MAX8901B Maxim Integrated Products, MAX8901B Datasheet - Page 11

• Manufacturer Part Number: MAX8901B
• Description: Highest Efficiency Supply
• Manufacturer: Maxim Integrated Products

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The high switching frequency of the MAX8901_
demands a high-speed rectification diode for optimum
efficiency. A Schottky diode is recommended due to its
fast recovery time and low forward voltage drop.
Ensure that the diode’s average and peak current
rating exceeds the average output current and peak
inductor current. In addition, the diode’s reverse
breakdown voltage must exceed V
current can be approximated from the following
equation:
Ceramic capacitors with X5R, X7R, or better dielectric
are recommended for stable operation over the entire
operating temperature range. The exact values of input
(C1) and output (C2) capacitors are not critical (see
Figures 4 and 5). The typical value for the input
capacitor is 1µF, and the typical value for the output
capacitor is 0.1µF (MAX8901A) or 1µF (MAX8901B).
Higher value capacitors can reduce input and output
ripple, but at the expense of size and higher cost. The
compensation capacitor (C3) stabilizes the converter
and controls soft-start. The compensation capacitor is
typically chosen to be 0.022µF for most applications.
Series WLEDs in a 2mm x 2mm TDFN Package
I
DIODE RMS
______________________________________________________________________________________
(
Schottky Diode Selection
)
=
I
LED
Capacitor Selection
Highest Efficiency Supply for 2 to 6
×
I
OUT
PEAK
. The RMS diode
The MAX8901A uses a sense resistor (R
from CS to GND to program the maximum WLED cur-
rent for 100% PWM duty cycle. The MAX8901A regu-
lates V
R
where I
milliamps. Maximum WLED current is programmed to
25mA using a 20Ω resistor.
Due to fast switching waveforms and high-current
paths, careful attention is required when the PCB layout
is designed. Minimize trace lengths between the IC and
the inductor, the diode, the input capacitor, and the
output capacitor. For the MAX8901A, minimize the
trace length between the IC and R
short, direct, and wide. Keep noisy traces, such as the
LX node trace, away from CS. The input bypass
capacitor (C
to the IC. The ground connections of C
should be as close together as possible. PGND and
GND should be connected together at the input
capacitor ground terminal. Refer to the MAX8901 EV Kit
for an example layout.
CS
Current-Sense Resistor (MAX8901A Only)
(in ohms) using the following equation:
CS
LED(MAX)
to 0.5V (typ) for 100% duty cycle. Calculate
IN
) should be placed as close as possible
R
is the maximum WLED current in
CS
=
I
LED(MAX)
500
CS
PCB Layout
CS
. Keep traces
IN
) connected
and C
OUT
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