ST1208
! Applications Information
Dimming Using EN1 and EN2
( See Figure 1. )
Input Ripple
( See Figure 4. )
For LED drivers, input ripple is more important than
output ripple. Input ripple depends on the source
supply’s impedance. Adding a lowpass filter to the input
further reduce input ripple. Figure 4 shows a C-R-C
filter used to reduce input ripple to less than 2mVp-p
when driving a heavy load or flash LED.
Use EN1 and EN2 inputs as a digital 2-bit number to
control on/off, 1/10, 3/10 and full current ( see Table1 ).
RSET programs the full current level ( see the Setting
the Output Current section ).
EN1 EN2
BRIGHTNESS
Shutdown
1/10 Brightness
3/10 Brightness
Full Brightness
LED CURRENT
ILED = 0
ILED = 16 X ISET
ILED = 48 X ISET
ILED = 160 X ISET
0
0
1
1
0
1
0
1
Table 1. Brightness Control
Dimming Using PWM into EN1
( See Figure 2. )
Use EN2 for shutdown and drive EN1 with a PWM
signal. Current can be varied from 1/10 to full. The
maximum frequency of PWM is 10 KHz.
PCB Layout and Routing
The ST1208 is a high-frequency switched-capacitor
voltage regulator. For best circuit performance, use a
solid ground plane and place CIN and COUT as close
to the ST1208 as possible. Also, place their ground
pads close together and as close as possible to GND.
Making the bias current clear can enhance stability of
the LED output current,
ILED = K·ISET· ( 0.9·PWM + 0.1 )
Where K = 160, PWM = 0% ~ 100%.
Dimming Using a Filtered PWM Signal
( See Figure 3. )
Component Selection
Use a high-frequency PWM signal to drive an R-C-R
filter on the SET pin. A 0% PWM duty cycle
corresponds to 20mA/LED, while a 100% PWM duty
cycle corresponds to 0mA/LED. At PWM frequencies
above 5 KHz, C may be reduced.
For high efficiency performance, how to choose the
component is the most important.
Low ESR capacitor can reduce the output voltage
ripple and increase the voltage of charge pump. The
ceramic capacitor with an X5R or X7R is the best.
The efficiency of 1x mode is over 15% higher than 1.5x
mode. Therefore, higher input voltages or the low
forward voltage of LED can make the ST1208 switch to
1x mode.
Figure 1. Typical Operating Circuit
Figure 2. Dimming Using PWM Signal into EN1
V1.1
6/12
2004/10/27
SITRONIX [ SITRONIX TECHNOLOGY CO., LTD. ]
