NELLCOR PURITAN BENNETT Symphony N-3000 Service Manual page 95

The transformer flyback pulse is rectified by CR7 and filtered by C12, C57,
and R5 to create VCCI. The other two transformer windings have three times
as many turns as the VCCI winding. These windings are rectified by CR5 and
CR6 to achieve ±15 volts during the flyback cycle. These supplies are then
regulated by U2 and U15 to ±12 volts.
Normal transformer signals are filtered out of the circuitry by C55, R110,
CR2, Q3, R38, C14, Q6, R53, and R49. However, the Q7 turn on transition is
a direct result of the ANALOG CLK signal. This creates the isolated clock
signal (ISO CLK) and guarantees a consistent output pulse time independent
of the pulse width modulation and inductive transient changes.
b. Timing
The ISO CLK is divided by U29 and decoded by U1 and U30 to create the
timing signals used throughout the isolated section of the circuit. The timing
diagram (Figure S4-2) shows the result of this decoding. One complete cycle
of the front end takes 16 ISO CLK cycles.
0 1
Analog Clk
Off/On
Red/Ir
Ir/Red
IRSMP L
REDSMP L
c. Microprocessor Control of Isolated Circuitry
Controller U10 (sheet 2 of schematic) is an Octal 8-bit DAC. The reference
voltage for U10 is 5 volts, created by U39. Two U10 outputs, VOUTA and
VOUTB, are used to program the LED outputs for the IR and RED channels.
The other six signal lines are used as logic lines. Five control amplifier gain:
PD0 and PD1 select the predemodulation gain, while DM0, DM1, and DM2
control the demodulated gain. The remaining signal line, CAL (U10, pin 18),
selects the calibration setting.
d. LED Drive
The LED drive circuitry is shown in the lower left corner of the schematic
diagram, sheet 2. Signal outputs from U10 — RED LED and IR LED — are
used to control LED brightness. These signals are multiplexed by U31 and
buffered by one-half of U32. A voltage divider circuit consisting of R101 and
R102 divide the multiplexed signal down to the proper voltage to use as a
reference for the other half of U32.
Additionally, R92 provides a path for stray current during the off period to
ensure that the LEDs are truly off. Components R97, R98, and Q12 provide
secondary protection against over-current drive by disabling the control op
amp if the voltage goes above 0.9 volts.
2 3 4 5 6 7
Figure S4-2: Timing Diagram
Technical Supplement
8 9 10 11 12 13 14 15
S-5