U® L LSI Computer Systems, Inc. 1235 Walt Whitman Road, Melville, NY 11747 (631) 271-0400 FAX (631) 271-0405
TOUCH CONTROL CONTINUOUS DIMMER with
AUTOMATIC GAIN CONTROL (AGC)
• Touch or pushbutton control of incandescent lamps.
• Touch Sensitivity guaranteed to 600pF Touch Plate Capacitance.
• Touch Operation independent of line plug polarity.
• Minimal external components.
• AGC Loop stabilizes immediately after Power-Up.
• Brightness state is Off after AC power applied.
• No change of Brightness state if AC power interrupted < 0.5 sec
• Advanced CMOS design for reliable operating characteristics
and low power.
• 50/60 Hz Line Frequency.
• 5V Operation (VDD - VSS).
• LS7540 (DIP); LS7540-S (SOIC) - See Figure 1 -
V DD (+V ) 3
7 V SS (-V )
• Screw-in and built-in adapter modules for converting table
and floor lamps to touch control for continuous dimming.
The LS7540 is a CMOS integrated circuit for controlling the
brightness of incandescent lamps with momentary touch. When
its TOUCH input is connected to a lamp body, the entire lamp
turns into a touch plate. A built-in AGC circuit allows for a wide
range of lamp sizes to be used, ranging from very small table
lamps to very large floor lamps. The brightness is controlled by
controlling the conduction angle of a triac in series with the lamp
and triggered by the TRIG/ output. Between off and maximum,
there are 124 steps of brightness levels, level zero being off and
level 124 being maximum. The phase reference of the Trig/ sig-
nal with respect to the AC line is maintained through the SYNC
The on/off and brightness control of the lamp is made with brief
touches of the lamp body as described below:
• In the off state a single touch will turn the lamp to full_on.
• At full-on a single touch will start a dim-cycle causing the
brightness to diminish gradually until the lamp turns off.
• During a dim-cycle a single touch will stop the dim-cycle and
lock in the brightness present at that time. Another touch will re-
start the dim-cycle from the present brightness causing the
brightness to ramp down towards off again. The start and stop of
dim-cycle can be repeated over and over again until the lamp
CLOCK Input (Pin 1)
An external resistor connected between this input and VDD, along
with an internal capacitor and oscillator stage, generates a clock
which is used for all timing functions. The recommended value of
this resistor for 50Hz and 60Hz operation is specified in the
SYNC Input (Pin 2)
50Hz or 60Hz AC input for zero crossing detection.
VDD (Pin 3) Supply voltage positive terminal.
TOUCH Input (Pin 4)
Input for sensing that a touch has been made on a lamp surface or
other touch plate. This action alters the state of the TRIG output as
described in the Touch Operation section of the General De-
CONTROL I/O (Pin 5)
An external R-C network connected between this pin and VDD es-
tablishes the controlling feedback for the AGC Loop.
RAMP Input/Output (Pin 6)
The ramp down rate of the brightness in the dimming cycle is regu-
lated by a resistor-capacitor (RC) pair connected to the RAMP in-
put. The ramp rate is equal to 4TRC per step, where TRC = 0.48RC.
Total ramp down time (TT) from maximum brightness to off is
equal to 496TRC.
R = 200kΩ, C = 0.22uF;
TRC = 0.48 x (200x103) x (0.22x10-6) = 0.02 sec.
Ramp rate = 4 x TRC = 0.08 sec/step;
Total ramp time, TT = 496 x TRC = 9.92 sec.
VSS (Pin 7) Supply voltage negative terminal.
TRIG/ output (Pin 8)
The TRIG/ output produces a negative pulse every half cycle of
the AC line period to turn on a triac in series with the lamp or any
other load device. The brightness of the lamp is regulated by regu-
lating the conduction angle, Ø (See Fig. 2) of the TRIG/ signal.
The conduction angles at maximum brightness (level 124) and
mimimum brightness (level 1) are 162° and 50° with 0.91° in-