TOP242-249
Processor Controlled Supply Turn On/Off
position. Inthecaseofproductswithadiskdrive, theshutdown
procedure may include saving data or settings to the disk. After
the shutdown procedure is complete, when it is safe to turn off
the power supply, the microprocessor releases the M pin by
turning the optocoupler U4 off. If the manual switch and the
optocouplers U3 and U4 are not located close to the M pin, a
capacitorCM maybeneededtopreventnoisecouplingtothepin
when it is open.
A low cost momentary contact switch can be used to turn the
TOPSwitch-GXpoweronandoffundermicroprocessorcontrol
that may be required in some applications such as printers. The
lowpowerremoteofffeatureallowsanelegantimplementation
of this function with very few external components as shown in
Figure 45. Whenever the push button momentary contact
switch P1 is closed by the user, the optocoupler U3 is activated
to inform the microprocessor of this action. Initially, when the
power supply is off (M pin is floating), closing of P1 turns the
power supply on by shorting the M pin of the TOPSwitch-GX
to SOURCE through a diode (remote on). When the secondary
output voltage VCC is established, the microprocessor comes
alive and recognizes that the switch P1 is closed through the
switch status input that is driven by the optocoupler U3 output.
Themicroprocessorthensendsapowersupplycontrolsignalto
hold the power supply in the on-state through the optocoupler
U4. If the user presses the switch P1 again to command a turn
off, the microprocessor detects this through the optocoupler U3
and initiates a shutdown procedure that is product specific. For
example,inthecaseoftheinkjetprinter,theshutdownprocedure
may include safely parking the print heads in the storage
The power supply could also be turned on remotely through a
local area network or a parallel or serial port by driving the
optocoupler U4 input LED with a logic signal. Sometimes it is
easier to send a train of logic pulses through a cable (due to AC
coupling of cable, for example) instead of a DC logic level as
a wake up signal. In this case, a simple RC filter can be used to
generate a DC level to drive U4 (not shown in Figure 45). This
remote on feature can be used to wake up peripherals such as
printers,scanners,externalmodems,diskdrives,etc.,asneeded
from a computer. Peripherals are usually designed to turn off
automatically if they are not being used for a period of time, to
save power.
VCC
(+5 V)
+
External
Wake-up
Signal
High Voltage
DC Input
Power
Supply
ON/OFF
Control
MICRO
PROCESSOR/
CONTROLLER
100 kΩ
U2
27 kΩ
1N4148
LOGIC
INPUT OUTPUT
LOGIC
1N4148
6.8 kΩ
TOPSwitch-GX
D
S
M
U4
CONTROL
C
U3
6.8 kΩ
CM
47 µF
F
U1
U3
LTV817A
P1 Switch
Status
P1
1 nF
U4
LTV817A
-
RETURN
PI-2561-033001
Figure 45. Remote ON/OFF using Microcontroller.
E
7/01
August 8, 2000
25
POWERINT [ Power Integrations ]
