Rainbow Electronics MAX1848 - Manual (Page 7)

The output current and the number of LEDs in each leg
affect the capacitance of C
COMP . Table 1 shows the
minimum C COMP values needed to stabilize the con-
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
exceed V
OUT .
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
between the IC and R
SENSE
, the inductor, the diode,
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
capacitor (C
IN
) should be placed as close to the IC as
possible. PGND and GND should be connected in only
one place as close to the IC as possible. The ground
connections of C
IN and C OUT should be as close
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
≅ ×
MAX1848
White LED Step-Up Converter in SOT23
_______________________________________________________________________________________ 7
INDUCTOR I LED
(mA)
NO. OF
LEDs
C COMP
(µF) L (µH) I PEAK ( m A )
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
R SENSE
5 Ω
25 Ω
Figure 2. Six LEDs in 3 x 2 Configuration