MAX1575 Maxim Integrated Products, MAX1575 Datasheet - Page 6

• Manufacturer Part Number: MAX1575
• Description: White LED 1x/1.5x Charge Pump
• Manufacturer: Maxim Integrated Products

White LED 1x/1.5x Charge Pump
for Main and Sub-Displays
The LED current at full (100%) brightness is set (up to
30mA) by a resistor, R
When the LEDs are enabled by driving ENM or ENS
high, the LED current initially goes to I
Dimming for the main display is done by pulsing ENM
low (500ns to 500µs pulse width). Dimming for the sub-
display is done by pulsing ENS low (500ns to 500µs
pulse width). Each pulse reduces the LED current by
10%, so after one pulse the LED current is 0.9 x I
The 10th pulse reduces the current by 5% so the LED
current reduces from 0.1 x I
11th pulse sets the LED current back to I
shows a timing diagram for EN_.
Because soft-start is longer than intitial t
ming pulses quickly upon startup (after initial t
avoid LED_ current transitioning through full brightness.
If dimming control is not required, EN_ work as simple
on/off controls. Drive ENM high to enable the main
LEDs, or drive ENM low to turn off the main LEDs. Drive
ENS high to enable the sub-LEDs, or drive ENS low to
turn off the sub-LEDs. Drive both ENM and ENS low to
put the IC in low-power shutdown mode.
Figure
6
OR ENS
_______________________________________________________________________________________
I
LED_
ENM
1. EN_ Timing Diagram
SHDN
0
ENM and ENS Dimming Controls
t
SOFT-START
INITIAL t
100%
≥50µs
R
SET
Setting the Output Current
SET
HI
=
, as follows:
0 6
.
I
1
LED
V
LED_
90%
×
228
_
2
80%
to 0.05 x I
LED_
3
70%
LED_
HI
.
, apply dim-
4
60%
.
LED
Figure
500ns TO 500µs
5
HI
50%
. The
LED
) to
t
LO
1
.
When both ENM and ENS are held low for 2ms or longer,
the MAX1575 is shut down and put in a low-current
mode. OUT is internally pulled to GND with 5kΩ during
shutdown.
If any LED fails as an open circuit, the output voltage is
limited to approximately 5V by gating on/off the charge
pump. In case any LED_ is floating or grounded, the
MAX1575 operates in the same overvoltage-protection
mode. To avoid overvoltage-protection mode when using
fewer than six LEDs, connect any unused LED_ to IN.
The MAX1575 includes a thermal-limit circuit that shuts
down the IC at approximately +160°C. The part turns
on after the IC cools by approximately 20°C.
When driving fewer than six LEDs, connect any unused
LED_ directly to IN
manner, the corresponding LED driver is disabled.
For LED drivers, input ripple is more important than out-
put ripple. Input ripple depends on the source supply’s
impedance. Adding a lowpass filter to the input further
reduces input ripple. Alternately, increasing C
cuts input ripple in half with only a small increase in foot-
print. The 1x mode always has very low input ripple.
6
≥500ns
40%
t
HI
7
30%
8
20%
Applications Information
9
Driving Fewer than Six LEDs
10%
(Figure
10
5%
Overvoltage Protection
11
3). When connected in this
100% 90%
Thermal Shutdown
www.DataSheet4U.com
Shutdown Mode
2ms (typ)
Input Ripple
t
SHDN
IN
to 2.2µF
SHDN