PAM2808 Datasheet, PDF (6/8 Pages) Power Analog Micoelectronics – 5W High Power HBLED Driver

PAM2808
5W High Power HBLED Driver
Application Information
External Capacitor Requirements
A 4.7µF or larger ceramic input bypass capacitor,
connected between V_IN and GND and located
close to the PAM2808, is required for stability. A
4.7 μF minimum value capacitor from V_O to GND is
also required. To improve transient response,
noise rejection, and ripple rejection, an additional
1 0 µF o r l a r g e r , l o w E S R c a p a c i t o r i s
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 2.5 V is used.
Regulator Protection
The PAM2808 features internal current limiting,
thermal protection and short circuit protection.
During normal operation, the PAM2808 limits
output current to about 2.5A. 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 40°C below the high temp trip
point, regulator operation resumes.
Thermal Information
The amount of heat 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)
Where:
l V_{I (avg)} is the average input voltage.
l V_O(avg) is the average output voltage.
l I _O(avg) is the average output current.
l I _(Q) is the quiescent current.
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
6
08/2011 Rev1.0