USB Dual-Channel Power Switch
Applications Information
Operation in Current Limit
If an excessive load is applied to either output of an
AAT4626, the load current will be limited by the
device's current limit circuitry. Refer to the "Current
Limit" curve in the Typical Characteristics section of
this datasheet. If a short circuit were to occur on the
load applied to either the A or B output, 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 output 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 fol-
lowing events occurs: the load current of the offend-
ing output is reduced to a level below the AAT4626's
current limit setting; the input power is removed; or
the output is turned off by a logic high level applied to
the EN pin of the fault channel.
die and the board onto which it is mounted. T
A(MAX)
is the maximum ambient temperature for the print-
ed 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.
AAT4626
Eq. 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,
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:
Eq. 1:
T
J(MAX)
= I
MAX2
· R
DS(ON)(MAX)
· R
ΘJA
+ T
A(MAX)
115°C - 85°C
Eq. 3:
I
MAX
= 130W
· 120
°
C/W
= 1.25A
If this system requires less than 1.4A, 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
Typical Characteristics curve "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
In Equation 1, I
MAX
is the maximum current
required by the load. R
DS(ON)(MAX)
is the maximum
rated R
DS(ON)
of the AAT4626 at high temperature.
R
θJA
is the thermal resistance between the device
8
4626.2006.05.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
