CAT6500 Datasheet, PDF (10/13 Pages) ON Semiconductor – 3.0 A Power Selector Switch

CAT6500
CIRCUIT DESCRIPTION AND OPERATING CONSIDERATIONS
Description
CAT6500 is an autonomous power selector switch
designed for portable device applications where either of
two power sources may be used for battery charging and
device operation. CAT6500 can operate in two distinct
modes, forward or reverse, depending on the states of the
RM_ENx inputs.
In forward mode, CAT6500 will automatically select from
the available power sources, PS1 or PS2, and direct one to
PWR_OUT.
In reverse mode, a system power rail connected to
PWR_OUT can source power to an external device attached
to either PS1 or PS2. This allows charging or powering of
other portable devices.
Power Source Selection
In forward mode, on−chip voltage detection circuitry
senses the presence of a suitable power source at power
inputs, PS1 and PS2. If both inputs are powered the
PRIORITY pin sets the preferred power source directs that
source to PWR_OUT. If only one of the two inputs is
powered then that power source is directed to PWR_OUT.
CAT6500 provides two status outputs SWx_STAT to
indicate the presence of a voltage at either PS1 or PS2. These
status outputs trigger at 1.7 V and are LOW true digital
outputs.
PRIORITY has an internal pull−up and defaults to a logic
HIGH if the pin is disconnected or left floating. Input
selection follows the truth table in Table 6.
CAT6500 draws its operating power from PS1 or PS2
when a voltage of 2.5 V or more is present. If no power is
present at PS1or PS2 or CAT6500 is in reverse mode, power
will be drawn from VCC.
CAT6500 provides overvoltage protection to circuitry
downstream from the chip by limiting input voltages to 7 V.
Should the voltage at PS1 or PS2 rise above 7 V then
PWR_OUT will be disconnected from the power source
until the voltage returns to safe levels.
CAT6500 provides similar protection for reverse polarity
voltages down to −5 V.
Reverse Mode Operation
The RM_ENx inputs allow CAT6500 to operate the power
switches in reverse mode where the PWR_OUT becomes
the supply powering PS1 and/or PS2. When RM_EN1 is
logic high, SW1 switch is turned on and PWR_OUT is
connected to PS1. When RM_EN2 is logic high, SW2
switch is turned on and PWR_OUT is connected to PS2. The
switch connection remains on until the PWR_OUT voltage
decreases all the way to 0 V (below 0.1 V typical) regardless
of the state of the associated RM_ENx input.
RM_EN is not affected by the voltage levels seen at PS1
or PS2 as PRIORITY and will not switch OFF automatically
if the voltage drops below 1.7 V as would CAT6500
otherwise do. This allows the power connection to be used
for signaling purposes as in USB On−The−Go where power
line signaling is used to request a transfer of bus Master
status between devices. When operating in reverse mode,
the SW1_STAT and SW2_STAT outputs are still active and
will reflect the switch conditions.
RM_EN1 and RM_EN2 are independent controls and can
be activated simultaneously, meaning both SW1 and SW2
can be conducting at the same time. This presents both
opportunities and hazards.
Having both switches ON allows for simultaneous
charging or powering of two devices from a single source;
a USB power source can charge and operate the device as
well as power an additional unit connected to the other PS
input. Or the device can power two external units attached
to PS1 and PS2.
The downside of this capability becomes apparent when
two operating power sources are present at the same time. If
both switches are ON the power sources will compete with
the stronger driving the weaker. For example; if a wall
charger is attached to PS1 and an active USB port to PS2,
with both SW1 and SW2 ON, the wall charger will likely
dominate and push power backwards into the USB port,
possibly elevating the USB bus voltage above allowable
limits.
Note: SW1 and SW2 are not current limited and can conduct
very high currents if short circuited. Current limiting
circuitry is advisable if short circuits are possible in the
intended application.
Entering and Exiting Reverse Mode
When entering or exiting Reverse Mode, it is
recommended that power applied to PWR_OUT be
sequenced with the enabling/disabling signal. It is best to
enter Reverse Mode with PWR_OUT at 0 V and apply
power after the logic control. Similarly on exiting Reverse
Mode, power should be taken to 0 V and then the switch
disabled.
1
RM_EN
0
PWR_OUT
V_OUT
0V
Figure 18. Entering and Exiting Reverse Mode
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