LX1994

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 6

High Efficiency LED Driver

Copyright

© 2003

Rev. 1.0d, 2006-02-09

TM

®

TH EORY OF O PERATION (CONTINUED)

Dimming Modes

Microsemi’s LX1994 provides two dimming options:

PWM or DC voltage input.

PWM dimming

A PWM signal applied to S/P pin (see figure 4).

This

PWM signal is scaled to the reference such that a N% duty

cycle PWM signal will produce an LED current of

()

{

}

N% • 10μA+ILS • RBRT

RFB

If a light sensor (such as Microsemi’s LX1970) is used, the

light sensor current is applied to the LS pin and adds to the

10µA internal current source; in this case the internal

current source determines the adjustment range in a pitch

black ambient. The PWM signal will scale the light sensor

signal allowing the dimming range to increase as the

ambient light increases.

DC dimming mode

In “DC dimming mode” (see figure 5) the BRT pin input

voltage can be applied directly to BRT pin with the S/P pin

pulled high or developed indirectly by applying a PWM

signal to the S/P pin and using a scaling resistor and filter

capacitor at the BRT pin.

The internal current source

produces a 10µA reference current that is scaled by the

resistance applied to the BRT pin.

Protection and IC Shutdown

OVP: The LX1994 provides OVP protections.

If the

voltage at the OVP pin exceeds the internal reference

voltage (1.2V), the converter will suspend switching. The

converter will attempt to regulate the OVP pin to its

nominal 1.2V.

IC Shutdown: To force the IC into shutdown mode, the

S/P pin must pull low for a duration longer 100µs. In

shutdown mode, the switch is off and the LED string

current typically reduces to a few nano amps of leakage

current.