction temperatures in the range of 150
o C, it is
recommended that the selected heat sink be chosen such that during maximum continuous load operation the junction temperature is
kept below that temperature.
Layout Consideration
The output capacitors must be located as close to the V
OUT
terminal of the device as possible. It is recommended to use a section
of a layer of the PC board as a plane to connect the V
OUT
pin to the output capacitors to prevent any high frequency oscillation that may
result due to excessive trace inductance.