ELM304
Example Application
The circuit of Figure 1 below shows how the
ELM304 would typically be connected. This is all that
is needed to generate an NTSC signal for 525 line
systems that use a nominal horizontal rate of 15734
Hz, and a vertical field rate of 59.94 Hz. The output
signal is an interlaced type, so 262.5 lines are
displayed during each field, for a total of 525 lines per
frame. Equalization pulses, vertical retrace blanking,
etc. are all generated by the ELM304.
resistor values may be required, but for many
applications, you may find the results to be acceptable.
The oscilloscope traces shown in Figures 2 and 4
were obtained from a circuit that used standard 5%
carbon resistors with values as shown in Table 1
(there was no trimming of resistor values, or the
oscillator frequency).
Figure 1 shows a switch connected to pin 4, but if
you only require one type of output signal, pin 4 can be
connected directly to either VSS or VDD.
Figure 1 does not show a power supply, as the
type used depends mostly on the application. For
portable use, this circuit can easily be powered by two
‘AA’ cells (the average current required is only about
10 mA at 3V, and the peak current is almost double
that). Since the output level depends directly on the
supply voltage however, you may wish to consider a
higher voltage supply and a 5V regulator connected to
keep the supply voltage constant. For the following
discussion, we are assuming that the supply voltage is
regulated at 5 volts.
One element of such a circuit that should be
considered if you are building it, is the use of short
VDD
3V
5V
R1
R2
330W
330W
680W
680W
Once the power supply voltage is determined, the
values of the summing resistors (R1 to R4) are then
chosen using Table 1 as a starting point. Depending
on the accuracy desired, some adjustment of these
R3
R4
220W
470W
430W
150W
Table 1 Typical Resistor Values
VDD
0.1µF
1
2
3
4
8
7
6
5
R1
R2
R3
3.58MHz
27pF
27pF
75W(unbalanced)
output signal
R4
Switch closed for
gray bars, and open
for a white screen
Figure 1. Typical Circuit Configuration
ELM304DSC
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