300MA HIGH PSRR LOW DROPOUT CMOS LINEAR REGULATOR

2007-7-4

4/11

FSP2134

APPLICATION INFORMATION

Capacitor Selection and Regulator Stability

Similar to any low dropout regulator, the external capacitors used with the FSP2134 must be carefully selected for

regulator stability and performance.

exceed 0.5 inch. Ceramic capacitors are suitable for the FSP2134. Capacitors with larger values and lower ESR

provide better PSRR and line-transient response.

The FSP2134 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 FSP2134 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 FSP2134. 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 FSP2134 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 FSP2134 features a 0.75Ω P-Channel MOSFET device as a pass transistor. The P-MOS pass transistor

enables the FSP2134 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 FSP2134 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 FSP2134 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 FSP2134 also has a built-in short circuit current

limit.