7

,

Power Analog Microelectronics Inc

www.poweranalog.com

PAM2804

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

10/2010 Rev1.0

Application Information

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 in 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 V

or Vout also increases the

rip ple curren t a s sho wn in e qua tio n 1. A

reasonable starting point for setting ripple

current is

I = 400mA (40% of 1A).

(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-resis tance inductor.

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 charac teristics of all

the ceramics for a given value and size.

Inductor Selection

Using Ceramic Input and Output Capacitors

IN

L

△

Thermal consideration

Thermal protection limits power dissipation in

the PAM2804. When the junction temperature

exceeds 150°C, the OTP (Over Temperature

Protection) starts the thermal shutdown and

turns the pass transistor off. The pass transistor

r e s u m e s

o p e r a t i o n

a f t e r

t h e

j u n c t i o n

temperature drops below 120°C.

F o r

c o n t i n u o u s

o p e r a t i o n ,

t h e

j u n c t i o n

temperature should be maintained below 125°C.

The power dissipation is defined as:

I is the step-down converter quiescent current.

The term tsw is used to estimate the full load

step-down converter switching losses.

Q

( )( )





D









OUT

OUT

L

IN

1

V

I =

V

1-

f

L

V

(

)

(

)

O

DSONH

IN

O

D SONL

2

D

O

SW

S O

Q

IN

IN

V R

+ V -V

R

P =I

+ t

F I +I

V

V