The output current and the number of LEDs in each leg

verter in worst-case conditions. If further stability analy-

sis is required, note that the error amplifier has 50µA/V

transconductance.

Inductor Selection

The value of the inductor depends on the maximum

output current to the LEDs. See Table 1 for inductance

values and peak current ratings for the inductor.

Schottky Diode Selection

The MAX1848’s high-switching frequency demands a

high-speed rectification diode. A Schottky diode is

required due to their fast recovery time and low for-

ward-voltage drop. Ensure that the diode’s average

and peak current rating exceed the average output cur-

rent and peak inductor current, respectively. In addi-

tion, the diode’s reverse breakdown voltage must

Applications Information

Connecting Four or Six LEDs

The MAX1848 can drive one, two, or three legs of LEDs

(Figure 2) as long as the total number of LEDs does not

exceed six. Each leg must contain the same number of

LEDs and the same sense-resistor value. Adding the

second or third leg does not affect the sense-resistor

value (see the Adjusting LED Current section).

Three legs of two LEDs is more efficient than two legs

of three LEDs (see Efficiency Graphs in the Typical

Operating Characteristics); however, a third sense

resistor is needed. Multiple legs can have slight current

mismatches due to component tolerances.

PC Board Layout

Due to fast-switching waveforms and high-current

paths, careful PC board layout is required. Protoboards

and wire-wrap boards should not be used for evalua-

tion. An evaluation kit (MAX1848EV kit) is available to

aid design.

When laying out a board, minimize trace lengths

the input capacitor, and the output capacitor. Keep

traces short, direct, and wide. Keep noisy traces, such

as the inductor's traces, away from CS. V+’s bypass

possible. PGND and GND should be connected in only

one place as close to the IC as possible. The ground

together as possible. The traces from V+ to the induc-

tor and from the Schottky diode to the LEDs may

be longer.

Refer to the MAX1848 EV kit for an example of proper

layout.

Chip Information

TRANSISTOR COUNT: 1290

I

I

I

DIODE(RMS)

OUT

PEAK

≅

×

White LED Step-Up Converter in SOT23

_______________________________________________________________________________________

7

INDUCTOR

(mA)

NO. OF

LEDs

(µF)

L (µH)

3

0.220

12

2

0.100

56

80

3

0.150

20

2

0.068

33

130

3

0.100

40

2

0.047

15

260

3

0.068

60

2

0.01

10

375

Table 1. Component Selection

V+

CTRL

COMP

LX

OUT

PGND

GND

CS

2.6V to 5.5V

LOGIC

OR DAC

3.3

µF

0.01

µF

10

µH

1

µF

D1

MAX1848

30

Ω

5

Ω

25

Ω

Figure 2. Six LEDs in 3 x 2 Configuration