Accutorr Plus Service Manual - Mindray

Detailed Description
It would be helpful to have a schematic (0387-00-0716) while reading the following
circuit descriptions.
Power
The power for the interface board is supplied by the system +5V. It is used for the
pull-up resistors (R1, R2 and R6) on the serial and control lines as well as the LED drive
resistors (R3 and R4) on optocoupler U1. It is also the input to isolation converters T1
and T2. In addition, the system +5V is the supply voltage for optocoupler U2 and the
logic gates in U3.
T1 converts the unisolated system +5V into an isolated and regulated source, +5DIG, for
many uses. The converter provides suitable isolation to the specified patient limit of
1500VAC. The majority of the current from +5DIG is delivered to the MS-3 board. It
also acts as the supply voltage for optocoupler U1 and the ESD protection array, U5. In
addition, the digital voltage supplies the LED drive resistor (R5) on optocoupler U2.
The second largest usage for the +5DIG is as an input for the LT1373. The LT1373 is a
high frequency current mode switching regulator running at approximately 250kHz. In
this design, it is configured as an extremely efficient dual flyback converter. It will
convert a single input voltage ranging from 2.7 to 25 volts into a positive and negative
output. This design utilizes the feedback to provide +/-15VAN to the MS-3 board. The
power transistor “switch” integral to the LT1373 alternately charges and discharges the
output capacitor C16. C12 and C13 provide the hold-up time for the output voltages.
L2-C14 and L3-C15 are additional low pass filters for the analog voltages.
Converter T2 generates an unregulated +/-9V (used for the front end). Schottky diodes
CR1 and CR2 provide reverse polarity protection for T2. C10 and C11 filter the isolated
+/-9V in order to reduce the ripple on each supply.
Caps C17 and C19 on the input of the two DC/DC converters are necessary to reduce the
ripple reflected back on to the system +5V. This noise is generated due to the transient
nature of the MS-3 loading.
Communication
The interface signals to and from the MS-3 are isolated (2500VAC) by two optocouplers,
U1 and U2. The optocouplers include integrated Schmitt triggers at the outputs. This
provides logic compatible waveforms without the need for external pull-up resistors.
NANDs are used in the transmission lines to provide the necessary inversions for proper
signal polarity and to force the optocouplers normally off. The low duty cycle of the
signals also holds the power requirements to reasonable levels.
Communications with the SpO2 Module take place by a CMOS level asynchronous serial
link from J1. The parameters are 8 data bits, no parity, one stop bit, with the data
having “true” polarity. The baud rate is fixed at 9600 bps.
Display Controller Connections
A four pin right angled connector (J4) is available to interface with the future Display
Controller board. The serial information received (TX and RX) will be used to generate
a pleth waveform. The control signal (SpO2_COMM*) will be an active low signal used
to enable a download to this future board.
Accutorr Plus Service Manual Revised 12/20/00
2-33
Chapter 2 - Theory of Operation