NIBP Module (P/N 0997-00-0501) Theory of Operation
2 - 24 0070-10-0441 Passport 2®/Passport 2 LT™ Service Manual
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.
2.4.5 Pump, M1
The pump control signal, M1EN, is provided via a PWM signal from the HC16Z1
microcontroller and is active high. This signal switches MOSFET Q3 (1/2), applying power
to the pump motor. Power to the pump (12VSW) is supplied from 12V2 via Q2. This allows
the pump to be disabled in case of a fault condition. (See section 1.1.1.3- Over-Pressure
Detection).
2.4.6 Dump Valve, V1
The dump valve, V1, is controlled by the V1EN signal provided by an active high signal from
the HC16Z1 microcontroller. This signal drives MOSFET Q3 (1/2), which switches power to
the valve coil. Power to the valve (12VSW) is supplied from 12V2 via Q2. This allows the
valve to be opened in case of a fault condition. This valve is a ‘normally open’ type, so coil
current must be supplied to cause the valve to close.
2.4.7 Linear Valve, V2
The bleed valve, V2, provides bleed down of the cuff pressure. The HC16Z1 software
maintains the pressure bleed rate at a nominal 6mmHg per second, for product specified
volumes and pressure ranges. The linear valvecan also be used to control the pump up rate
when small volume cuffs are used in the neonatal mode.
Note that this valve is a ‘normally open’ type, so coil current must be supplied to cause the
valve to close.
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.
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