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Functional Block Diagram
V
IN
EP1084
V
OUT
+
+
Current
Limit
Thermal
Shutdown
1.25V
+
-
FB
Introduction
The EP1084 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 V
IN
-1.4V. The EP1084 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 EP1084 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 EP1084 can be programmed to any voltages in the range of 1.25V to V
IN
-1.4V with the addition of
R1 and R2 external resistors according to the following formula:
The EP1084 keeps a constant 1.25V between the output pin and the FB pin. By placing a resistor R1
across these two pins a constant current flows through R1, adding to the I
FB
current and into the R2
resistor producing a voltage equal to the (1.25/R1)*R2+I
FB
*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 EP1084 is 10mA, R1 is typically selected to be 121Ω resistor so that it automatically satisfies the
minimum current requirement. Notice that since I
FB
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 I
FB
is
only 0.3% of the nominal set point.
Jul. 2006
5/8
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