MAX1848ETA-T Maxim Integrated Products, MAX1848ETA-T Datasheet - Page 7

• Manufacturer Part Number: MAX1848ETA-T
• Description: LED Drivers High-Efficiency Unif orm-Brightness White
• Manufacturer: Maxim Integrated Products

Specifications of MAX1848ETA-T

• Number Of Segments: 3
• Operating Supply Voltage: 2.6 V to 5.5 V
• Maximum Power Dissipation: 1951 mW
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: TDFN EP
• Minimum Operating Temperature: - 40 C

The output current and the number of LEDs in each leg
affect the capacitance of C
minimum C
verter in worst-case conditions. If further stability analy-
sis is required, note that the error amplifier has 50µA/V
transconductance.
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.
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
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.
Table 1. Component Selection
(mA)
I
LED
12
20
40
60
OUT
I
COMP
DIODE(RMS)
NO. OF
.
LEDs
Applications Information
3
2
3
2
3
2
3
2
_______________________________________________________________________________________
values needed to stabilize the con-
Connecting Four or Six LEDs
White LED Step-Up Converter in SOT23
Schottky Diode Selection
≅
C
0.220
0.100
0.150
0.068
0.100
0.047
0.068
0.01
COMP
(µF)
I
OUT
COMP
Inductor Selection
×
I
. Table 1 shows the
PEAK
L (µH)
56
33
15
10
INDUCTOR
I
PEAK
130
260
375
80
( m A )
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
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
possible. PGND and GND should be connected in only
one place as close to the IC as possible. The ground
connections of C
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.
TRANSISTOR COUNT: 1290
Figure 2. Six LEDs in 3 x 2 Configuration
2.6V to 5.5V
OR DAC
LOGIC
3.3μF
0.01μF
IN
) should be placed as close to the IC as
IN
V+
CTRL
COMP
and C
10μH
MAX1848
SENSE
PGND
Chip Information
GND
OUT
OUT
, the inductor, the diode,
CS
LX
PC Board Layout
1μF
D1
should be as close
R
SENSE
5Ω
25Ω
30Ω
7