PAM2846 Datasheet, PDF (10/14 Pages) Power Analog Micoelectronics – Integrated 6 String LED Boost Type Driver

PAM2846
Integrated 6 String LED Boost Type Driver
Application Information
Inductor Selection
The inductance, peak current rating, series
resistance, and physical size should all be
considered when selecting an inductor. These
factors affect the converter's operating mode,
efficiency, maximum output load capability,
transient response time, output voltage ripple,
and cost.
The maximum output current, input voltage,
output voltage, and switching frequency
determine the inductor value. Very high
inductance minimizes the current ripple, and
therefore reduces the peak current, which
decreases core losses in the inductor and I²R
losses in the entire power path. However, large
inductor values also require more energy storage
and more turns of wire, which increases physical
size and I²R copper losses in the inductor. Low
inductor values decrease the physical size, but
increase the current ripple and peak current.
Finding the best inductor involves the
compromises among circuit efficiency, inductor
size, and cost.
When choosing an inductor, the first step is to
determine the operating mode: continuous
conduction mode (CCM) or discontinuous
conduction mode (DCM). When CCM mode is
chosen, the ripple current and the peak current of
the inductor can be minimized. If a small-size
inductor is required, DCM mode can be chosen.
In DCM mode, the inductor value and size can be
minimized but the inductor ripple current and
peak current are higher than those in CCM.
Capacitor Selection
An input capacitor is required to reduce the input
ripple and noise for proper operation of the
PAM2846. For good input decoupling, low ESR
(equivalent series resistance) capacitors should
be used at the input. At least 2.2μF input
capacitor is recommended for most applications.
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, voltage rating
(50V) capacitor is chosen.
Diode Selection
PAM2846 is high switching frequency convertor,
which demands high speed rectifier. It's
indispensable to use a Schottky diode rated at
2A, 60V with the PAM2846. Using a Schottky
diode with a lower forward voltage drop can
improve efficiency of the convertor. The voltage
rating of the diode should be greater than the
output voltage.
Methods for Setting LED Current
There are three methods for setting and
adjusting the LED current outlined here. The
methods are:
1) RSET only
2) PWM Input at PWMD
3) Single wire logic signal at ENA
>10ms
0
12345
27 28 29 30 31 32
ENA
t _SOFT-START
I_{LED_}
shutdown
32/32
31/32
500ns<t_LO <10μs
30/32 _{29/32 28/32}
t_HI>500ns
32/32
5/32
4/32 _{3/32 2/32 1/32}
Figure 1. Timing diagram for EN
Power Analog Microelectronics,Inc
www.poweranalog.com
10
t
shutdown
200μs(typ)
shutdown
07/2008 Rev 1.2