11

 2005 Semtech Corp.

www.semtech.com

SC604A

POWER MANAGEMENT

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Note: 1) “N” is the number of NMOS transistors used for brightness control.

Table 4 - Summary of LED Current Control

Figure 3 - PWM Example Circuit

Method 4. LED current may also be controlled by applying a PWM

signal to any of the CTRL2, CTRL1 and CTRL0 inputs. The circuit in

Figure 3 turns 4 LEDs on and off by applying a PWM signal to the

CTRL0 input. This circuit uses resistor R

current and the average LED current is then proportional to the

percentage of on-time when the CTRL0 pin is a logic low. Average

LED current is approximately equal to:

I

I

The recommended PWM frequency is between 100Hz and 500Hz.

Due to start up delay and ramp up time, frequency >500Hz will

result in error in the average value of I

naturally cause the LEDs to blink visibly.

In PWM applications where ILED4 is not used, keep ILED4 pin 13

open. Connecting ILED4 to ground can result in the charge pump

operating in open loop mode. Connecting ILED4 to V

but will cause shutdown current I

V

Methods for Setting LED Current (Cont.)