SC604A
POWER MANAGEMENT
Methods for Setting LED Current (Cont.)
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 RSET to set the on state
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:
IAVG = (tON ILED_ON)/(tON + tOFF)
ꢁ
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 ILED. Frequency <100Hz can
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 VIN will work
but will cause shutdown current IQ to increase to approximately
VIN / 100k.
Table 4 - Summary of LED Current Control
Brightness
Control
LED Current Control Method
Figure of Reference
LED Current Range
0.5mA < ILED < 30mA
0.5mA < ILED < 30mA
0.5mA < ILED < 30mA
0.5mA < ILED < 30mA
Method 1
RSET Only
Circuit on Page 1
Figure 1 on Page 10
Figure 2 on Page 10
Figure 3 on page 11
Fixed Brightness
Infinite
Method 2
Method 3
Method 4
Analog Reference VADJ
NMOS Switched Parallel
Resistors
2N Stepped
Current Levels(1)
PWM CTRLx Input(s)
Infinite
Note: 1) “N” is the number of NMOS transistors used for brightness control.
2005 Semtech Corp.
www.semtech.com
11
SEMTECH [ SEMTECH CORPORATION ]
