Application Information

11

T h e PA M 3 1 3 1 f a m i l y o f l o w - d r o p o u t ( L D O )

increasing R1+R2 will reduce stability. In

regulators have several features that allow them to

general, R1 and R2 in the 10’s of kΩ will produce

apply to a wide range of applications. The family

adequate stability, given reasonable layout

operates with very low input voltage (1.4V) and low

p r e c a u t i o n s .

T o

i m p r o v e

s t a b i l i t y

dropout voltage (typically 300mV at full load),

characteristics, keep parasitics on the ADJ pin

making it an efficient stand-alone power supply or

to a minimum, and lower R1 and R2 values.

post regulator for battery or switch mode power

supplies. The 3A output current make the PAM3131

Thermal Information

family suitable for powering many microprocessors

and FPGA supplies. The PAM3131 family also has

The amount of heat that an LDO linear regulator

low output noise (typically 40µVRMS with 2.2µF

generates is: P =(V -V )I .

D

IN

O

O

output capacitor), making it ideal for use in telecom

equipment.

A l l i n t e g r a t e d c i r c u i t s h a v e a m a x i m u m

allowable junction temperature (T max) above

J

External Capacitor Requirements

which normal operation is not assured. A system

d e s i g n e r

m u s t

d e s i g n

t h e

o p e r a t i n g

A 2.2µF or larger ceramic input bypass capacitor,

environment so that the operating junction

connected between V and GND and located close

IN

temperature (T ) does not exceed the maximum

J

to the PAM3131, is required for stability. A 1.0μF

junction temperature (T max). The two main

J

minimum value capacitor from V to GND is also

O

environmental variables that a designer can use

required. To improve transient response, noise

to improve thermal performance are air flow and

rejection, and ripple rejection, an additional 10µF

e x t e r n a l h e a t s i n k s . T h e p u r p o s e o f t h i s

or larger, low ESR capacitor is recommended at the

information is to aid the designer in determining

output. A higher value, low ESR output capacitor

the proper operating environment for a linear

may be necessary if large, fast-rise-time load

regulator that is operating at a specific power

transients are anticipated and the device is located

level.

several inches from the power source, especially if

the minimum input voltage of 1.4V is used.

In general, the maximum expected power

(P (max)) consumed by a linear regulator is

D

Regulator Protection

computed as:

The PAM3131 features internal current limiting,

(1)

thermal protection and short circuit protection.

Where:

During normal operation, the PAM3131 limits

l

V

is the average input voltage.

I (avg)

output current to about 4.5A. When current limiting

l

V

is the average output voltage.

O(avg)

engages, the output voltage scales back linearly

l

I

is the average output current.

O(avg)

until the over current condition ends. While current

l

I

is the quiescent current.

(Q)

limiting is designed to prevent gross device failure,

care should be taken not to exceed the power

For most LDO regulators, the quiescent current

d i s s i p a t i o n r a t i n g s o f t h e p a c k a g e . I f t h e

is insignificant compared to the average output

temperature of the device exceeds 150°C, thermal-

current; therefore, the term V

xI

can be

I(avg)

(Q)

protection circuitry will shut down. Once the device

neglected. The operating junction temperature

has cooled down to approximately 50°C below the

is computed by adding the ambient temperature

high temp trip point, regulator operation resumes.

(T ) and the increase in temperature due to the

A

The short circuit current of the PAM3131 is about

regulator' s power dissipation. The temperature

1A when its output pin is shorted to ground.

rise is computed by multiplying the maximum

expected power dissipation by the sum of the

Output Adjustment

thermal resistances between the junction and

the case (R

), the case to heatsink (R

), and

θJC

θCS

The PAM3131 uses external feedback resistors to

t h e h e a t s i n k t o a m b i e n t ( R

) . T h e r m a l

generate an output voltage from 0.9V to 3.3V. V

is

θ S A

adj

resistances are measures of how effectively an

trimmed to 0.9V and V

is given by the equation:

OUT

object dissipates heat. Typically, the larger the

V

= V

( 1 + R1 / R2 )

OUT

adj

device, the more surface area available for

power dissipation so that the object 's thermal

Feedback resistors R1 and R2 should be high

resistance will be lower.

e n o u g h to ke e p q u i e sce n t cu rre n t l o w, b u t

(

)(

)

(

)

(

)(

)

(

)

D

I avg

O avg

O avg

I avg

Q

P max= V

-V

×I

+V

×I

Power Analog Microelectronics, Inc

www.poweranalog.com

PAM3131

3A Adjustable Low Voltage

Low Dropout CMOS Regulator

10/2010 Rev1.1