AAT4626_08 Datasheet, PDF (8/15 Pages) Advanced Analogic Technologies – USB Dual-Channel Power Switch

SmartSwitch^TM
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 threshold, 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 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 current limit circuit from one of the two
outputs will regulate 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 temperature while
current limit is not active can be calculated using
Equation 1:
Eq. 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 maximum rated R_DS(ON) of the
AAT4626 at high temperature. R_θJA is the thermal resis-
tance between the device die and the board onto which it
is mounted. T_A(MAX) is the maximum ambient temperature
for 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.
PRODUCT DATASHEET
AAT4626
USB Dual-Channel Power Switch
Eq. 2: I_MAX ⁼
T_SD(MIN) - T_A(MAX)
R · R _DS(ON)(MAX)
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 operate 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 approxi-
mately 100°C/W. R_DS(ON)(MAX) = 130W. Using Equation 2,
Eq. 3: I_MAX ⁼
115°C - 85°C
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 pro-
tects the source power supply from transient current
effects generated by the application load circuits. 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 dramatically reduce the load switch
input transient response effects seen by the power sup-
ply and other circuitry upstream from the AAT4626.
The second purpose of the input capacitor is to prevent
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 operates 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-volt-
age lockout threshold. 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 circuit should be slew rate limited when turned on.
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4626.2008.02.1.3