ISSUED DATE :2005/07/27
REVISED DATE :2006/03/29C
GTM
CORPORATION
Functional Description
Introduction
The G3U1084 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 2.5V. The G3U1084 only needs 1.5V
differential between Vin and Vout to maintain output regulation, the output voltage tolerances are also extremely tight and
they include the transient response as port 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 G3U1084 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 G3U1084 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:
G3U1084ADJ
The G3U1084 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 requirement of the G3U1084 is 10mA, R1 is typically
selected to be 121ꢁ 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=121ꢁ and R2=200ꢁ the
error due to Iadj is only 0.3% of the nominal set point.
Load Regulation
Since the G3U1084 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 G3U1084 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 G3U1084 incorporates an internal shutdown that protects the device when the junction temperature exceeds the
maximum allowable junction temperatures. Although this device can operate with junction temperatures in the range of
150ć, it is recommended that the selected heat sink be chosen such that during maximum continuous load operation the
junction temperature is kept below the temperature.
Layout Consideration
The output capacitors must be located as close to the Vout 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 Vout pin to the output capacitors to prevent any high frequency
oscillation that may result due to excessive trace inductance.
G3U1084
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