AAT4625
USB Single-Channel Power SwitchSmartSwitchTM
PRODUCT DATASHEET
8 4625.2009.11.1.2
www.analogictech.com
Applications Information
Operation in Current Limit
If an excessive load is applied to the output of the
AAT4625, 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, there would be
a demand for more current than allowed by the internal
current limiting circuit and the voltage at the device out-
put will drop. This causes the AAT4625 to dissipate more
power than in normal operation, causing the die tempera-
ture to increase. When die temperature exceeds the inter-
nal over-temperature threshold, the AAT4625 will shut
down. After shutting down, the AAT4625 cools to a level
below the over-temperature threshold, at which point it
will start up again. The AAT4625 will continue to cycle off
and on until one of the following events occurs: the load
current is reduced to a level below the AAT4625’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.
Thermal Considerations
Since the AAT4625 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) will regulate the
current available to the load. In these applications, the
maximum current available without 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: TJ(MAX) = IMAX2 × RDS(ON)(MAX) × RθJA + TA(MAX)
In Equation 1, IMAX is the maximum current required by
the load. RDS(ON)(MAX) is the maximum rated RDS(ON) of the
AAT4625 at high temperature. RθJA is the thermal resis-
tance between the device die and the board onto which it
is mounted. TA(MAX) is the maximum ambient temperature
for the printed circuit board assembly under the AAT4625
when the load switch is not dissipating power. Equation 1
can be transformed to provide IMAX; Refer to Equation 2.
Eq. 2: =
IMAX
TSD(MIN) - TA(MAX)
RDS(ON)(MAX) · RΘJA
TSD(MIN) is the minimum temperature required to activate
the device over-temperature protection. The typical
thermal limit temperature specification is 125°C for the
AAT4625; for calculations, 115°C is a safe minimum
value.
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 por-
table device has a sealed case and the area of the
printed board assembly is relatively small, causing RθJA to
be approximately 120°C/W. Using Equation 2,
=
IMAX = 1.4
115 - 85
130 · 120
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 circuit. If a
short circuit is suddenly applied to the output of an
AAT4625, 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
“Short Circuit Through 0.3Ω." A properly sized input
capacitor can dramatically reduce the load switch input
transient response effects seen by the power supply and
other circuitry upstream from the AAT4625.
The second purpose of the input capacitor is to prevent
transient events generated by the load circuit from
affecting the operation of the AAT4625. For example, if
an AAT4625 is used in a circuit that operates from a 5V
power supply with poor step load response, turning on
the load switch could cause the input power supply to
drop below the AAT4625's under-voltage lockout thresh-
old. This drop in voltage would cause the AAT4625 to
turn off until the input power supply voltage level recov-
ers. 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.
Output Capacitor
In order to insure stability while the device current limit
is active, a small capacitance of approximately 1μF
should be used. When the AAT4625 is activated using