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MAX1575

White LED 1x/1.5x Charge Pump for Main and Sub-Displays

Maxim Integrated 

White LED 1x/1.5x Charge Pump

for Main and Sub-Displays

Setting the Output Current

The LED current at full (100%) brightness is set (up to

30mA) by a resistor, RSET, as follows:

RSET

=

0.6V × 234

ILED_

ENM and ENS Dimming Controls

When the LEDs are enabled by driving ENM or ENS

high, the LED current initially goes to ILED_.

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 ILED.

The 10th pulse reduces the current by 5% so the LED

current reduces from 0.1 x ILED_ to 0.05 x ILED. The

11th pulse sets the LED current back to ILED_. Figure 1

shows a timing diagram for EN_.

Because soft-start is longer than intitial tHI, apply dim-

ming pulses quickly upon startup (after initial tHI) to

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.

Shutdown Mode

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.

Overvoltage Protection

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.

Thermal Shutdown

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.

Applications Information

Driving Fewer than Six LEDs

When driving fewer than six LEDs, connect any unused

LED_ directly to IN (Figure 3). When connected in this

manner, the corresponding LED driver is disabled.

Input Ripple

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 CIN to 2.2µF

cuts input ripple in half with only a small increase in foot-

print. The 1x mode always has very low input ripple.

INITIAL tHI

≥50μs

0

ENM

OR ENS

tSOFT-START

100%

ILED_ SHDN

12345

tLO

90%

80%

70%

500ns TO 500μs

60% 50%

Figure 1. EN_ Timing Diagram

6

7

8

9 10 11

tHI

≥500ns

tSHDN

100% 90% 2ms (typ)

40% 30% 20% 10% 5%

SHDN

6 _______________________________________________________________________________________

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