True Shutdown™ Mode
When EN1 and EN2 are grounded, the MAX1573 is in
shutdown, and the charge pump examines whether the
input voltage is greater than or less than the output volt-
age and shorts the transfer capacitor nodes to either IN
or OUT as necessary. The output is high impedance in
either case.
Thermal Shutdown
The MAX1573 includes a thermal-limit circuit that shuts
down the IC at about +160°C. Turn-on occurs after the
IC cools by approximately 20°C.
Setting the Output Current
SET controls the LED bias current. Current flowing into
LED1, LED2, LED3, and LED4 is a multiple of the current
flowing out of SET. Set the output current as follows:
where K = 22, 66, or 219 (depending upon EN1 and
EN2, see Table 1), and R
SET
is the resistor connected
between SET and GND (see the Typical Operating
Circuit).
Applications Information
Dimming Using EN1 and EN2
Use EN1 and EN2 inputs as a digital 2-bit number to
control on/off, 1/10, 3/10, and full current (see Table 1).
R
SET
programs the full current level (see the Setting the
Output Current section).
Dimming Using PWM into EN1
Use EN2 for shutdown and drive EN1 with a PWM sig-
nal. Current can be varied from 1/10 to full. The wave-
forms in the Typical Operating Characteristics show the
response time of dimming. EN2 keeps the part on, elimi-
nating any soft-start delay that would impede PWM con-
trol, allowing a PWM frequency up to 50kHz (Figure 1).
Dimming Using a Filtered PWM Signal
Use a high-frequency PWM signal to drive an R-C-R fil-
ter on the SET pin (Figure 2). A 0% PWM duty cycle
corresponds to 18.7mA/LED, while a 100% PWM duty
cycle corresponds to 0mA/LED. At PWM frequencies
above 5kHz, C3 may be reduced.
Input Ripple
For LED drivers, input ripple is more important than out-
put ripple. Input ripple depends on the source supply’s
impedance. Adding a lowpass filter to the input further
reduces input ripple. Figure 3 shows a C-R-C filter used
to reduce input ripple to less than 2mV
P-P
when driving
a 75mA load. Alternately, increasing C
IN
to 2.2µF or
4.7µF yields input ripple of 17mV
P-P
or 9mV
P-P
, respec-
tively, with only a small increase in footprint. The 1x
mode always has very low input ripple.
