PAM2804 Datasheet, PDF (8/11 Pages) Power Analog Micoelectronics – 1A Step-Down Constant Current, High Efficiency LED Driver

PAM2804
1A Step-Down Constant Current, High Efficiency LED Driver
Application Information
For the condition where the step-down converter
is in dropout at 100% duty cycle, the total device
dissipation reduces to:
P_D =I_O2 R_{DSON H} +I_QV_IN
Since R , _DS(ON) quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range. The maximum power dissipation
depends on the thermal resistance of IC
package, PCB layout, the rate of surrounding
airflow and temperature difference between
junction and ambient. The maximum power
dissipation can be calculated by the following
formula:
P_D
=
T_J(MAX)
θ
-T_A
JA
Where TJ(max) is the maximum allowable
junction temperature 125°C.T_A is the ambient
temperature and θ_JA is the thermal resistance
from the junction to the ambient. Based on the
standard JEDEC for a two layers thermal test
board, the thermal resistance θ_JA of SOT23-5
package is 250°C/W. The maximum power
dissipation at T_A = 25°C can be calculated by
following formula:
P_D=(125°C-25°C)/250°C/W=0.4W
Setting the Output Current
The internal feedback(FB) voltage is 0.1V
(Typical). The output current is ca lculated as
below:
ILED=0.1/Rs
The output Current is given by the following
table.
R s(Ω)
ILE D(m A )
0.286
350
0.143
700
0.1
1000
As the input voltage approaches the LED forward
voltage, the PAM2804 turns the P-chan nel
transistor continuously on. In this mode the
Voltage drop on LED is equal to the input voltage
minus the voltage drop across the P - channel
transistor, Inductor and current resistor:
V _LEDDROP = V _IN – ILED (R _dson + R _L+ R _S)
where R_dson = P-channel switch ON resistance,
ILED = LED current, R_L = Inductor DC resistance,
R = Inductor DC resistance.
S
Thermal Shutdown
When the die temperature exceeds 150°C, a
reset occurs and the reset remains until the
temperature decrease to 120°C, at which time
the circuit can be restarted.
100% Duty Cycle Operation
Power Analog Microelectronics, Inc
www.poweranalog.com
8
10/2010 Rev1.0