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

PAM2808
Application Information
PWM Dimming
The PAM2808 can be used to dim LED current dimming by driving the EN pin via PWM waveform. The VOUT pin current is then effectively
switched on and off causing the LED current to turn on and off. The recommended PWM frequency is 200Hz (see Figure 5 for details).
External Capacitor Requirements
A 4.7μF or larger ceramic input bypass capacitor, connected between VIN and GND and located close to the PAM2808, is required for stability.
A 4.7µF minimum value capacitor from VO 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 2.5V 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 temperature trip point, regulator operation resumes.
Thermal Information
The amount of heat generates is:
PD ^{ }VIN ^ VO^IO
All integrated circuits have a maximum allowable junction temperature (TJ MAX) above which normal operation is not assured. A system designer
must design the operating environment so that the operating junction temperature (TJ) does not exceed the maximum junction temperature
(TJ MAX). The two main environmental variables that a designer can use to improve thermal performance are air flow and external heat sinks. 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 (PD(MAX)) consumed by a linear regulator is computed as:
  _PDMAX  _VI(AVG)  _VO(AVG) xI_O(AVG)  _VI(AVG) xI_(Q)
Where:
VI(AVG) is the average input voltage
VO(AVG) is the average output voltage
IO(AVG) is the average output current
I(Q) is the quiescent current
The quiescent current is insignificant compared to the average output current; therefore, the term VI(AVG) x I(Q) can be neglected. The operating
junction temperature is computed by adding the ambient temperature (TA) 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 (RθJC), the case to heatsink (RθJS), and the heatsink to ambient (RθJA). 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.
PAM2808
Document number: DS36402 Rev. 2 - 2
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September 2015
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