UM_8009C_059
73S8009C Demo Board User Manual
Rev. 1.3
13
4.4
ON/OFF Switch Operation
The ON/OFF switch uses a pushbutton to toggle between turning the 73S8009C on and off. The switch
input contains a debounce circuit for protection. The 73S8009C defaults to the OFF state when the
power source is applied. When the 8009C is in the OFF state, a switch closure turns on the 73S8009C.
When the 73S8009C is ON, a switch closure does not turn off the 73S8009C by itself, but it activates the
OFF_REQ signal by setting it high. The 73S8009C does not shut off until the OFF_ACK is set high. The
purpose of this sequence is to allow the host processor to perform any necessary shut down tasks before
losing power. When the host is finished, it can set the OFF_ACK signal high to shut off the 73S8009C. If
there is no need for the host to perform any shutdown tasks, the OFF_ACK pin can be left open and it
follows the state of the OFF_REQ output by means of an internal resistor connection between the
OFF_REQ and OFF_ACK pins.
When power is applied to VBUS, the 73S8009C automatically turns on and the ON/OFF switch is
overidden. However, care must be taken as the ON_OFF input is internally latched while the VBUS
is applied. When VBUS is removed, the latched state of the ON/OFF switch input dictates the state
of the 73S8009C. If the switch input was not closed, the state of this latch will not change. It will be
in the same state before the VBUS power was applied. If it has changed it holds the last toggled
state. The OFF_REQ output follows this toggling. If the OFF_REQ output is high when VBUS
power is removed and the OFF_ACK is high, the 73S8009C shuts off.
