PAM2842 Datasheet, PDF (11/17 Pages) Power Analog Micoelectronics – High Power LED Driver

PAM2842
High Power LED Driver
Chose
so
DI_L
=
V_INV_O
LF(V_IN + V_O )
ΔI_L=0.4I_L1
L = ^2.5VI2N
FI_O(V_IN + V_O )
Capacitor Selection
An input capacitor is required to reduce the input
ripple and noise for proper operation of the
PAM2842. For good input decoupling, Low ESR
(equivalent series resistance) capacitors should
be used at the input. At least 10μF input capacitor
is recommended for most applications. And close
the IC Vin-Pin we should add a bypass capacitor,
usually use a 1μF capacitor.
A minimum output capacitor value of 10μF is
recommended under normal operating
conditions, while a 22μF or higher capacitor may
be required for higher power LED current. A
reasonable value of the output capacitor depends
on the LED current. The total output voltage ripple
has two components: the capacitive ripple caused
by the charging and discharging on the output
capacitor, and the ohmic ripple due to the
capacitor's equivalent series resistance. The ESR
of the output capacitor is the important parameter
to determine the output voltage ripple of the
converter, so low ESR capacitors should be used
at the output to reduce the output voltage ripple.
The voltage rating and temperature
characteristics of the Output capacitor must also
be considered. So a value of 10μF, 50V voltage
rating capacitor is chosen.
Consider from discharge aspect: IxΔt=CxΔV
In
boost
and
sepic
topology,
C_O
=
I_OD
FV_RIPPLE
In buck topology,
C_O
= ^{IO(1- D)}
FV_RIPPLE
V : _RIPPLE Output voltage allowable ripple.
Consider from equivalent series resistance:
V =I xC _{ripple-esr co.ripple}
oesr
In sepic topology, there is a series capacitor Cs
between L1 and L2 (see application schematic), it
flows the current:
I_Cs(RMS) = I_O
V_O
V_IN
The ripple voltage is
DV_Cs
=
I_OD
FC_S
The voltage rating must be higher than input
voltage.
Because the Cs capacitor will flow the large RMS
current, so this topology is suitable for small
power application.
Diode Selection
PAM2842 is a high switching frequency converter
which demands high speed rectifier. It's
indispensable to use a Schottky diode rated at 3A,
40V with the PAM2842. Using a Schottky diode
with a lower forward voltage drop is better to
improve the power LED efficiency.
In boost topology, the voltage rating should be
higher than Vout and in buck topology, the voltage
rating higher than Vin, the peak current is
I_DMAX
= I_L
+
DI_L
2
in sepic topology, the voltage rating should be
higher than Vin+Vout, the peak current is
I =I +I _{DMAX L1peak L2peak}
The average current of the diode equals to Io.
Work frequency selection
PAM2842 working frequency is decided by
resistor connect to the RT pin, it can be calculated
by follow equation:
F_SW
=
10¹²
(Hz)
24 ´ (RT + 12K)
From the equations, we can see when working
frequency is high, the inductance can be small.
It's important in some size limit application. But
we should know when the working frequency is
higher, the switching loss is higher too. We must
pay attention to thermal dissipation in this
application.
Methods for Setting LED Current
There are two methods for setting and adjusting
the LED current:
1) Rsense only
2) PWM signal with external components
a) Use the COMP pin
b) Use the Sense pin
Power Analog Microelectronics,Inc
www.poweranalog.com
11
09/2008 Rev 1.1