TOP233P datasheet(17/36 Pages) POWERINT

17 / 36 page

TOP232-234

7/01

Processor Controlled Supply Turn On/Off

A low cost momentary contact switch can be used to turn the

TOPSwitch-FX power on and off under microprocessor control

that may be required in some applications such as printers. The

low power remote off feature allows an elegant implementation

of this function with very few external components as shown in

Figure 27. 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-FX 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.

The microprocessor then sends a power supply control signal to

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, in the case of the inkjet printer, the shutdown procedure

may include safely parking the print heads in the storage

position. In the case of products with a disk drive, the shutdown

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

capacitor C

M may be needed to prevent noise coupling to the pin

when it is open.

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 27). This

remote on feature can be used to wake-up peripherals such as

printers, scanners, external modems, disk drives, etc., as needed

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.

Figure 27. Remote ON/OFF Using Microcontroller.

PI-2561-040401

VCC

(+5 V)

RETURN

CONTROL

High Voltage

DC Input

TOPSwitch-FX

1N4148

LTV817A

6.8 k

Ω

1 nF

100 k

Ω

6.8 k

Ω

LTV817A

27 k

Ω

1N4148

µF

P1 Switch

Status

Power

Supply

ON/OFF

Control

External

Wake-up

Signal

LOGIC

INPUT

LOGIC

OUTPUT

MICRO

PROCESSOR/

CONTROLLER

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