300MA HIGH PSRR LOW DROPOUT CMOS LINEAR REGULATOR
FSP2132
APPLICATION INFORMATION
Capacitor Selection and Regulator Stability
Similar to any low dropout regulator, the external capacitors used with the FSP2132 must be carefully selected for
regulator stability and performance.
Using a capacitor, CIN, whose value is＞1μF at the FSP2132 input pin, the amount of the capacitance can be
increased without limit. Please note that the distance between CIN and the input pin of the FSP2132 should not
exceed 0.5 inch. Ceramic capacitors are suitable for the FSP2132. Capacitors with larger values and lower ESR
provide better PSRR and line-transient response.
The FSP2132 is designed specifically to work with low ESR ceramic output capacitors in order to save space and
improve performance. Using an output ceramic capacitor whose value is ＞2.2μF with ESR＞5mΩ ensure stability.
A 10nF bypass capacitor connected to BYP pin is suggested for suppressing output noise. The capacitor, in series
connection with an internal 200kΩ resistor, forms a low-pass filter for noise reduction. Increasing the capacitance will
slightly decrease the output noise, but increase the start-up time.
Load Transient Considerations
The figure11 shows the FSP2132 load transient response. It shows two components the output response: a DC shift
from the output impedance due to the load current change and transient response. The DC shift is quite small due to
excellent load regulation of the FSP2132. The transient spike, resulting from a step change in the load current from
1mA to 300mA, is 20mV. The ESR of the output capacitor is critical to the transient spike. A larger capacitance along
with smaller ESR results in a smaller spike.
Shutdown Input Operation
The FSP2132 is shutdown by pulling the EN input low, and is turned on by tying the EN input to VIN or leaving the
EN input floating.
Internal P-Channel Pass Transistor
The FSP2132 features a 0.75Ω P-Channel MOSFET device as a pass transistor. The P-MOS pass transistor
enables the FSP2132 to consume only 65μA of ground current during low dropout, light load, or heavy load
operations. This feature increases the battery operation life time.
Dropout Voltage
A regulator’s minimum dropout voltage determines the lowest usable supply voltage. The FSP2132 has a typical
300mV dropout voltage. In battery powered systems, this will determine the useful end-of-life battery voltage.
Current Limit and Short Circuit Protection
The FSP2132 features a current limit, which monitors and controls the gate voltage of the pass transistor. The output
current can be limited to 400mA by regulating the gate voltage. The FSP2132 also has a built-in short circuit current
limit.
Thermal Considerations
Thermal protection limits power dissipation in the FSP2132. When the junction temperature exceeds 150℃, the
OTP (Over Temperature Protection) starts the thermal shutdown and turns the pass transistor off. The pass
transistor resumes operation after the junction temperature drops below 120℃.
For continuous operation, the junction temperature should be maintained below 125℃. The power dissipation is
defined as :
PD=(VIN-VOUT)*IO+VIN*IGND
The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of
surrounding airflow and temperature difference between junction and ambient. The maximum power dissipation can
be calculated by the following formula:
PD(MAX)=(TJ(MAX)-TA)/θJA
Where TJ(MAX) is the maximum allowable junction temperature 125℃. TA is the ambient temperature and θJA is the
thermal resistance from the junction to the ambient.
For example, θJA is 250℃/W for the SOT23-5L package, based on the standard JEDEC 51-3 for a single layer
thermal test board. The maximum power dissipation at TA=25℃ can be calculated by the following formula:
PD(MAX)= (125℃-25℃)/250=0.4W
It is also useful to calculated the junction temperature of the FSP2132 under a set of specific conditions. In this
example let the input voltage VIN=3.3V, the output current IO=300mA and the case temperature TA=40℃ measured
by a thermal couple during operation. The power dissipation for the VO=2.8V version of the FSP2132 can be
calculated as:
PD=(3.3V-2.8V)*300mA+3.3V*70μA≌150mW
And the junction temperature, TJ, can be calculated as follows:
TJ=TA+PD*θJA
TJ=40℃+0.15W*250℃/W
=77.5℃＜TJ(MAX) =125℃
4/11
2007-8-6
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
