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:
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 MAX8596X). To set the
maximum current, calculate R
SENSE
when V
CTRL
is at
its maximum as follows:
For the MAX8595X, R
SENSE
= 1.65 / (5 x I
LED(MAX)
)
For the MAX8596X, 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 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 V
OUT
. The
RMS diode current can be approximated from:
Applications Information
Compensation Information
The dominant pole (f
DP
) of the error amplifier is given by:
The output pole is given by
where R
OUT
is the sum of R
SENSE
and the incremental
series resistance of the white LED string.
Continuous conduction introduces a right-half-plane
zero determined by
.
.
