Kurzweil K2000 - K2000 Audio;Power Board Theory - Part 4; Power Supply and Regulator Theory; Backup Voltage Supply and LCD Backlight Theory

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SECTION 5
AUDPWR2.DOC 12/15/92
5.1.2.4 ANALOG REGULATORS
VR5, C186 (F-1) provide +5Vana and VR2, CLB8 (G-1) provide +5Vref; both circuits are sourced by
VR7, C207 (+12Vana @ TP23, D-1 & E-1).
VR3, C193 (D-1) provide +12VDC for the optional hard disk at TP19 (H-1) and pin 1 of J18 (1-1).
R200 is required to lightly load VR3 when the hard disk is not installed; this prevents
oscillation.
VR6, C206 (D-2) provide -12VDC for the audio circuitry at TP24 (E-2) and becomes -12Vana.
VR5 and VR4 provide quiet +5 and -% volt power to the analog switches in the audio path. VR2
provides an isolated and quiet +5 volt reference to the AID and D/A converters on the scanner
microprocessor and sever performance controls.
VR1, C182 (D-2) outputs -15Vbias for the LCD Vee generator and muting circuit. This regulator
may cease regulating at low line voltage, however the only effects are a possible reduction in
LCD contrast and premature clipping of large negative signal peaks.
5.1.2.5 DIGITAL REGULATORS
The circuit of D25 (dual Shottky, B-2) and C202 (B-3) provide high current availability of +8VDC
(unregulated) @ TP17 (C-2). This current source is used by the digital regulator circuit(s) .
Due
to the lower forward voltage drop of a Shottky rectifier, less heat is dissipated by the
rectifier and the transformer voltage can be lower; however because of the heavy current drain on
the +5 volt supply (up to 3.5 amps), it is necessary to heats ink D25 to the rear panel. C200 and
C201 suppress rectifier switching noise.
The circuit of U62 (C-3) and Q18 (E-3, a VMOS power FET) makes an ideal pass transistor for a
linear regulator and provides +5VDC for all digital circuitry at the output end of L5 (1-2).
This circuit is wired as a voltage fOllower, thus the final output voltage at the source of Q18
is forced (by the gain of the op-amp, U62) to equal the reference voltage at its non-inverting
input. This VOltage can be monitored at TP16 (H-2). The output is adjusted at R180 for exactly
5.00 volts (C-4) to optimize performance of the custom VLSI chips on the Engine Board.
It is extremely important for the Engine board to know when there is sufficient voltage for
proper operation so as to reset itself. The mute circuit(s) must also be asserted when there is
insufficient voltage. Simply put, when the output of U62 is within 1.2 volts of its supply
(close to its saturation point), Q16 and Q17 cut off saturating Q15 and discharging C176 quickly.
The reset circuit (described later) immediately asserts RESET to all microprocessors and mu~e
circuit(s). •
R192 and C174 stabilize the feedback loop to prevent oscillation of the regulator. C178 and C180
improve regulation during sudden load changes. R193 provides a minimum load if the power supply
is ever operated with no other components on the board.
5.1.2.6 BACKUP VOLTAGE SUPPLY
Three AA alkaline cells provide power to three RAM chips during power off. Standard alkaline
cells are used instead of the more expensive lithium cells because power drain is approximately
the same; thus reducing cost to the customer.
When power is off, current from the battery passes through R182 and D16 (1-3) to the BUVdd line
which is connected to the power pins of the 3 RAM chips (one on the Audio/Power Board and the
remaining two on the Engine Board). D17 prevents battery power from backing into unwanted
circuitry.
With power up, Q19 provides power to the RAM chips. D17 ensures gradual application of full
VOltage to the RAMs during power-up. C203 (E-7) provides 30 - 60 seconds of RAM power to allow
for battery change when needed.
5.1.2.7
LCD BACKLIGHT POWER SUPPLY
The backlight for the LCD requires approximately 100 volts rms, at approximately 360Hz operation.
The circuit of Q25 and Tl (G-4 to H-5) fulfill this need. The backlight cable must be plugged
into J2l (1-5) for Q25 to oscillate. A reasonably good sine wave appears at J2l-4. C208 and
R218 provide the positive feedback route.
Q24 and surrounding circuitry provide a controlled higher voltage from the unregulated +8 volts
so as to improve LCD brightness. The output voltage can be adjusted to balance between
brightness and the acoustic noise level from the panel by changing the value of R216 (F-5).
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