AAT4626 Datasheet, PDF (7/14 Pages) Advanced Analogic Technologies – USB Dual-Channel Power Switch

Applications Information
Operation in Current Limit
If an excessive load is applied to the either output
of an AAT4626, the load current will be limited by
the AAT4626's current limit circuitry. Refer to the
"Current Limit" figure in the typical characteristics
section of this data sheet. If a short circuit were to
occur on the load applied to either the A or B out-
put, there would be a demand for more current
than what is allowed by the internal current limiting
circuit and the voltage at the device output will
drop. This causes the AAT4626 to dissipate more
power than in normal operation, causing the die
temperature to increase. When die temperature
exceeds the internal over temperature threshold,
the AAT4626 will shut down both the A and B out-
put channels. After shutting down, the AAT4626
cools to a level below the over temperature thresh-
old, at which point it will start up again. The
AAT4626 will continue to cycle off and on until one
of the following events occurs; the load current of
the offending output is reduced to a level below the
AAT4626's current limit setting, the input power is
removed, or until the output is turned off by a logic
high level applied to the EN pin of the fault channel.
Thermal Considerations
Since the AAT4626 has internal current limit and
over temperature protection, junction temperature
is rarely a concern. If an application requires a
large load current in a high temperature operating
environment, there is the possibility that the over
temperature protection circuit rather than the cur-
rent limit circuit from one of the two outputs will reg-
ulate the current available to the load. In these
applications, the maximum current available with-
out risk of activation of the over temperature circuit
can be calculated. The maximum internal temper-
ature while current limit is not active can be calcu-
lated using Equation 1.
T_J(MAX) = I_MAX² × R_DS(ON)(MAX) × R_θJA + T_A(MAX)
In Equation 1, I_MAX is the maximum current
required by the load. R_DS(ON)(MAX) is the maxi-
mum rated R_DS(ON) of the AAT4626 at high temper-
ature. R_θJA is the thermal resistance between the
device die and the board onto which it is mounted.
T_A(MAX) is the maximum ambient temperature for
4626.2002.1.0.93
AAT4626
USB Dual-Channel Power Switch
the printed circuit board assembly under the
AAT4626 when the load switch is not dissipating
power. Equation 1 can be transformed to provide
I_MAX; Refer to Equation 2.
I_MAX=
T_SD(MIN) - T_A(MAX)
R_DS(ON)(MAX) × R_θJA
T_SD(MIN) is the minimum temperature required to
activate the device over temperature protection.
The typical thermal limit temperature specification
is 125°C for the AAT4626, for calculations, 115°C is
a safe minimum value to use.
For example, a portable device is specified to oper-
ate in a 50°C environment. The printed circuit board
assembly will operate at temperatures as high as
85°C. This portable device has a sealed case and
the area of the printed board assembly is relatively
small causing R_θJA to be approximately 100°C/W.
R_DS(ON)(MAX) = 130W. Using Equation 2,
I_MAX=
115°C - 85°C
130W × 120°C/W
=
1.4 A
If this system requires less than 1.4 A, the thermal
limit will not activate during normal operation.
Input Capacitor
The input capacitor serves two purposes. First, it
protects the source power supply from transient
current effects generated by the application load cir-
cuits. If a short circuit is suddenly applied to either
output of an AAT4626, there is a microsecond long
period during which a large current can flow before
the current limit circuit becomes active. Refer to the
characteristic curve named "Short Circuit Through
0.3Ω." A properly sized input capacitor can dramat-
ically reduce the load switch input transient
response effects seen by the power supply and
other circuitry upstream from the AAT4626.
The second purpose of the input capacitor is to pre-
vent transient events generated by the load circuits
from effecting the operation of the AAT4626. For
example, if an AAT4626 is used in a circuit that oper-
ates from a 5 volt 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
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