MAX1561 Datasheet, PDF (7/10 Pages) Maxim Integrated Products – High-Efficiency, 26V Step-Up Converters for Two to Six White LEDs

High-Efficiency, 26V Step-Up Converters
for Two to Six White LEDs
PWM OR
ANALOG
DIMING
INPUT
2.6V TO 5.5V
2.2µF
IN
PWM CONTROL
LX
PGND
f_OSC
SLOPE COMP
OVERVOLTAGE OUT
CURRENT SENSE
PROTECTION
CTRL ^450kΩ
50kΩ
g_m
MAX1561
MAX1599
COMP
170mV
SHUTDOWN TIMER
GND
CS
SHUTDOWN
L1
C_OUT
0.1µF
UP TO 26V
2 TO 6 LEDS
C_COMP
0.1µF
R_SENSE
7.5Ω
Figure 1. Functional Diagram
Design Procedure
Adjusting LED Current
Adjusting the MAX1561/MAX1599s’ output current
changes the brightness of the LEDs. An analog input
(CTRL) and the sense-resistor value set the output cur-
rent. Output current is given by:
I_LED
=
V_CTRL
10 × R_SENSE
The V_CTRL voltage range for adjusting output current is
0.24V to 1.62V. To set the maximum current, calculate
R_SENSE when V_CTRL is at its maximum as follows:
R_SENSE
=
10
1.62V
× I_LED(MAX)
Power dissipation in R_SENSE is typically less than 5mW,
making a standard chip resistor sufficient.
PWM Dimming Control
CTRL is also used as a digital input allowing LED bright-
ness control with a logic-level PWM signal applied direct-
ly to CTRL. The frequency range is from 200Hz to
200kHz, while 0% duty cycle corresponds to zero cur-
rent and 100% duty cycle corresponds to full current.
The error amplifier and compensation capacitor 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
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. Larger value capacitors can be used to reduce
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