PAM2842 Datasheet, PDF (10/17 Pages) Power Analog Micoelectronics – High Power LED Driver

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PAM2842
Application Information (cont.)
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 V_OUT and in buck topology, the voltage rating higher than V_IN, the peak current is
ID(MAX)
=
IL
+
ΔI_L
2
in sepic topology, the voltage rating should be higher than V_IN +V_OUT, the peak current is
ID(MAX) ⁼ IL1(PEAK ) ⁺ IL2(PEAK )
The average current of the diode equals to I_O.
Work Frequency Selection
PAM2842 working frequency is decided by resistor connect to the RT pin, it can be calculated by follow equation:
FSW
=
10¹²
(Hz)
24x(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) R_SENSE only
2) PWM signal with external components
a) Use the COMP pin
b) Use the Sense pin
● Method 1: LED Current Setting with Resistor R_SENSE
The most basic means of setting the LED current is connecting a resistor between R_SENSE+ and R_SENSE-. The LED current is decided by ISET
Resistor R_SENSE.
I_LED = 0.1/ R_SENSE
For flowing the large current, must pay attention to power dissipation on the resistor.
R_SENSE has two positions to select: high side current sense and low side current sense. In buck topology it just has high side current sense. In
other topology we recommend use low side current sense for easier PCB layout.
● Method 2: LED Current Setting with PWM Signal Using COMP Pin
This circuit uses resistor Rsense to set the on state current and the average LED current, then proportional to the percentage of off-time when
the COMP pin is logic high. Here use a invert component 2N7002 (Q1) to isolate and invert the PWM signal (See Figure 1).
Average LED current is approximately equal to:
IAVG
=
TOFF ILED
TON ⁺ TOFF
PAM2842
Document number: DSxxxxx Rev. 1 - 2
Figure 1. PWM Dimming Use COMP Pin
10 of 17
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October 2012
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