Passport 2®/Passport 2 LT™ Service Manual 0070-10-0441 2 - 49
Theory of Operation Passport 2 NIBP Module (P/N 0670-00-0730 or 0670-00-0746-01)
The linear valve’s bleed rate is current controlled by the HC16Z1 software by writing to a
Maxim MAX5352 12-bit DAC (U12) connected to a Burr-Brown OPA336 CMOS
operational amplifier (U13), in turn connected to an International Rectifier IRF7303 Power
MOSFET (Q6).
The DAC output voltage is divided by 5 (which results in a full-scale control voltage of 0.5V
nominal) which is applied to the opamp’s, U13, non-inverting input. A current sensing
resistor, R95, is used to generate a low voltage proportional to the linear valve current. This
voltage is fed back to the opamp’s inverting input. The opamp controls the Q6 FET gate
voltage to maintain the desired current setting and corresponding valve orifice. A5V is used
to generate a small offset voltage using R94 and R92 to insure the FET is fully off for a DAC
code of 0.
R95 is chosen to guarantee the full-scale current fully closes the valve under worst case
conditions, including coil winding tolerance.
The IRF7303 MOSFET part (Q6), was chosen due to its lead-frame design which offers
superior power dissipation capabilities. As such, the maximum junction rise when driving the
0.526W, 274 ohm linear valve coil under the worst case conditions, does not warrant any
heat-sinking to achieve reasonable reliability.
2.15.8 Over-Pressure Detection
There are two methods of keeping the pressure from getting too high. The first method is the
HC16Z1 software based monitoring of the pressure transducer, PT1. When the software
monitors pressure higher than, 300mmHg in Adult mode, 200mmHg in Pediatric or
150mmHg in Neonate mode, the measurement cycle will halt and dump valve, V1 and
bleed valve, V2 shall open to release the pressure in the cuff.
In the event of an over-pressure condition that is not corrected by the HC16Z1 software and
the measurement channel circuitry, the over-pressure channel serves as a fully redundant
backup system to disable the pump and valves, thus relieving the pressure in the cuff. The
hardware over pressure limit detect circuitry will deactivate the over pressure signal
12VEN*, cutting off power to the pump (M1) and valves (V1 and V2), reverting them to their
‘normally open’ state, independent of software. This action places the NIBP PCB in a
‘shutdown’ state that can be exited only by cycling the system power.
To achieve this secondary over-pressure detection, the PCB uses a pressure transducer (PT2)
separate from the measurement channel, a secondary microprocessor (U4), and power-cut
circuitry (Q1, Q2). This circuit is also powered by a separate voltage (T5V) than used by the
AD7714 and measurement transducer.
An over-pressure test shall be performed yearly through service diagnostics to verify the
circuit is operating within the prescribed parameters.
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