PL1084J3 Datasheet, PDF (4/9 Pages) Cystech Electonics Corp. – 5A Low Dropout Postive Voltage Regulator

Spec. No. : C512J3
CYStech Electronics Corp. Issued Date : 2007.07.04
Revised Date : 2014.12.29
Page No. : 4/9
Functional Description
Introduction
The PL1084 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 PL1084 only needs 1.5V
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 5A load step. The
PL1084 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 PL1084 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=1.25V typically, Iadj=40μA typically
The PL1084 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 PL1084. 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 40μA, 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 PL1084 is only a 3 terminal device, it is not possible to provide true remote sensing of 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
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 represent 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 PL1084 requires the use of an output capacitor as part of the frequency compensation in order to make the
regulator stable. The addition of 150μF aluminum electrolytic or a 22μF solid tantalum capacitor will ensure stability
for all operating conditions.
When the adjustment terminal is bypassed with a capacitor to improve the ripple rejection, the requirement for an
output capacitor increases. The value of 22μF tantalum or 150μF aluminum covers all cases of bypassing the
adjustment terminal. Without bypassing the adjustment terminal, smaller capacitor can be used with equally goo
result.
To ensure good transient response with heavy load current changes, capacitor values on the order of 100μF are used
in the output of many regulators. To further improve stability and transient response of these devices, larger values of
output capacitors can be used.
Thermal Design
The PL1084 incorporates an internal 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℃, 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 excess trace inductance.
PL1084J3
CYStek Product Specification