ELM711 Datasheet, PDF (5/7 Pages) ELM Electronics – Christmas LEDs Driver

ELM711
Example Application
This section shows how the ELM711 might be
typically used. Basically, all that needs to be done is
apply power to the IC and it will create a sequence of
outputs for you. There are some concerns over the
external connections that should be discussed first,
however.
Whenever interfacing to CMOS circuitry, you need
to protect against a problem called ‘latchup’. This may
occur when excessive current is allowed to flow into
one of the IC’s pins, typically an input. This can
happen at almost any time, but generally occurs if
there are long wires connected directly to the IC, with
no form of current limiting. Latchup will cause
excessive currents to flow, which may damage
components (such as the ELM711).
Figure 1 shows how a pushbutton will typically be
connected to the input of the ELM711. This works well
if the wires to the switch are short (less than about 12
inches long). If the wires are longer than that, currents
can more easily be induced into the wires, and into the
ELM711 pin. Since there is no current limiting, the
currents can possibly be large enough to exceed the
latchup threshold, and the IC may be damaged.
Simply adding a series resistor such as that shown in
Figure 2 is all that is generally needed to prevent
latchup. Note that we also show a 4.7KΩ pullup
resistor connected to the switch as well. This pullup
resistor helps to pass some of the induced currents
around the ELM711 input, and also provides a lower
impedance input so that the voltages caused by the
induced currents are reduced, and less likely to
provide false inputs. (The ELM711 input pin also uses
a relatively long debounce time, so that it is less
susceptible to noise.)
The ELM711 may be used to control other
devices, but it was designed with LEDs in mind. When
on, LEDs require a forward voltage of about 2 to 3
volts as shown in Table 2. These voltages will remain
fairly constant over a wide range of currents, and can
be used in calculations as if they are constant.
If the ELM711 is to use a 3 to 5 volt power supply,
it is clear that it can only be directly connected to 1 or
possibly 2 LEDs on each output, as there isn’t enough
voltage available to power a longer string of LEDs.
This could result in a sparse Christmas tree unless
something can be done.
The circuit of Figure 3 shows a transistor driving
several LEDs from a power supply of Vp volts. As long
as the sum of the LED voltages is less than Vp, the
1
8
2
7
3
6
4
5
Figure 1. Typical Pushbutton Connection
1
8
2
7
4.7KΩ
3
6
4.7KΩ
4
5
Figure 2. Protection for very long PB leads
Colour
Red
Yellow
Green
Blue
White
Typical Vf
1.8
1.9
2.0
3.0
2.9
Table 2. Typical LED Forward Voltage Drops
ELM711DSA
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