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MAX1574

180mA, 1x/2x, White LED Charge Pump in 3mm x 3mm TDFN

Maxim Integrated 

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

in 3mm x 3mm TDFN

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

When VIN is higher than VOUT, the MAX1574 operates in

1x mode and VOUT is pulled up to VIN. The internal cur-

rent regulators regulate the LED current. As VIN drops,

VLED_ eventually falls below the switchover threshold of

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

When the input voltage rises above VOUT by approxi-

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-

put current is set to 5% of the maximum set by RSET. In

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,

RSET, as follows:

RSET

=

0.6V × 393

ILED _

EN Dimming Control

When the LEDs are enabled by driving EN high, the

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

rent up to ILED_. Dimming is then done by pulsing EN

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

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

rent is 0.9 x ILED. The tenth pulse reduces the current by

5%, so the ILED_ current reduces from 0.1 x ILED_ to 0.05

x ILED. The eleventh pulse sets the LED current back to

ILED_. Figure 1 shows a timing diagram for EN.

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.

INITIAL tHI

0

≥ 50µs

EN

tSOFT-START

100%

ILED_

SHDN

12345

tLO

90%

80%

70%

500ns TO 500µs

60% 50%

6 7 8 9 10 11

tHI

≥ 500ns

40% 30% 20% 10% 5%

tSHDN

100% 90% 2ms (typ)

SHDN

Figure 1. EN Timing Diagram

6 _______________________________________________________________________________________

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