8

,

Power Analog Microelectronics Inc

www.poweranalog.com

PAM2804

1A Step-Down Constant Current, High Efficiency LED Driver

10/2010 Rev1.0

Application Information

For the condition where the step-down converter

is in dropout at 100% duty cycle, the total device

dis sipation reduces to:

Since R

, 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

d e p e n d s o n t h e t h e r m a l r e si st a n c e o f I C

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:

Where TJ(ma x) is the m aximu m allo wable

junction temperature 125°C.T

is the ambient

temperature and

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

of SOT23-5

package is 250°C/W. The maximum power

dissipation at T

= 25°C can be calculated by

following formula:

P =(125°C-25°C)/250°C/W=0.4W

The in te rna l feedb ack(FB) voltage is 0.1 V

(Typical). The output current is ca lculated as

below:

ILED=0.1/Rs

The output Current is given by the following

table.

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

= V –ILED (R

+ R + R )

where R

= P-channel switch ON resistance,

ILED = LED current, R = Inductor DC resistance,

R = Inductor DC resistance.

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.

DS(ON)

A

JA

JA

A

D

LEDDROP

IN

dson

L

S

dson

L

S

θ

θ

Setting the Output Current

100% Duty Cycle Operation

Thermal Shutdown

2

D

O

DSON H

Q

IN

P =I R

+I V

J(MAX)

A

D

JA

T

-T

P =

θ

Rs(

Ω)

ILED(mA)

0.286

350

0.143

700

0.1

1000