Power System Circuit And Description - NEC MultiSync LCD1920NX User And Service Manual

2.9 Power system circuit and description

The LCD1920NX uses an external power adaptor to provide the DC+12 voltage. In order to greatly reduce
the power consumption when in power saving stage, the circuit is used not only to convert the +12V to +5V,
but also to control the +5V ON/OFF status. When +12Vdc current enters W09 12V enters the whole system.
There are two regulators that convert the 12V to 5V. They are +5Vx and VLCD. The comparator of IC20 will
make the output of two regulators as a feed back to compare to the reference voltage. The compared result
will then control ON/OFF of IC19.
Time to achieve the voltage regulation affects ZD04 will create the reference voltage. Q13, Q14 and MC4049
are basically used to speed up the MOSFET ON/OFF switching time in order to reduce IC19 power
consumption. This product is also equiped with over-voltage protection circuit. When voltage of +5V at W09
is over 6Volts, IC20 Pin 14 will be pull high and Power MOS will be turned off, the voltage of +5Vx is not
support. The same theory of VLCD.
The +5V and VLCD source provide most of the required power for the LCD1920NX and LCD panel. When
system enter the power saving stage, the above two power sources will turn off through IC25 Pin 3. The +5V
source provides power to the CPU and SYNC processor, so it must be isolated from other power source.
When in power saving stage, IC25 Pin 3 will be set to low level to make the +5V and VLCD to be at cut off.
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