AAT4626 Datasheet, PDF (9/15 Pages) Advanced Analogic Technologies – USB Dual-Channel Power Switch

from effecting the operation of the AAT4626. For
example, if an AAT4626 is used in a circuit that oper-
ates from a 5V power supply with poor step load
response, it is possible that turning on the load
switch could cause the input power supply to droop
below the AAT4626's under-voltage lockout thresh-
old. This drop in voltage would cause the AAT4626
to turn off until the input power supply voltage levels
recover. 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 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 when a short circuit is sud-
denly applied to a device that is already on. Refer to
the Typical Characteristics curve "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 downstream 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 informa-
tion section. When both the A and B outputs of the
AAT4626 are in the off state, the respective outputs
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 5V TTL levels, as well
4626.2006.05.1.2
AAT4626
USB Dual-Channel Power Switch
as CMOS-compatible levels ranging from 2.5V to
5V. The ENA or ENB function control voltage levels
should not exceed the input supply level applied to
the IN pin.
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 properly oper-
ate. 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 10ms delay on the fault
function is intended to prevent capacitive loads con-
nected to one of the load switch outputs from acti-
vating its 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 AAT4626 to sig-
nal ancillary circuitry that is powered by voltage lev-
els 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.
As shown in 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-volt-
age lockout control circuit. The under-voltage lock-
out prevents the output MOSFET devices from turn-
ing on until V_IN exceeds the typical UVLO threshold
of 2.3V. During operation, the device will automati-
cally shut down if V_IN falls below the UVLO threshold
and the fault flags will be toggled.
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