Voltage Regulator Circuit; Over Voltage Protection Circuit; Switching Regulator Circuit - Epson LQ-860 Technical Manual

2.2.7 +5 V Switching Regulator Circuit
Figure 2-19 shows the +5 VDC switching regulator. This circuit employs a hybrid type switching regulator
the external output section.
4
t -
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I START
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, CIRCUIT
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L
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2.2.7.1 Activation
When the +35 VDC rises, the start circuit OF IC20 is activated, and a reference voltage is applied to the
positive terminal of comparator CMP in IC20. Since the negative side of CMP still remains at O [V] at this
time, the CMP output goes HIGH, Trf is turned on so that current flows from the emitter to the collector,
and the +5 V line turns on.

Voltage Regulator Circuit

2.2.7.2
Resistors R4 and R5 in IC20 regulate the +-5 V output voltage. L20 and C25 compose a differentiation
circuit and prevent noise and abnormal oscillation.
The output voltage is fed back from two pin:; of IC20 and is delivered to R4 and R5 and then input to the
negative side of CMP.
The voltage input to the negative side regulates the output voltage by controlling the on-time of Trl. It is
compared with the reference voltage, and if it is larger than +5 V, it switches on the CMP output so as
to turn on Ttl.
This operation is called Pulse Width Modulation (PWM) control. Figure 2-20 shows the PWM sequence.
ST R
2 0 0 0 5
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– Reference AC ,
Figure 2-19.
+5 V Switching Regulator Circuit
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R3
R4
R5
2-25
To over Voltage
Protection Circuit
+5 v