UL
®
LSI/CSI
LSI Computer Systems, Inc. 1235 Walt Whitman Road, Melville, NY 11747
V
DD
(+V )
BCV
TEST
SYNC
1
2
3
4
LS7642
LS7642FO
(631) 271-0400 FAX (631) 271-0405
October 2009
A3800
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.
7642-102009-1
PIN ASSIGNMENT - TOP VIEW
LSI
FIGURE 1
8
7
6
5
TRIG/
V
SS
(-V )
EXT
SENS/
INPUT/OUTPUT DESCRIPTION:
V
DD
(Pin 1) Supply voltage positive terminal.
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-
plied from the center tap of a potentiometer connected between V
DD
and V
SS
. In non-manual instrumentation application where a uC is the
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
the applied power supply. For V
DD
= 5.0V this translates to a range be-
tween 1.25V for the minimum brightness and 4.4V for the maximum
brightness. There are 81 discrete brightness steps of 1.4
o
/39mV in-
crements in the dynamic range of 1.25V and 4.4V of the BCV voltage.
The corresponding conduction angles are 45
o
at the dimmest and
159
o
at the brightest.
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
case is 1.4
o
/33.3ms at 60Hz or 1.4
o
/40ms at 50Hz.
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.
V
SS
(Pin 7) Supply voltage negative terminal.
LS7642
LS7642FO
LSI [ LSI COMPUTER SYSTEMS ]
