Highest Efficiency Supply for 2 to 6

Series WLEDs in a 2mm x 2mm TDFN Package

10

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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.

charged to 1.25V with a 60µA current source. When

4µA current source, and the internal MOSFET begins

above 2.25V, the duty cycle is at its maximum. The

final duty cycle required by the load. Typical shutdown

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

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.

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 includes 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)

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

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-

(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:

I

V

V

V

x L

PEAK

OUT(MAX)

IN(MIN)

IN(MIN) x .2

=

×

+

I

x

LED MAX

()

900

2

1