Covidien Force FX, Service Manual

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Footswitch Board
Principles of Operation
Force FX Electrosurgical Generator C Service Manual
4-17
of the corresponding optoisolator. This current generates an IR beam that causes the
associated photo-transistor to conduct. The collectors of the transistors are connected to
input pins of an I/O port on the microcontroller where they activate the desired mode of
operation.
Audio Circuit
The audio system consists of an audio oscillator, tone control signals, a volume control
potentiometer, an audio amplifier, and a speaker. A reference voltage (V
ref
) is used
throughout the audio circuit and is generated by dividing the +12 V power supply down
to about 2 V by R9 and R8.
The audio circuit annunciates the presence of RF output and provides an auditory
indication of alarm conditions. A potentiometer adjusts the volume of RF output
activation tones. The speaker volume cannot be turned off entirely. The volume of the
tone issued during alarm conditions is not adjustable.
The audio oscillator is enabled when UP_TONE\ or RF_TONE\ is pulled low. Diodes D1 and
D2 provide a wired OR function for the two signals. Because UP_TONE\ and RF_TONE\
are +5 V (logic level) signals, resistors R4 and R6 divide the +12 V audio power supply
down to about 4.85 V to prevent D1 and D2 from sourcing current into the Voltage
Translator U22 pins, which interfaces with FPGA U16-B, on the Control board.
When either UP_TONE\ or RF_TONE\ is enabled low, the voltage at the noninverting input
of U1B is pulled below the V
ref
threshold present at U1B inverting input, the open
collector output of U1B is turned on, grounding R31 and allowing U6A to oscillate.
U6A is a relaxation oscillator whose frequencies are determined by the RC time constants
of R30, C35, and C18. This design allows the oscillator to produce two distinct
frequencies that can be selected by the state of the LO_TONE signal.
• When LO_TONE is not asserted, R30 and C35 determine the frequency of operation
of the oscillator (approximately 900 Hz).
• When LO_TONE is asserted (+5 V), the voltage at the noninverting input of U1A
exceeds the 2 V V
ref
signal at the inverting input, turning on its output transistor. This
effectively connects C18 in parallel with C35 to produce a higher RC time constant
for the oscillator, which results in a lower audio frequency of approximately 700 Hz.
The ALARM signal selects the user-controlled audio volume or the fixed alarm level
volume. U1C and U1D are configured in an exclusive OR arrangement in which the state
of the output transistors of U1C or U1D is complementary. In other words, the output
transistor of one of these two devices is always on, but both cannot be on
simultaneously.
Under normal operating conditions, the ALARM signal is low, allowing the U1C output to
float while the U1D output transistor is turned on. The output of U1D creates a voltage
divider through R11, R12 (the volume control potentiometer), and R32 to attenuate the
audio signal to levels acceptable for input to the audio amplifier.