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White LED Current Regulator with 1x/1.5x
High-Efficiency Charge Pump
I LED _ = 230 × ? SET ?
Detailed Description
The MAX1570 is a complete charge-pump buck/boost
converter requiring only four small ceramic capacitors.
The MAX1570 utilizes a proprietary 1x/1.5x fractional
charge-pump topology to drive up to five white LEDs
with regulated constant current for uniform intensity.
The MAX1570 operates with a 1MHz fixed frequency.
An external resistor (R SET ) programs the full-scale LED
current, while two digital inputs control on/off and pro-
vide brightness control.
Output Regulation
The MAX1570 operates in 1x charge-pump mode until
just above dropout. Then the MAX1570 switches to 1.5x
charge-pump mode to regulate the voltage at LED1
and maintain constant LED brightness even at very low
battery voltages. Using this topology, there is no LED
brightness change during the 1x/1.5x switchover,
which guarantees no flicker on the display. The
switchover scheme has low hysteresis, minimizing
operation in the less efficient 1.5x mode. The 1x mode
produces almost no ripple, while the 1.5x mode regu-
lates the output voltage by controlling the rate at which
the transfer capacitors are charged. In this way, the
switching frequency remains constant for reduced
input ripple and stable noise spectrum.
Soft-Start
The MAX1570 includes soft-start circuitry to limit inrush
current at turn-on. When starting up with an output volt-
age that is not near the input voltage, the output capac-
itor is charged directly from the input with a DAC
ramped current source (with no charge-pump action)
until the output voltage is near the input voltage. Once
this occurs, the charge pump determines if 1x or 1.5x
mode is required. In the case of 1x mode, the soft-start
is terminated and normal operation begins. In the case
of 1.5x mode, soft-start operates until LED1 reaches
regulation. In case of an overload condition, soft-start
repeats every 2.1ms. If the output is shorted to ground,
the output current is limited by the MAX1570 fractional-
switching technique and then the device hits thermal
shutdown once the die temperature reaches -160°C.
True Shutdown Mode
When EN1 and EN2 are grounded, the MAX1570 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 MAX1570 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, LED4, and LED5 is 230 times
greater than the current flowing out of SET. Set the out-
put current as follows:
? V ?
? R SET ?
where V SET = 0.2V, 0.4V, or 0.6V (depending upon EN1
and EN2, see Table 1), and R SET is the resistor con-
nected 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, 1/3, 2/3, and full current (see Table 1).
EN1 and EN2 control the voltage at SET (V SET ).
Adjusting the SET voltage controls the current (I SET )
through the SET resistor (R SET ). Increasing V SET or
reducing R SET increases I SET , which then increases the
LED current (I LED_ ).
The charge-pump feedback threshold at LED1 is
increased as the current is increased to prevent
dropout in the current regulators while improving effi-
ciency at lower current settings. LED1 is regulated at
0.2V, 0.3V, or 0.4V when EN1 and EN2 are adjusted for
1/3, 2/3, or full current, respectively (see Table 1). A
higher threshold improves LED-to-LED current match-
ing, while a lower threshold improves efficiency by
allowing the 1x mode at lower input voltages.
Dimming Using PWM into EN1
Use EN2 for shutdown and drive EN1 with a PWM signal.
Current can be varied from 1/3 to full. The waveforms in
the Typical Operating Characteristics show the response
time of dimming. EN2 keeps the part on, eliminating any
soft-start delay that would impede PWM control, allowing
a PWM frequency up to 5kHz (Figure 1).
Dimming Using a Filtered-PWM Signal
Use a high-frequency PWM signal to drive an R-C-R filter
on the SET pin (Figure 2). A 0% PWM duty cycle corre-
sponds to 20mA/LED, while a 100% PWM duty cycle
corresponds to 0mA/LED. At PWM frequencies above
5kHz, C3 may be reduced.
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