iC-NT
LIGHT CHAIN PULSE DRIVER
Rev A0, Page 7/8
APPLICATIONS INFORMATION
Light curtain
The circuit in Figure 5 shows iC-NT chained to a light curtain where consecutive LEDs emit clock-driven light
pulses.
When discussing the function of iC-NT, it is assumed that all flipflops in IC1..ICn have been reset, for example
after the operating voltage has been switched on. The signal DIN1= hi activates the IC1 current sink which is
switched to LED1 with the CLK rising edge. With DIN1= lo, the next CLK rising edge resets FF1, turns off the
LED and deactivates the current sink in IC1. Simultaneously, FF1 sends the stored information to FF2. FF3 also
accepts this information via the CLK trailing edge (provided that SHEN= hi) and activates the current sink in the
next component, IC2, via the output driver. The pulse diagram in Figure 6 is also valid for the subsequent
components in the chain, i.e. the ICs switched as a light curtain make up a clock-driven shift register which
passes on the input information.
The typical timing of a CLK signal, shown in Figure 6, is characterized by two successive pulses which
determine the length of a light pulse t
w
from 1..3µs, followed by a longer activation time of t
en
≥5µs.
In general,
the CLK pulse interval is determined and lengthened by the activation time required by the receiver or by more
extensive, system-set default options.
Because of the high LED pulse currents, the PCB layout of the light curtain sender must be designed to avoid
large voltage drops on the supply lines. The high, short-term pulse current is provided by back-up capacitors
C1..Cn at the pulse driver ICs; these should have a low inductance due to the high current increase rate. The
leads to the LED anode and to iC-NT’s GND pin should be as short as possible. The capacitors selected should
ensure that the voltage drop caused by the light pulse is less than 1V, i.e. that C1..Cn= 1µF for a light pulse of
1A x 1µs, for example. In practice, the voltage at the IC drops much less during a light pulse, as charge from
the back-up capacitors of neighboring ICs also flows into the chip. A low-inductance capacitance distribution can
be achieved more economically by placing further smaller capacitors in parallel.
Since only one device is activated at a time within one section of a light curtain, several iC-NTs may share the
external resistor RSET, needed to set the pulse current. This parallel chain circuit should be limited to ca. 5 ICs
due to the increasing capacitive loading at pin ISET.
Fig. 5: Schematic of a Chain Configuration
ICHAUS [ IC-HAUS GMBH ]
