AAT4625_06 Datasheet, PDF (8/15 Pages) Advanced Analogic Technologies – USB Single-Channel Power Switch

AAT4625
USB Single-Channel Power Switch
Applications Information
Operation in Current Limit
If an excessive load is applied to the output of an
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 what is allowed by the internal current limiting
circuit and the voltage at the device output will
drop. This causes the AAT4625 to dissipate more
power than in normal operation, causing the die
temperature to increase. When die temperature
exceeds the internal over-temperature threshold,
the AAT4625 will shut down. After shutting down,
the AAT4625 cools to a level below the over-tem-
perature 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 cur-
rent 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: 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 AAT4625 at high temperature.
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 the print-
8
ed circuit board assembly under the AAT4625
when the load switch is not dissipating power.
Equation 1 can be transformed to provide I_MAX;
Refer to Equation 2.
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 AAT4625; 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
120°C/W. Using Equation 2,
I_MAX ⁼
115 - 85
130 · 120
=
1.4A
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
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 dra-
matically 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 pre-
vent 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 oper-
ates from a 5V power supply with poor step load
response, turning on the load switch could cause the
4625.2006.04.1.2