YSI 5000, Руководство по обслуживанию

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YSI Incorporated Model 5000/5100 Service Manual 28
transformer T1. Voltage regulation on the isolated supply (5ISO) will range from 5.1 to 5.5
volts.
DISPLAY INTERFACE
The display module is connected directly to the microprocessors bus. A chip select line, (CS3#)
is used to select the display and address line A0 selects command or data transfers. The font
select pin is driven by micro output pin P4.1. The LC Display drive voltage is generated from a
pulse width modulated output of the CPU. As the contrast is adjusted in software, the duty cycle
changes are used to control the DC current in transformer T3. See the power supply section.
POWER SUPPLY
POWER CONTROL
Power on is initiated by pressing the momentary power button. This pulls pin 13 of U2 low and
causes pin 11 to go hi. With both pins 8 and 9 of U2 hi, pin 10 will go low and bring the SMPS
controller U3 out of shutdown, so the switcher begins to operate, and latches pin 11 hi so power
stays on after the switch is released. Any time the power button is pressed an opto coupler is
energized that pulls a dedicated line on the keyboard I/O chip low and causes an interrupt. The
main micro, U5 can then turn power off by pulling the WDFO low through optocoupler U8.
SUPPLY GENERATION
The switching power supply circuit generates the main 5 volt supply and an auxiliary isolated 5
volt supply. The low current linear 5 volt supply is fed from the main through a RC filter.
NEGATIVE SUPPLY
Negative bias voltage for the LCD display is generated by switching a p-channel fet with the
PWM2 output from the micro. The inverting inductor/ diode/ capacitor configuration will output
a negative voltage dependent on the pulse width of PWM2 (-6 to -10.2v).
Should the PWM not operate or be temporarily shut off the leading RC will guarantee that the fet
does not remain on and drawing excessive current. In a fault condition the RC will cause the
FET to switch off in a maximum of 18.3us causing a peak current of 300mA Max. The p-FET is
rated for 100mA continuous and 400mA pulsed.
The negative linear supply is generated by U17 and its associated capacitors.
ANALOG
TEMPERATURE
The resistor divider network (R59, R60) output voltage ranges from 0.0615 volts at 50°C to
0.453 volts at -5°C. U22’s gain stage is set for 8. Gain error is calibrated out of the temperature
measurement by performing a gain=8 reading on a precision fixed voltage divider( R40 and
R41). The actual gain result (close to 8) is stored for use in calculating probe temperature.
Output from the amplifier ranges from 0.493 to 3.627 volts.
PRESSURE SENSOR
The pressure sensor produces a differential voltage on two output pins. A/D conversions must be
done on the high (channel 4) and low (channel 3) outputs, the ultimate result being the difference
in the two conversions adjusted to gain 1 (% of AVCC from BARO+ to BARO-). The user is
expected to provide a calibration point close to typical operating temperatures.