MAX1553/MAX1554

High-Efficiency, 40V Step-Up

Converters for 2 to 10 White LEDs

Maxim Integrated | 8

www.maximintegrated.com

Adjusting the LED Current

Adjusting the output current changes the brightness of

the LEDs. The LED current is set by the voltage at BRT

range for adjusting output current is 0 to 1.25V. Over

this range, the LED current is found from the following

equation:

greater than 1.72V does not increase the output current

above the level at 1.72V. See the LED Current vs. BRT

Voltage graph in the

Typical Operating Characteristics

section. To set the maximum LED current, calculate R1

age. Power dissipation in R1 is typically less than 5mW;

therefore, power dissipation in a standard chip resistor

is not a concern.

PWM Dimming Control

The BRT input is also used as a digital input allowing

LED brightness control with a logic-level PWM signal

applied directly to BRT. The frequency range is from

100Hz to 10kHz, and the duty cycle range is 0 to 100%.

A 0% duty cycle corresponds to the minimum current,

and a 100% duty cycle corresponds to full current. See

the LED Current vs. BRT Duty Cycle graph in the

Typical Operating Characteristics section. The BRT

resistor and SS capacitor form a lowpass filter, so PWM

dimming results in DC current to the LEDs without the

need for additional RC filters.

Capacitor Selection

A 0.47µF ceramic output capacitor (C2) is recommend-

ed for most applications. For circuits driving six or

fewer LEDs, use a 4.7µF ceramic input capacitor (C1).

For circuits driving more than six LEDs, use a 10µF

input capacitor (C1). For best stability over a wide tem-

perature range, use capacitors with an X5R, X7R, or

better dielectric.

Inductor Selection

The MAX1553 has a 480mA inductor current limit and

can drive up to six LEDs at 20mA or nine LEDs at

15mA. Inductor values from 4.7µH to 47µH work satis-

factorily. Larger values provide the best efficiency while

small inductor values allow the smallest inductor size. A

good choice for best efficiency is the TOKO D62 or

D62L series at 47µH. For smallest size, the Murata

LQH32C at 4.7µH works well.

The MAX1554 has a 970mA inductor current limit and

can drive up to 10 LEDs at 20mA. Inductor values from

4.7µH to 22µH work satisfactorily. A good choice for

high efficiency and small size when driving 9 or 10

LEDs is the TOKO D62 series at 22µH.

When large inductor values are used to optimize effi-

ciency, the MAX1553/MAX1554 operate with continuous

inductor current. With large inductor values (typically

greater than 10µH), stability, input, and output ripple

are improved by connecting a capacitor in parallel with

the LEDs (C4 in Figures 1, 2, and 3).

To prevent saturation, use an inductor with a current

rating that matches the device’s LX current limit.

However, if size is particularly important, it is some-

times acceptable to operate the inductor 10% into satu-

ration. For best efficiency, the inductor’s DC resistance

should also be as low as possible.

Diode Selection

The MAX1553/MAX1554s’ high switching frequency

demands a high-speed rectification diode (D1) for opti-

mum efficiency. A Schottky diode is recommended due

to its fast recovery time and low forward-voltage drop.

Ensure the diode’s average and peak current ratings

exceed the average output current and peak inductor

current. In addition, the diode’s reverse breakdown

R

V

xI

BRT MAX

LED MAX

1

017

667

=

+

()

()

.

.

I

V

xR

LED

BRT

=

667

1

.

.

LX

SS

R1

10

Ω

R3

10k

Ω

R2

330k

Ω

FB

ON

OFF

BRT

GND

3.15V TO 5.5V

INPUT

C1

10

μF

C3

0.1

μF

C2

0.47

μF

50V

L1

22

μH

A915BY-220M

D1

CMDSH1-60M

OV

EN

D2–D11

WHITE

LEDs

PWM

OR

DC CONTROL

MAX1554

C4

3300pF

Figure 3. Circuit with the MAX1554 Driving 10 White LEDs