Rainbow Electronics MAX8596Z - Manual (Page 7)

Design Procedure
Adjusting LED Current
Adjusting the output current of the MAX8595Z/
MAX8596Z changes the brightness of the LEDs. An
analog input (CTRL) and the sense-resistor value set
the output current. Output current is given by:
I LED = V CTRL / (5 x R SENSE )
The V
CTRL
voltage range for adjusting output current is
0.24V to 1.65V (or 1.72V for the MAX8596Z). To set the
maximum current, calculate R
SENSE when V CTRL is at
its maximum as follows:
For the MAX8595Z, R
SENSE
= 1.65 / (5 x I
LED(MAX)
)
For the MAX8596Z, R SENSE = 1.72 / (5 x I LED(MAX) )
Power dissipation in R SENSE 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. C
COMP stabilizes
the converter and controls soft-start. Connect a 0.1µF
capacitor from COMP to GND. For stable operation,
C
OUT
must not exceed 10 times C
COMP
.
Inductor Selection
Inductor values range from 10µH to 47µH. A 22µH
inductor optimizes the efficiency for most applications
while maintaining low 12mV
P-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:
Schottky Diode Selection
The high switching frequency of the MAX8595Z/
MAX8596Z 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 V
OUT
. The
RMS diode current can be approximated from:
Applications Information
PC Board Layout
Due to fast switching waveforms and high-current
paths, careful PC board layout is required. An evalua-
tion kit (MAX8596ZEVKIT) is available to speed 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 LX node trace, away from CS. The IN bypass
capacitor (C
IN
) 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 C
IN and C OUT 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
I I I
DIODE RMS OUT PEAK ( )
= ×
I
V I
V
V s
L
PEAK
OUT MAX LED MAX
IN MIN
IN MIN =
Ч
Ч
+
Ч
Ч
( ) ( )
( )
( )
.
.
0 9
0 9
2
µ
MAX8595Z/MAX8596Z
High-Efficiency, 32V Step-Up Converters
with T A Derating Option for 2 to 8 White LEDs
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