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STMicroelectronics Confidential
When HSync pulses are received, the voltage on pin 9 will increase until the local oscillator matches
the incoming frequency. Since the range for pin 9 voltage is between 1.3V and 6.2V, the relative
capture frequency range (ratio of maximum to minimum frequency) is:
4.1.7
Frequency Precision
All the terms in the f
0
equation have a spread. Both voltages are derived from the same reference,
their ratio will remain quite accurate. In regards to the other variables, we have to consider a
tolerance of ±1% for R, ±5 or 10% for C and ±6% for the numerical factor which incorporates a
current ratio.
Consequently the free-running frequency can vary from ±12 or 17% of the designed value, using
safety margins can reduce the usable relative frequency range by 2 x 12% or 2 x 17%; or ±9% if you
can afford a ±2% capacitor.
An other consequence of the free-running frequency spread is to make the design of a B+ converter
more difficult, because in free-running mode, the B+ converter may have to follow the oscillator
frequency down to low values. (This may be avoided if fake HSync pulses are sent by the MCU at a
convenient frequency, as soon as there is no sync coming through the input plugs.)
6.2
1.3
--------
4.77
=
