PAM3131 Datasheet, PDF (11/14 Pages) Power Analog Micoelectronics – 3A Adjustable Low Voltage Low Dropout CMOS Regulator

PAM3131
3A Adjustable Low Voltage
Low Dropout CMOS Regulator
Application Information
The PAM3131 family of low-dropout (LDO)
regulators have several features that allow them to
apply to a wide range of applications. The family
operates with very low input voltage (1.4V) and low
dropout voltage (typically 300mV at full load),
making it an efficient stand-alone power supply or
post regulator for battery or switch mode power
supplies. The 3A output current make the PAM3131
family suitable for powering many microprocessors
and FPGA supplies. The PAM3131 family also has
low output noise (typically 40µVRMS with 2.2µF
output capacitor), making it ideal for use in telecom
equipment.
External Capacitor Requirements
A 2.2µF or larger ceramic input bypass capacitor,
connected between V_IN and GND and located close
to the PAM3131, is required for stability. A 1.0μF
minimum value capacitor from V_O to GND is also
required. To improve transient response, noise
rejection, and ripple rejection, an additional 10µF
or larger, low ESR capacitor is recommended at the
output. A higher value, low ESR output capacitor
may be necessary if large, fast-rise-time load
transients are anticipated and the device is located
several inches from the power source, especially if
the minimum input voltage of 1.4V is used.
Regulator Protection
The PAM3131 features internal current limiting,
thermal protection and short circuit protection.
During normal operation, the PAM3131 limits
output current to about 4.5A. When current limiting
engages, the output voltage scales back linearly
until the over current condition ends. While current
limiting is designed to prevent gross device failure,
care should be taken not to exceed the power
dissipation ratings of the package. If the
temperature of the device exceeds 150°C, thermal-
protection circuitry will shut down. Once the device
has cooled down to approximately 50°C below the
high temp trip point, regulator operation resumes.
The short circuit current of the PAM3131 is about
1A when its output pin is shorted to ground.
Output Adjustment
The PAM3131 uses external feedback resistors to
generate an output voltage from 0.9V to 3.3V. V_adj is
trimmed to 0.9V and V_OUT is given by the equation:
V_OUT = V_adj ( 1 + R1 / R2 )
Feedback resistors R1 and R2 should be high
enough to keep quiescent current low, but
increasing R1+R2 will reduce stability. In
general, R1 and R2 in the 10’s of kΩ will produce
adequate stability, given reasonable layout
p r e c a u t i o n s . To i m p r o v e s t a b i l i t y
characteristics, keep parasitics on the ADJ pin
to a minimum, and lower R1 and R2 values.
Thermal Information
The amount of heat that an LDO linear regulator
generates is: P_D=(V_IN-V_O)I_O.
All integrated circuits have a maximum
allowable junction temperature (T_J max) above
which normal operation is not assured. A system
designer must design the operating
environment so that the operating junction
temperature (T_J) does not exceed the maximum
junction temperature (T_J max). The two main
environmental variables that a designer can use
to improve thermal performance are air flow and
external heatsinks. The purpose of this
information is to aid the designer in determining
the proper operating environment for a linear
regulator that is operating at a specific power
level.
In general, the maximum expected power
(P_D(max)) consumed by a linear regulator is
computed as:
( ) PDmax= VI⁽ avg⁾ -VO ⁽ avg⁾ ×IO ⁽ avg⁾ +VI⁽ avg⁾ ×I⁽ Q⁾₍₁₎
Where:
l V_{I (avg)} is the average input voltage.
V i l _O(avg) s the average output voltage.
l I_O(avg) is the average output current.
l I_(Q) is the quiescent current.
For most LDO regulators, the quiescent current
is insignificant compared to the average output
current; therefore, the term V_I(avg) xI_(Q) can be
neglected. The operating junction temperature
is computed by adding the ambient temperature
(T_A) and the increase in temperature due to the
regulator' s power dissipation. The temperature
rise is computed by multiplying the maximum
expected power dissipation by the sum of the
thermal resistances between the junction and
the case (R ) _θJC , the case to heatsink (R_θCS), and
the heatsink to ambient (R_θSA). Thermal
resistances are measures of how effectively an
object dissipates heat. Typically, the larger the
device, the more surface area available for
power dissipation so that the object 's thermal
resistance will be lower.
Power Analog Microelectronics,Inc
www.poweranalog.com
11
10/2010 Rev1.1