Xerox Phaser 3450 Service Manual page 145

CIRCUIT DESCRIPTION
11.2.4 Video Controller
11.2.4.1 Power On Reset Circuit
Reset circuit initializes PPC(U26) and SPGPi(U27) when the power turns on and prevents unstable CPU operation until
the power reaches standard level. This circuit is composed of R-C circuit, which decides the value of
XC61FN3112MR(U20) Reset IC and Reset time. Reset takes place 0.2seconds after the power is on and when the power
level falls below 3.0V. Then, CPU(U2) carries out initialization accordingly. 'RESET' signal is set at High followed by voltage
change approximately after 50~150ms.
Vcc
3.0 V
Reset*
Time
Treset = 137ms
Treset = 180ms
As shown in the diagram, when the power reaches 3.0V after initial power(Vcc) is on, Reset signal is sent to CPU after
180ms by reset circuit. At this time CPU initializes itself and the surrounding elements. Also, it prevents various
malfunctions by sending the reset signal when the Vcc falls below 3.0V during operation.(Initial operation voltage of IC :
3.3±5% 0.5V) Since IC guarantees operation at minimum 3.1V, before the level falls below the minimum value, the circuit
is designed to activate reset function at 3.0V.
11.2.4.2 CPU
CPU is 32bit microprocessor SPGPi(U27, 352pin). CPU receives 12.5MHz OSC Clock and operates at 50MHz. All
surrounding IC's are controlled by CPU. (Refer to Data Book for detailed CPU Specification.)
11.2.4.3 CPU Circuit
OSC(12.5MHz) is applied. Clock is applied to CPU #A5 using CY25814. The DC connected Bead and Resistance re used
for EMI counter measures.
11.2.4.4 Fuser Control / Thermistor Circuit
To settle the toner to the surface of a paper, this circuit adequately controls the temperature of heat lamp. It is composed
of thermistor, LM393(voltage comparison) and transistor for switching. Basic principle is that the thermistor attains
resistance value that is inversely proportional to the heat lamp temperature. The voltage value distributed between the
resistance R8 and R9 is read at #69 AD0_IN of LPEC1. This voltage value becomes active(inactive) when Fuser signal is
High(low) according to the set temperature table and the Rom program. Based on the Q2 switching, if the 'FUSERON*'
signal is low, TLP3061(U201) turns on. AC voltage is applied to the heat lamp by turning on the bidirectional
Thyristor(THY201) or shut off by turning off the bidirectional Thyristor(THY201). LM393 is the hardware type safety feature
to prepare for abnormal heat lamp control. When the temperature of thermistor rises to 210 °C, #1 pin of LM393 falls to
low level, which turns the 'FUSERON*' signal to High.(Q3 is forced Off). Therefore, the heat lamp is forced to go Off.
Service Manual
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