Datasheet

Application information STCC2540
16/33 DocID024711 Rev 5
5.5 Charging detection
The STCC2540 device continuously monitors current drawn by the portable device. While
programmed in CDP or DCP mode, the STCC2540 device sends a charging flag to the
system if the current through the USB power switch is above the charging threshold, set
at 20 mA typ.: open drain active low output CHARGING
is asserted.
Note: In CDP mode the charging flag is asserted only if the CDP handshaking between the
portable device and the host is performed before the current is above the threshold.
If a device is charging (CDP mode) while the system is turned off and battery supplied, the
risk is that the charging process stops before the end of charge due to the system power-off.
To avoid this, the flag CHARGING
can be used by the host to prevent the switching-off of
the high power DC-DC converter when the host goes to S5 state, even if battery supplied.
After the charging current drops below the threshold and the deglitch delay expires, the
CHARGING
output is deasserted.
5.6 Power switch
Overcurrent conditions
When an overcurrent condition is detected, the device maintains a constant output current
and reduces the voltage accordingly. There are two overload conditions:
 The first occurs when a short-circuit or a partial short-circuit is already present when
the device is being powered-up or enabled. The output voltage is held near zero
potential with respect to ground and the STCC2540 device ramps the output current to
I
OS
until the overload condition is removed or the device starts thermal cycle.
 The second condition is when a short-circuit, a partial short-circuit, or a transient
overload occurs while the device is already enabled and powered-on. The STCC2540
device responds to overcurrent condition within time t
IOS
. The current-sense amplifier
is overdriven during this time and momentarily disables the internal current limit
MOSFET. It then recovers and ramps the output current to I
OS
. Similar to previous
conditions, it limits output current to I
OS
until the overload condition is removed or
thermal cycle starts.
Thermal protection
The STCC2540 device has an internal thermal sensing circuit which monitors the operating
temperature of the circuit. It disables power switch operation if die temperature exceeds
temperature threshold T
STOP
, set at 150 °C. The switch turns on if the temperature has
cooled down by 20 °C.
Undervoltage lockout
The UVLO circuit disables the circuit until the input voltage reaches the UVLO turn-on
threshold. Built-in hysteresis prevents unwanted on/off cycling due to input voltage drop
during turn-on.