Power Supply - Sharp QA-1650 Service Manual

9.6.EEPROM control function
IC2(M.P.U.) has the EEPROM for writing timing parameters of all computers that can be connected to QA-
1650.
The timing setting value is written into the EEPROM under the following four conditions:
1. ALL RESET functioning
2. RESET functioning
3. Turning the power off
4. Changing the computer connected
Writing into the EEPROM when turning the power off,however,is carried out only when the setting value is
altered by key entry.
10.POWER SUPPLY
The AC adaptor that comes with the QA-1650 supplies a regulated power voltage VIN (12 VDC) to the QA-
1650. From VIN, power 5 V (V.p,, D-Vog, and A-V,,), 10 V, -10 V, power 12V for LCD, power for PLL
(VVCO) and fan motor (Vfan) are generated.
10.1.DC-DC Converter for 5 V (V,,,,, and V,,,.)
C200 in Fig. 23 is a controller for a fixed frequency pulse width modulation (PWM) switching regulator and
has two constant voltage controllers.One controller is used to generate a 5V power supply (Vepy» D-Vog)
from the input power supply VIN (12 VDC) (the other one is for V,.,.).
1C200 has a saw-tooth wave oscillator built in whose oscillating frequency is determined by capacitor C301
connected to pin 1 and resistor R303 connected to pin 2. In this circuit configuration, a saw-tooh wave of
approximately 75 kHz is observed at pin 1 of IC200. Basically this saw-tooth wave is compaed with a
control signal observed at pin 5 of IC200. Only while the voltage of the saw-tooth wave is lower than the
control signal voltage, pin 7 of IC200 becomes "H", as a result external transistors Q201 and Q2t0 become
conductive.The output voltage is fed back to pin 3 of IC200 after dividing R300, R301, and VR3(0.
A reference voltage of 2.5 V is outputted from pin 3 of IC200. An error amplifier in IC200 makes a compari-
son between the half of the reference voltage and the feedback signal to pin 3 and generates a control signal
(pin 5 of IC200). As explained above the output voltage is stabilized by the pulse width from pin 7, which is
generated by comparing the control signal with the saw-tooth wave.
TP206(V.py) is adjusted to 5.1 V with VR300 while TP208(D-V,,.) is 5.0V. This is because the ltad on the
Vo, is greater than the load on V,,,, and the forward voltages of D212, D213, and D214 are highe than that
of D211.
The voltage supplied to pin 6 of 1C200 is called a dead time control signal. It controls the maxmaum ON
period of the output transistor in IC200. In the QA-1650 configuration this control to have the mainaum ON
period is realized on 1C200 by shorting pin 6 and pin 16.
10.2.DC-DC Converter for 5 V (V, .,.)
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