PAM3112 datasheet(7/16 Pages) PAM

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Application Information

PAM3112

300mA CMOS Linear Regulator

Capacitor Selection and Regulator Stability

Load Transient Considerations

Shutdown Input Operation

Internal P-Channel Pass Transistor

Input-Output ( Dropout ) Voltage

Current Limit and Short Circuit Protection

Thermal considerations

Similar to any low dropout regulator, the external

capacitors used with the PAM3112 must be

carefully selected for regulator stability and

performance.

A capacitor C

of more than 1 F can be used at

the PAM3112 input pin, while there is no upper

limit for the capacitance of C . Please note that

the distance between C and the input pin of the

PAM3112 should not exceed 0.5 inch. Ceramic

capacitors a re suitable f or the PAM3112.

Capacitors with larger values and lower ESR

(equivalent series resistance) provide better

PSRR and line-transient response.

The PAM3112 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

A 10nF bypass capacitor connected to BYP pin is

suggested for suppressing output noise. The

capacitor, in series connection with an internal

200k

resistor, forms a low-pass filter for noise

reduction. Increasing the capacitance will slightly

decrease the output noise, but increase the start-

up time.

Curve

of the PAM3112 load-transient response

shows two components of 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 PAM3112. 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 is smaller spike.

The PAM3112 is shut down by pulling the EN input

low and turned on by tying the EN input to VIN or

leaving the EN input floating.

The PAM3112 features a 0.75

device as a pass transistor. The P-MOS

pass transistor enables the PAM3112 to consume

only 65 A of ground current during low dropout,

light-load, or heavy-load operation. These

features increase the battery operation life time.

A regulator's minimum input-output voltage

difference (or dropout voltage) determines the

lowest usable supply voltage. The PAM3112 has a

typical 300mV dropout voltage. In battery-

powered systems, this will determine the useful

end-of-life battery voltage.

The PAM3112 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

PAM3112 also has a built-in short circuit current

limit.

Thermal protection limits power dissipation in the

PAM3112. When the junction temperature

exceeds 150°C, 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°C.

For

c ontinuous

operation,

the

junction

temperature should be maintained below 125°C.

The power dissipation is defined as:

P = (V -V

)*I +V *I

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:

OUT

GND

2.2μF with ESR>5mΩ ensures stability.

Ω P-Channel

MOSFET

= (T

-T )/θ

D(MAX)

J(MAX)

on page 6

10/2008 Rev 1.3

Power Analog Microelectronics Inc

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