ST1208 datasheet(6/12 Pages) SITRONIX

6 / 12 page

ST1208

V1.1

6/12

2004/10/27

! Applications Information

Dimming Using EN1 and EN2

( See Figure 1. )

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

LED CURRENT

Shutdown

ILED = 0

1/10 Brightness

ILED = 16 X ISET

3/10 Brightness

ILED = 48 X ISET

Full Brightness

ILED = 160 X ISET

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.

ILED = K·ISET· ( 0.9·PWM + 0.1 )

Where K = 160, PWM = 0% ~ 100%.

Dimming Using a Filtered PWM Signal

( See Figure 3. )

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.

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.

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,

Component Selection

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

ST1208 Datasheet(PDF) 6 Page - Sitronix Technology Co., Ltd.

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