Diodes PAM2845 - Manual (Page 9)

PAM2845
Document number: DSxxxxx Rev. 1 - 2
9 of 14
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October 2012
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PAM2845
A Product Line of
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Application Information
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 I
2 R 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 I 2 R
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 PAM2845. 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.
Diodes Selection
PAM2845 is high switching frequency convertor, which demands high speed rectifier. It's indispensable to use a Schottky diode rated at 2A, 60V
with the PAM2845. Using a Schottky diode with a lower forward voltage drop is better to improve the power LED efficiency, and its voltage rating
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
2) PWM Input at PWMD
3) PWM Input at ENA
Figure 1