MAX1553/MAX1554

High-Efficiency, 40V Step-Up

Converters for 2 to 10 White LEDs

Maxim Integrated | 6

www.maximintegrated.com

PIN

NAME

FUNCTION

1

GND

Ground

3

EN

4

BRT

Brightness-Control Input. Either an analog or PWM control signal can be used. The LED current can be

controlled over a 10 to 1 range. The PWM signal must be between 100Hz and 10kHz, and must have an

amplitude greater than 1.72V.

5FB

Feedback Input. Connect to the cathode of the LED string and connect a resistor from FB to GND to set the

LED current.

6SS

Soft-Start Timing-Control Input. Connect a capacitor from SS to GND to control soft-start timing. See the Soft-

Start section for information on selecting the soft-start capacitor. SS is pulled to ground with an internal 200

Ω

switch when EN is low.

7OV

Overvoltage Sense. Connect to a resistor-divider from the anode of the LED string to set the overvoltage

threshold. See Figures 1, 2, and 3.

8

LX

Inductor Connection. Connect to the inductor and diode. LX is high impedance when EN is low.

—

EP

Exposed Pad. Connect to GND.

Pin Description

Detailed Description

Control Scheme

The MAX1553/MAX1554 utilize a minimum off-time, cur-

rent-limited control scheme. If the voltage at FB drops

below the regulation threshold, the internal low-side

MOSFET turns on and the inductor current ramps up to

the current limit. Once the current-limit comparator

trips, the low-side MOSFET turns off for the minimum

off-time (250ns). After 250ns, if the voltage at FB is

above the regulation threshold, the low-side MOSFET

stays off. If the voltage at FB is below the regulation

point, the low-side MOSFET turns back on and the

cycle repeats. By using a regulation control scheme

that is not fixed frequency and that can skip pulses, the

MAX1553/MAX1554 operate with very high efficiency.

Soft-Start

Soft-start is provided on the MAX1553/MAX1554 to min-

imize inrush current. The soft-start time is set with an

external capacitor, C3 (Figures 1, 2, and 3). Use the fol-

lowing equation to solve for C3:

vides a soft-start time of 20ms.

Shutdown

The MAX1553/MAX1554 feature a low-current shut-

down feature. When EN is low, the IC turns off, reduc-

ing its supply current to approximately 0.1µA. For

Overvoltage Protection

The MAX1553/MAX1554 have an adjustable overvolt-

age- protection circuit. When the voltage at OV reaches

the overvoltage threshold (1.25V typ), the protection cir-

cuitry prevents the internal MOSFET from switching,

allowing the output voltage to decay.

The peak output voltage in an overvoltage-protection

event is set with a resistor-divider from the output con-

nected to OV (R2 and R3 in Figures 1, 2, and 3). Select

a value for R3 (10k

Ω is recommended), then solve for

R2 using the following equation:

RR

x

V

V

OUT PEAK

OV

23

1

=−

⎛

⎝⎜

⎞

⎠⎟

()

C

x

tSS

3

=