Capacitor Selection and Regulator Stability

Shutdown Input Operation

Input-Output ( Dropout ) Voltage

Current Limit and Short Circuit Protection

Thermal considerations

Similar to any low dropout regulator, the external

capacitors used with the PAM3108 must be

carefully selected for regulator stability and

performance.

A capacitor C

of more than 10µF can be

employed in the 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

PAM3108 should not exceed 0.5 inch. Ceramic

c a p a c it o r s a r e s u i t a b l e f o r t h e PA M 3 1 0 8 .

Capacitors with larger values and lower ESR

(equivalent series resistance) provide better

PSRR and line-transient response.

The PAM3108 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

>

µ

The PAM3108 is shut down by pulling the EN

input low, and is turned on by tying the EN input to

CTRL or leaving the EN input floating.

A regulator's m inimu m input-output voltage

difference (or dropout voltage) determines the

lowest usable supply voltage. The PAM3108 has

a typical 150mV dropout voltage.

The PAM3108 features a current limit, which

monitors and controls the gate voltage of the

pass transistor. The output current can be limited

to 3A by regulating the gate voltage. The

PAM3108 also has a built-in short circuit current

limit.

Thermal protection limits power dissipation in the

PA M 3 1 0 8 . W h e n t h e j u n c t i o n t e m p e r a t u r e

exceeds 170°C, the OTP (Over Temperature

Protection) starts the thermal shutdown and turns

the pass transistor off. The pass transistor

r e s u m e s

o p e r a t i o n

a f t e r

t h e

j u n c t i o n

temperature drops below 130°C.

F o r

c o n t i n u o u s o p e r a t i o n , t h e

j u n c t i o n

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:

Where T

is the maximum allowable junction

t e m p e r a t u r e 1 2 5 °C , T

i s t h e a m b i e n t

temperature and

is the thermal resistance

from the junction to the ambient.

IN

IN

IN

D

IN

OUT

O

IN

GND

J(MAX)

A

10 F with ESR>5m

Ω ensures stability.

P

= (T

-T )/θ

θ

D(MAX)

J(MAX)

A

JA

JA

Application Information

7

,

Power Analog Microelectronics Inc

www.poweranalog.com

PAM3108

Maximum 1.5A, Ultra Low Dropout Regulator

07/2011 Rev1.0