FSP2131 datasheet(4/11 Pages) FOSLINK

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300MA HIGH PSRR LOW DROPOUT CMOS LINEAR REGULATOR

2007-4-19

4/11

FSP2131

APPLICATION INFORMATION

Capacitor Selection and Regulator Stability

Similar to any low dropout regulator, the external capacitors used with the FSP2131 must be carefully selected for

regulator stability and performance.

Using a capacitor, CIN, whose value is ＞1μF at the FSP2131 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 FSP2131 should not

exceed 0.5 inch. Ceramic capacitors are suitable for the FSP2131. Capacitors with larger values and lower ESR

provide better PSRR and line-transient response.

The FSP2131 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 FSP2131 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 FSP2131. 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.

Internal P-Channel Pass Transistor

The FSP2131 features a 0.75Ω P-Channel MOSFET device as a pass transistor. The P-MOS pass transistor

enables the FSP2131 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 FSP2131 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 FSP2131 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 FSP2131 also has a built-in short circuit current

limit.

Thermal Considerations

Thermal protection limits power dissipation in the FSP2131. 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 SOT23-3L 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 FSP2131 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 FSP2131 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℃

For this operating condition, TJ, is lower than the absolute maximum operating junction temperature, 125℃, so it is

safe to use the FSP2131 in this configuration.

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