PAM2804 Datasheet, PDF (7/11 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
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.
Inductor Selection
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_IN or Vout also increases the
ripple current as shown in equation 1. A
reasonable starting point for setting ripple
current is △I_L = 400mA (40% of 1A).
DI_L
=
(f
1
)(L
) ^{V OUT}
 _{ 1-}
VO UT
VIN

 _
(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.
temperature should be maintained below 125°C.
The power dissipation is defined as:
( ) _{( ) PD=IO2}
V_OR_DSONH +
V_IN -V_O
V_IN
R_{D SONL +}
t_SWF_SI_O +I_Q
V_IN
I_Q is the step-down converter quiescent current.
The term tsw is used to estimate the full load
step-down converter switching losses.
Using Ceramic Input and 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 (Over Temperature
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
Power Analog Microelectronics, Inc
www.poweranalog.com
7
10/2010 Rev1.0