MX841 datasheet(4/6 Pages) IXYS

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Preliminary

MX841

03/10/05

MX841

IXYS

Functional Description

The MX841 is specifically designed to be

operated from a single cell battery source. The

step-up voltage necessary for the constant

current source to drive up to 5 white LED’s in

series, or multiple strings of white LED’s, is

achieved with minimal external components. (See

Application Schematics on pages 1&4).

Setting the LED Current

The regulated output current is determined by the

selection of the sense Resistor connected from

the ISENSE input to ground, and the maximum user

defined voltage of the CTRL input. The desired

output current, (typically 15mA to 20mA for white

LEDs), is derived using the following formula for

the RSENSE value: RSENSE Ω = (VCTRL X 1/13) / ILED.

Current Regulation / Light Intensity Control

The

ISENSE

voltage

fed

into

ErrorAmp/Comparator. The user defined Control

Voltage applied at the CTRL input is scaled by a

gain factor of 1/13 and compared to the ISENSE

voltage. If the scaled control voltage is greater

than the ISENSE voltage, the 1.0MHz oscillator

signal driving the N-Channel MOSFET will be

enabled. Once the ISENSE voltage reaches the

scaled CTRL voltage the oscillator signal is

modulated

and

off

the

ErrorAmp/Comparator to maintain the desired

regulated output current.

Light intensity is controlled by reducing the

voltage at the CTRL input. As the CTRL input

voltage is reduced below the user defined

maximum value, the oscillator signal driving the

N-Channel MOSFET is interrupted by the

ErrorAmp/Comparator. The pulse skip modulating

of the N-Channel MOSFET causes a reduction in

the regulated current equal to the ratio of the

reduced CTRL voltage to the maximum CTRL

voltage.

Sleep Mode

The MX841 goes into sleep mode when the VCTRL

input is less than 100mV. All circuits except VCTRL

detection are shutdown and the supply current

falls to 6uA. Once the VCTRL input rises above

260mV the MX841 resumes normal operation.

Over Voltage Detection

The MX841 will go into a shutdown state to avoid

an over voltage condition if the current path

between the SW output and the RSENSE input is

interrupted. A voltage reference and comparator

circuit evaluates the voltage potential at the OVD

input and shuts down the oscillator and N-

Channel MOSFET if an over voltage condition is

detected. Normal operation is resumed once the

OVD input voltage falls below the reference

voltage, (20V typical).

Drawing No. 17023

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