PAM2841 Datasheet, PDF (10/16 Pages) Power Analog Micoelectronics – 1.5ASW Current, 40V Precision WLED Driver

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PAM2841
Application Information (cont.)
Dimming Control
There are 4 different types of dimming control methods:
1). Using an External PWM Signal to EN Pin
With the PWM signal applied to the EN pin, the PAM2841 is alternately turned on or off by the PWM signal. The LEDs operate at either zero or
full current. The average LED current changes proportionally with the duty cycle of the PWM signal. A 0% duty cycle turns off the PAM2841 and
leads to zero LED current. A 100% duty cycle generates full current.Also the recommend dimming frequency is between 100Hz and 200Hz.
I_AVE = I_STATE * (T_ON –T_STARTUP)/(T_ON +T_OFF)
Where T_ON: on time of a period
T_STARTUP: 0.85ms
T_OFF: off time of a period
I_STATE: on state current (full current)
2). Using an External PWM Signal to NMOS Gate.
Figure. 1
When PWM signal is at high level, N MOSFET turned on, then pull comp pin down, then the LED current should be zero. When PWM signal is at
low level, N MOSFET turned off. The circuit uses resistor R1 to set the on state current. The average LED current changes proportionally with
the duty cycle of the PWM signal. A 100% duty cycle turns off the PAM2841 and leads to zero LED current. A 0% duty cycle generates full
current.
I_AVE = I_STATE * (T_ON –T_STARTUP)/(T_ON +T_OFF)
Where T_ON: on time of a period
T_STARTUP: 0.08ms
T_OFF: off time of a period
I_STATE: on state current (full current)
Also the recommend frequency is between 100 and 500Hz. Frequency<100Hz can naturally causes LEDs to blink visibly.
PAM2841
Document number: DSxxxxx Rev. 1 - 5
Figure. 2
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September 2013
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