MAX8901B 데이터 시트보기 (PDF) - Maxim Integrated

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MAX8901B

Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package

Maxim Integrated 

MAX8901A/MAX8901B

Highest Efficiency Supply

for 2 to 6 Series WLEDs in a

2mm x 2mm TDFN Package

Schottky Diode Selection

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 VOUT. The RMS diode

current can be approximated from the following

equation:

IDIODE (RMS) = ILED × IPEAK

Capacitor Selection

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.

Current-Sense Resistor (MAX8901A Only)

The MAX8901A uses a sense resistor (RCS) connected

from CS to GND to program the maximum WLED cur-

rent for 100% PWM duty cycle. The MAX8901A regu-

lates VCS to 0.5V (typ) for 100% duty cycle. Calculate

RCS (in ohms) using the following equation:

RCS

=

500

ILED(MAX)

where ILED(MAX) is the maximum WLED current in

milliamps. Maximum WLED current is programmed to

25mA using a 20Ω resistor.

PCB Layout

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 RCS. Keep traces

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

LX node trace, away from CS. The input bypass

capacitor (CIN) should be placed as close as possible

to the IC. The ground connections of CIN and COUT

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.

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