PAM2804 Datasheet, PDF (6/10 Pages) Power Analog Micoelectronics – 1A Step-Down Constant Current, High Efficiency LED Driver

PAM2804
Application Information
PWM Dimming
The PAM2804 can be used to dim LED current dimming by driving the EN pin via PWM waveform. The SW pin current is then effectively
switched on and off causing the LED current to turn on and off. The recommended PWM frequency is 500Hz (see Figure on page 5 for details).
Inductor Selection
The basic PAM2804 application circuit is shown in Page 1. External component selection is determined by the load requirement, selecting L first
and then CIN and COUT.
For most applications, the value of the inductor will fall within the range of 1μH to 4.7μH. Its value is chosen based on the desired ripple current.
Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple
current as shown in equation 1. A reasonable starting point for setting ripple current is ΔIL = 400mA (40% of 1A).
I_L

f
1
L
VOUT
¹

VOUT
VIN

Equation (1)
The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation.
Thus, a 1.4A rated inductor should be enough for most applications (1A + 400mA). For better efficiency, choose a low DC-resistance inductor.
Using Ceramic Input Output Capacitors
Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and
low ESR make them ideal for switching regulator applications. Using ceramic capacitors can achieve very low output ripple and small circuit size.
When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best
temperature and voltage characteristics of all the ceramics for a given value and size.
Thermal Consideration
Thermal protection limits power dissipation in the PAM2804. When the junction temperature exceeds +150°C, the OTP (Overtemperature
Protection) starts the thermal shutdown and turns the pass transistor off. The pass transistor resumes operation after the junction temperature
drops below +120°C.
For continuous operation, the junction temperature should be maintained below +125°C. The power dissipation is defined as:
  _{  PD  IO2}
VO RDS(ON)H ^
VIN ^ VO
VIN
RDS(ON)L

tSW ^F_S^I_O
 _IQ
VIN
IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load step-down converter switching losses.
For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:
PD ^ IO2 RDS(ON)H ^ IQ VIN
PAM2804
Document number: DS36403 Rev. 3 - 2
6 of 10
www.diodes.com
September 2015
© Diodes Incorporated