Detailed Description

The MAX1574 charge pump drives up to three white

LEDs with regulated constant current for uniform inten-

sity. By utilizing adaptive 1x/2x charge-pump modes

and very-low-dropout current regulators, it achieves

180mA output drive capability and high efficiency over

the 1-cell lithium-battery input voltage range. Fixed-fre-

quency switching of 1MHz allows for tiny external com-

ponents, and the regulation scheme is optimized to

ensure low EMI and low input ripple.

1x to 2x Switchover

130mV, and the MAX1574 starts switching in 2x mode.

mately 50mV, the MAX1574 switches back to 1x mode.

Soft-Start

The MAX1574 includes soft-start circuitry to limit inrush

current at turn-on. When starting up, the output capacitor

is charged directly from the input with a ramped current

source (with no charge-pump action) until the output volt-

age approaches the input voltage. Once this occurs, the

charge pump determines if 1x or 2x mode is required. In

the case of 1x mode, the soft-start is terminated and nor-

mal operation begins. During the soft-start time, the out-

the case of 2x mode, soft-start operates until the lowest of

LED1 to LED3 reaches regulation. If an overload condi-

tion occurs, soft-start repeats every 2.1ms. If the output is

shorted to ground, the output current is limited by the

MAX1574 switching technique.

Setting the Output Current

The LED current at full brightness is set by a resistor,

EN Dimming Control

When the LEDs are enabled by driving EN high, the

MAX1574 goes through soft-start, bringing the LED cur-

low (500ns to 500µs pulse width). Each pulse reduces

the LED current by 10%, so after one pulse the LED cur-

If dimming control is not required, EN works as a simple

on/off control. Drive EN high to enable the LEDs, or

drive EN low for shutdown.

Shutdown Mode

When EN is held low for 2ms or longer, the MAX1574

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 about 5V by gating on/off the charge pump.

If any LED_ is floating or grounded, the MAX1574 oper-

ates in the same overvoltage-protection mode. To avoid

overvoltage-protection mode when using fewer than

three LEDs, connect any unused LED_ to IN (Figure 3).

Thermal Shutdown

The MAX1574 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.

R

V

I

SET

LED

.

_

=

×

06

393

180mA, 1x/2x, White LED Charge Pump

in 3mm x 3mm TDFN

6

_______________________________________________________________________________________

EN

SHDN

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

5%

100% 90%

SHDN

1

11

10

9

8

7

6

5

4

3

2

≥ 500ns

500ns TO 500µs

0

≥ 50µs

Figure 1. EN Timing Diagram