MAX8595XETA(2004) Ver la hoja de datos (PDF) - Maxim Integrated

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MAX8595XETA
(Rev.:2004)

High-Efficiency, 36V Step-Up Converters with TA Derating Option for 2 to 9 White LEDs

Maxim Integrated 

High-Efficiency, 36V Step-Up Converters

with TA Derating Option for 2 to 9 White LEDs

Design Procedure

Adjusting LED Current

Adjusting the output current of the MAX8595X/

MAX8596X changes the brightness of the LEDs. An

analog input (CTRL) and the sense-resistor value set

the output current. Output current is given by:

ILED = VCTRL / (5 x RSENSE)

The VCTRL voltage range for adjusting output current is

0.24V to 1.65V (or 1.72V for the MAX8596X). To set the

maximum current, calculate RSENSE when VCTRL is at

its maximum as follows:

For the MAX8595X, RSENSE = 1.65 / (5 x ILED(MAX))

For the MAX8596X, RSENSE = 1.72 / (5 x ILED(MAX))

Power dissipation in RSENSE is typically less than 10mW,

allowing the use of a small surface-mount resistor.

PWM Dimming Control

CTRL is also used as a digital input allowing LED

brightness control with a logic-level PWM signal

applied directly to CTRL. The frequency range is from

200Hz to 200kHz, while 0% duty cycle corresponds to

zero current and 100% duty cycle corresponds to full

current. The error amplifier and compensation capaci-

tor form a lowpass filter so PWM dimming results in DC

current to the LEDs without the need for any additional

RC filters; see the Typical Operating Characteristics.

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

and output capacitors are not critical. The typical value

for the input capacitor is 2.2µF, and the typical value for

the output capacitor is 0.1µF. Higher value capacitors

can be used to reduce input and output ripple, but at

the expense of size and higher cost. CCOMP stabilizes

the converter and controls soft-start. Connect a 0.1µF

capacitor from COMP to GND. For stable operation,

COUT must not exceed 10 times CCOMP.

Inductor Selection

Inductor values range from 10µH to 47µH. A 22µH

inductor optimizes the efficiency for most applications

while maintaining low 12mVP-P input ripple. With input

voltages near 5V, a larger value of inductance can be

more efficient. To prevent core saturation, ensure that

the inductor-saturation current rating exceeds the peak

inductor current for the application. Calculate the peak

inductor current with the following formula:

IPEAK

=

VOUT(MAX) × ILED(MAX)

0.9 × VIN(MIN)

+

VIN(MIN) × 0.9µs

2×L

Schottky Diode Selection

The high switching frequency of the MAX8595X/

MAX8596X demands a high-speed rectification diode

(D1) for optimum efficiency. A Schottky diode is recom-

mended due to its fast recovery time and low forward-

voltage drop. Ensure that the diode’s average and

peak current rating exceed 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:

IDIODE(RMS) = IOUT × IPEAK

Applications Information

PC Board Layout

Due to fast switching waveforms and high-current

paths, careful PC board layout is required. An evalua-

tion kit (MAX8596XEVKIT) is available to speed design.

When laying out a board, minimize trace lengths

between the IC and RSENSE, the inductor, the diode,

the input capacitor, and the output capacitor. Keep

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

as the LX node trace, away from CS. The IN bypass

capacitor (CIN) should be placed as close to the IC as

possible. PGND and GND should be connected direct-

ly to the exposed paddle underneath the IC. The

ground connections of CIN and COUT should be as

close together as possible. The traces from IN to the

inductor and from the Schottky diode to the LEDs can

be longer.

Chip Information

TRANSISTOR COUNT: 2143

PROCESS: BiCMOS

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