USB Single-Channel Power Switch
input power supply to drop below the AAT4625's
under-voltage lockout threshold. This drop in voltage
would cause the AAT4625 to turn off until the input
power supply voltage level recovers. Since this cycle
would be self-perpetuating, the entire circuit could be
seen to be unstable. In the very rare case where
capacitor cost is prohibitive and the input capacitor is
omitted, the output load circuit should be slew rate
limited when turned on.
datasheet. When the AAT4625 is in the off state, the
output is an open circuit and the device quiescent
current consumption is reduced to less than 1µA.
The EN threshold voltage is set to allow the
AAT4625 to be controlled by 5V TTL levels, as well
as CMOS-compatible levels ranging from 2.5V to 5V.
The EN function control voltage level should not
exceed the input supply level applied to the IN pin.
AAT4625
Output Capacitor
In order to insure stability while the device current
limit is active, a small capacitance of approximately
1µF should be used. When the AAT4625 is activat-
ed using the ON function, there are no momentary
current transients, as in the case when a short cir-
cuit is suddenly applied to a device that is already
on. Refer to the "Turn-On/Off Response" charac-
teristics curve. Regardless of output capacitor size,
output current is limited to the value allowed by the
threshold determined by the internal current limiting
circuitry. Refer to the internal current limit threshold
specifications stated in the Electrical Characteristics
section of this datasheet. This permits very large
output capacitors to be used.
For example, USB ports are specified to have at
least 120µF of downstream capacitance from their
controlling power switch. An output capacitance as
large as 1000µF would not disturb the input power
supply to an AAT4625 used to control a USB port.
Fault Flag Output
The AAT4625 features an active low fault flag out-
put. The fault flag is provided to alert the system
if the over-current or over-temperature circuits
become active or if the load switch is not receiv-
ing a sufficient voltage level to properly operate.
If either the current limit or over-temperature cir-
cuits in any combination are constantly active for
more than approximately 10ms, the FAULT pin is
pulled to ground internally through an open drain
device. The 10ms delay on the FAULT function is
intended to prevent capacitive loads connected to
the load switch output from activating the FAULT
flag when the device is turned on. The placement
of a pull-up resistor between the FAULT pin and
the IN pin is recommended. Reasonable values
for the pull-up resistor should range from 10kΩ to
100kΩ. Since the FAULT flag is an open drain ter-
minal, it may be pulled up to any voltage that is
not greater than the level present on the IN pin.
This is done to allow the AAT4625 to signal ancil-
lary circuitry that is powered by voltage levels less
than the level on the IN pin.
If a fault flag delay greater than 10ms is required,
addition delay may be added by use of an RC filter.
An RC filter can be added to the fault flag output,
as shown in Figure 1.
V+
100kΩ
R1
10kΩ
1
2
3
C1
0.1μF
4
EN Input
The AAT4625 is available in both active high and
active low enable versions. For specific part num-
bers, refer to the Ordering Information section of this
USB Controller
Over-Current
Flag Input
AAT4625
EN
FLG
GND
NC
OUT
IN
OUT
NC
8
7
6
5
Figure 1: Fault Flag Delay RC Filter.
4625.2006.04.1.2
9
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
