AAT4626 Datasheet, PDF (8/14 Pages) Advanced Analogic Technologies – USB Dual-Channel Power Switch

recovers. Since this cycle would be self-perpetuat-
ing, 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 cir-
cuit should be slew rate limited when turned on.
Output Capacitor
In order to insure stability while the device current
limit is active, a small capacitance of approximately
1µF should be used on each output. When either
output of the AAT4626 is activated using the
EN(A/B) function, there are no momentary current
transients as in the case of when a short circuit sud-
denly applied to a device that is already on. Refer to
the characteristic curve named "Turn-On/OFF
Response". Regardless of output capacitor size,
output current on either output 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 per-
mits very large output capacitors to be used.
For example, USB ports are specified to have at
least 120µF of down stream capacitance from their
controlling power switch. An output capacitance as
large as 1000µF would not disturb the input power
supply to an AAT4626 used to control a USB port.
EN Inputs
The AAT4626 has two enable inputs, ENA and
ENB. These two enable inputs allow the AAT4626
to independently control each respective output.
The device is available in both active high EN
enable and active low (EN) enable versions. For
specific part numbers, refer to the ordering infor-
mation section. When both the A and B outputs of
the AAT4626 are in the off state, the respective out-
puts are an open circuit and the device quiescent
current consumption is reduced to less than 1µA.
The ENA and ENB threshold voltages are set to
allow the AAT4626 to be controlled by 5 volt TTL
levels as well as CMOS compatible levels ranging
from 2.5 volts to 5 volts. The ENA or ENB function
control voltage levels should not exceed the input
supply level applied to the IN pin.
AAT4626
USB Dual-Channel Power Switch
Fault Flag Output
The AAT4626 features an active low fault flag
(FLGA and FLGB) output for each A and B output
channel. The fault flags are provided to alert the
system if the over current or over temperature cir-
cuits become active or if the load switch is not
receiving a sufficient voltage level to operate prop-
erly. If either the current limit or over temperature
circuits in any combination are constantly active for
more than approximately ten milliseconds, the
FLG(A/B) pin is pulled to ground internally through
an open drain device. The 10 millisecond delay on
the fault function is intended to prevent capacitive
loads connected to one of the load switch outputs
from activating it's respective flag when the device
is turned on. The placement of a pull up resistor
between the FLGA or FLGB pin and the IN pin is
recommended. Reasonable values for the pull up
resistor should range from 10kΩ to 100kΩ. Since
the fault flags are open drain terminals, they 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 ancillary circuitry that is powered
by voltage levels less than the level on the IN pin.
If a fault flag delay greater then 10ms is required,
addition delay may be added by use of an RC filter.
Referencing to Figure 1, an RC filter can be added
to the fault flag output.
Reverse Voltage
The AAT4626 is designed to control current flowing
from IN to OUT. If a voltage is applied to OUT
which is greater than that on IN, a large resulting
reverse current may flow, potentially damaging the
AAT4626.
Under Voltage Lockout
The AAT4626 has been designed with an under
voltage lockout (UVLO) control circuit. The under
voltage lockout prevents the output MOSFET
devices from turning on until VIN exceeds the typi-
cal UVLO threshold of 2.3 volts. During operation,
the device will automatically shutdown if VIN falls
below the UVLO threshold and the fault flags will
be toggled.
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