VOLTAGE CONTROLLED LIGHT DIMMER with SOFT ON/OFF

FEATURES:

• Analog input for direct control of illumination

• Soft turn-on or turn-off with touch or pushbutton

• Controls high-voltage Halogen or Incandescent lamps

• Controls low-voltage halogen lamps via

magnetic or electronic transformers

• Automatic safety shutdown for magnetic transformers

• Controls CFLs and fluorescent lamps via dimming ballasts

• Extension input for remote operation

• Suited for 50Hz or 60Hz AC

• Single 5V power supply

•

LS7642, LS7642FO (DIP); LS7642-S, LS7642FO-S (SOIC)

- See Figure 1 -

GENERAL DESCRIPTION

LS7642 is an innovative light dimmer allowing direct control of il-

lumination with analog voltage. The analog voltage applied at the

brightness control input directly controls the conduction angle of

a triac in series with the lamp. For manual dimming applications

such as, wall switches, floor and table lamps etc, a sliding or ro-

tary potentiometer with a center tap can serve as the controlling

element. For automated applications such as uC controlled il-

lumination, dawn, dusk and daylight emulation in chicken coops

etc., the control voltage can be provided by the uC with built-in or

external D/A converter creating accurate control of dimming pat-

terns. Turn-on and turn-off of the lamp is made with a touch or

push-button or uC signal applied at the SENSE/ input or the EXT

input. In both turn-on and turn-off events, the brightness is softly

ramped up and ramped down, respectively.

SAFETY SHUTDOWN:

A typical electronic dimmer may not operate properly with the in-

ductive load encountered when driving a magnetic transformer-

coupled low-voltage halogen lamp. The inductive load may cause

a phenomenon called half-waving wherein the triac fires in al-

ternate AC half cycles leading to thermal destruction of the load

transformer. This problem is addressed by the

LS7642 in two

steps as described below:

1. Delayed triac turn-off.

When a trigger pulse is due to occur at a conduction angle which

coincides with the on state of the triac, the trigger pulse is de-

layed until the triac has turned off. This eliminates the underlying

cause of half-waving.

2. Delayed triac turn-on.

With inductive loads, the holding current needed to maintain the

triac on state may seriously lag behind the gate trigger signal

causing the triac to turn off prematurely. If the frequency of oc-

currences of the delayed turn-on exceeds a preset threshold, a

shutdown is initiated by turning off the trigger pulses. The safety

shutdown threshold value is accumulated in a 4-bit up/down

counter. The count increments for every occurrence of delayed

turn-on and decrements once every 8 AC cycles. If the count

reaches 15, the safety shutdown is invoked.

INPUT/OUTPUT DESCRIPTION:

BCV (Pin 2) Brightness control input. The voltage applied at the BCV

input controls the triac conduction angle and hence the brightness of

the lamp. The brightness varies in direct proportion to the applied volt-

age. For manual control of brightness, the BCV voltage can be sup-

controlling agent, the BCV voltage can directly be provided by the uC.

The dynamic range of the BCV voltage is between 25% and 88% of

tween 1.25V for the minimum brightness and 4.4V for the maximum

crements in the dynamic range of 1.25V and 4.4V of the BCV voltage.

TEST (Pin 3) For factory use only, should be left unconnected.

SYNC (Pin 4) The AC line frequency is applied to this input for syn-

chronizing all internal timings relative to the AC zero crossing. The

load on/off status information is also derived from this input for en-

forcing the safety shutdown protocol.

SENS/ (Pin 5) A momentary logic low applied to this input either with

a touch or a push-button switch, causes the state of the TRIG/ output

to switch; the lamp is turned on if the pre-touch state was off and

turned off if the pre-touch state was on. When turning on the bright-

ness is softly ramped up from off through minimum to the brightness

set by the BCV input. When turning off the brightness is softly ramped

down from the pre-touch brightness to off. The ramp rate for either

EXT (Pin 6) The EXT input has the same functionality as the SENS/

input. It is designed to discriminate ac pick-ups when remotely locat-

ed touch plates are connected to the dimmer units with long cables. A

logic high is the active level at the EXT input.

October 2009

7642-102009-1

LSI/CSI

LSI Computer Systems, Inc. 1235 Walt Whitman Road, Melville, NY 11747 (631) 271-0400 FAX (631) 271-0405

UL

®

A3800

LS7642

LS7642FO

1

2

3

4

8

7

6

5

EXT

BCV

SYNC

FIGURE 1

TRIG/

SENS/

PIN ASSIGNMENT - TOP VIEW

TEST