Biomedical Engineering Reference
In-Depth Information
onboard the PIC. The lines are optically isolated by way of IC25 and IC27. The LEDs in
the optoisolators are driven by FETs Q21, Q22, Q23, and Q24. The optoisolator outputs
are bu
ered and inverted through IC21B, IC21C, IC28A, and IC28B. Proper RS232 lev-
els are generated through a TTL-to-RS232 interface chip (IC26). The circuit connected to
the programming GUI computer is powered by
ff
5 V derived from the
15-V Condor
power supply through linear regulator IC23.
The shock-box controller knows the state of the power line through optoisolator IC22.
The LED in this optoisolator is powered from the
15-V output of the Condor power sup-
ply through resistor R41. The output of the optoisolator is conditioned and inverted by
IC21A and fed to the PIC through pin 5. A dual-color LED (D5/D6) controlled from pins
3 and 4 of the PIC is used as an indicator of the high-voltage capacitor charge state (red,
charging; green, ready to de
brillate; red blinking, charge fault). A red LED (D7) inter-
faced to pin 7 of the PIC is used to indicate a lead-impedance overrange condition.
fi
D/A and A/D Converters
The PIC communicates with two D/A converters (one to measure impedance, the other to
measure the level of high-voltage charge) and a D/A converter (used to determine the set
point for the high-voltage power supply) through a SPI serial interface (pins 18, 23, and 24).
Chip enables to address these converters are generated through pins 8, 9, and 10. As shown
in Figure 8.44, a LTC1451 D/A converter (IC33) is interfaced to the shock-box microcon-
troller via a three-line serial (SPI) bus (
enable). IC33 is operated with its internal reference.
The maximum of 4.096 V output from this D/A limits the voltage output of the HV power
supply to approximately 860 V, preventing accidental overcharge of the capacitor bank.
IC33 is powered through the switched
5-V (isolated) line. For this reason, the enable
line is controlled through a FET used as an isolation mechanism between the converter IC
and the microcontroller. Two MAX187 A/D converters (IC30, IC31) are interfaced to the
shock-box microcontroller via serial (SPI) bus (
separate enables) to measure the voltage
across the capacitor bank as well as the analog output of the impedance measurement circuit.
The converters use their internal references and are powered through the switched
5-V
(isolated) line. For this reason, the enable lines are controlled through FETs (Q25 and Q26),
which are used as isolation mechanisms between the converter ICs and the microcontroller.
Isolated Manual-Mode Pushbuttons
A photovoltaic isolator powered from the module's battery in a pulsed regime is controlled
by the microcontroller to generate an isolated supply to power the LEDs of optoisolators
controlled by the manual de
brillation control switches. This is done to maintain full isola-
tion of the manual-control switches from the applied part, yet still be able to operate the
module in manual mode whenever the main power line fails. As shown in the circuit of
Figure 8.45, photovoltaic isolator IC40 charges two 10-
fi
µ
F capacitors in parallel. When
power is
first applied to the shock-box circuit, charging occurs for 4 s. From that point, the
capacitors are topped off
fi
every 10 s through a 1-s pulse. The series diode prevents the capac-
itors from being discharged through the relatively high dark leakage current of the photo-
voltaic isolator during the no-charge periods.
Whenever one of the switches (SW1 or SW2) is pressed, the energy stored in the capaci-
tors is used to pulse the LED of the photocoupler (in IC42) associated with the switch
depressed. Activation of the photocoupler is conditioned through the associated inverter/
bu
ff
op (IC41A or IC41B). Upon
wake-up, the shock-box microcontroller reads the status of the latches. If a switch closure is
found, the microcontroller begins the behavior requested (i.e., charge to 50 J or de
ff
er (IC21D or IC21E) and latched by the associated
fl
ip-
fl
fi
brillate),
recharges the isolated 10-
µ
F capacitors for 4 s, and then resets the latches (IC41A and IC41B).
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