MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
10 ______________________________________________________________________________________
Startup and Soft-Start
The MAX8901_ use a single input (ON) to enable the IC
and to control WLED intensity. After ON is driven high,
the MAX8901_ enter soft-start and gradually increase
the WLED current to full brightness.
When the MAX8901_ are first turned on, C
COMP
is
charged to 1.25V with a 60µA current source. When
V
COMP
rises above 1.25V, C
COMP
is charged with a
4µA current source, and the internal MOSFET begins
switching at a reduced duty cycle. When V
COMP
rises
above 2.25V, the duty cycle is at its maximum. The
V
COMP
where the IC exits soft-start depends on the
final duty cycle required by the load. Typical startup
timing characteristics are shown in the
Typical
Operating Characteristics
section.
A PWM signal (MAX8901A) or serial pulses (MAX8901B)
are applied to dim the WLEDs. See the
PWM Dimming
Control (MAX8901A)
and
Serial-Pulse Dimming Control
(MAX8901B)
sections for more details.
Shutdown
The MAX8901_ enter shutdown when V
ON
is held low
for more than 1.33ms (typ), or when an output overvolt-
age condition is engaged. In shutdown, the supply cur-
rent is reduced to 0.01µA (typ) by powering down the
entire IC except for the ON voltage-detection circuitry.
C
COMP
is discharged to GND during shutdown,
allowing the device to reinitiate soft-start when it is
re-enabled. Although the internal n-channel MOSFET
does not switch in shutdown, there is still a DC current
path between the input and the WLEDs through the
inductor and Schottky diode. In shutdown, the minimum
forward voltage of the WLED string must exceed the
maximum input voltage to ensure that the WLEDs
remain off. Typically, with two or more WLEDs, the for-
ward voltage is large enough to keep leakage current
low, less than 1µA (typ). Shutdown timing characteris-
tics are shown in the
Typical Operating Characteristics
.
Thermal Shutdown
The MAX8901_ include a thermal shutdown feature that
protects the IC by turning it off when the die temperature
reaches +160°C (typ). After thermal shutdown occurs,
the MAX8901_ must be restarted by toggling the ON pin
low, then high, or by cycling the input voltage.
PWM Dimming Control (MAX8901A)
After V
IN
is above UVLO, apply a PWM signal to ON for
a WLED current that is proportional to the PWM signal’s
duty cycle (0% duty cycle corresponds to zero LED
current and 100% duty cycle corresponds to full LED
current). Restrict PWM frequency to between 30kHz to
500kHz for a maximum I
LED
current accuracy. If dim-
ming control is not required, ON works as a simple on/off
control. Drive ON high to enable the IC, or drive ON low
for shutdown. A resistor connected from CS to GND pro-
grams maximum I
LED
. See the
Current-Sense Resistor
(MAX8901A Only)
section for more details.
Serial-Pulse Dimming Control (MAX8901B)
After the MAX8901B is enabled by driving ON high for
the minimum initial ON high pulse width (40µs, min),
soft-start is engaged and brings WLED current to maxi-
mum brightness. After soft-start is completed, dim the
MAX8901B by pulsing ON low (500ns to 500µs pulse
width). Each pulse reduces the WLED current by
0.75mA. The maximum WLED current is 24.75mA for
the MAX8901B (0.75mA/step). The overall dimming
range for serial-pulse dimming control is from maximum
WLED current down to 1/33 of maximum WLED current.
The WLEDs are turned off by holding ON low for at
least 1.33ms (typ). See Figure 1 for the serial-pulse
dimming control timing diagram.
Applications Information
Inductor Selection
Recommended inductor values range from 10µH to
47µH. A 22µH inductor optimizes the efficiency for most
applications while maintaining low input voltage ripple.
With input voltages near 5V, a larger value of
inductance may 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:
where I
LED(MAX)
is in mA and L is in µH.
