PAM2842 datasheet(12/17 Pages) PAM

12 / 17 page

2N7002

COMP

PAM2842

PW M signal

Ton

Toff

PAM2842

High Power LED Driver

Power Analog Microelectronics Inc

www.poweranalog.com

09/2008 Rev 1.1

Method 1: LED Current Setting with Resistor

Rsense

Method 2: LED Current Setting with PWM

Signal Using COMP Pin

Figure 1. PWM Dimming Use COMP Pin

Method 3: LED Current Setting with PWM

Signal using Sense Pin

Figure 2. PWM Dimming Use Sense Pin in Low

Side Current Sense

Figure 3. PWM Dimming Use Sense Pin in High

Side Current Sense

The most basic means of setting the LED current

is connecting a resistor between Rsense+ and

Rsense-. The LED current is decided by ISET

Resistor Rsense.

=0.1/ R

For flowing the large current, must pay attention

to power dissipation on the resistor.

Rsense has two position 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.

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:

Also, the recommended PWM frequency is

between 100Hz and 200Hz.

Frequency <100Hz can cause the LEDs to blink

visibly. As the COMP pin connects to a capacitor,

it needs rise time. If frequency >200Hz, the

average LED current will have a large error when

duty cycle is small (<50%).

It maybe generate the audible noise in this

dimming condition.

This method is turn PWM signal to DC voltage, the

output current can be adjusted. Because the LED

current is a adjustable DC value, it will cause LED

color drift.

Low side current sense and high side current

sense circuit is different. Please see Figure 2 and

3. It use the internal reference voltage, so PWM

dimming signal voltage is not considered, just

meet the request of the MOSFET driving voltage.

The RC filter (R1,R2,C1,C2) value is decided by

dimming frequency, the divider resistor (R3,R4) is

decided by dimming range.

Because final adjusted is a DC value, this method

can avoid audible noise effectively and achieve

better EMI performance than the second method.

LED

sense

OFF LED

AVG

OFF

RSense

VDD_5V

Sense+

RTN

PWM-DIM

RSense

VDD_5V

Sense-

PWM-DIM

Sense+

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