AAT4618
DATA SHEET
Current Limited Load Switch
8Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
201942A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 14, 2012
Output Capacitor
In order to insure stability while current limit is active, a
low capacitance (approximately 0.47μF) is required. No
matter how large the output capacitor, output current is
limited to the value set by the AAT4618 current limiting
circuitry, so very large output capacitors can be used.
For example, USB ports are specified to have at least
120μF of capacitance downstream from their controlling
power switch. The current limiting circuit will allow an
output capacitance of 1000μF or more without disturbing
the upstream power supply.
ON (Enable Input)
In many systems, power planes are controlled by inte-
grated circuits which run at lower voltages than the
power plane itself. The enable input (ON) of the AAT4618
has low and high threshold voltages that accommodate
this condition. The threshold voltages are compatible
with 5V TTL and 2.5V to 5V CMOS systems.
When switching the AAT4618 on into a capacitive load,
the AAT4618 will charge the output capacitive load at a
rate no greater than the current limit setting.
Attaching Loads
Capacitive loads attached to an enabled AAT4618 will
charge at a rate no greater than the current limit set-
ting.
FAULT Output
The FAULT flag is provided to alert the system if an
AAT4618 load is not receiving sufficient voltage to oper-
ate properly. If current limit or over-temperature circuits
in any combination are active for more than approxi-
mately 4ms, the FAULT flag is pulled to ground through
approximately 100. The filtering of voltage or current
transients of less than 4ms prevents capacitive loads
connected to the AAT4618 output from activating the
FAULT flag when they are initially attached. Pull-up resis-
tances of 1k to 100k are recommended. Since FAULT
is an open drain terminal, it may be pulled up to any
unrelated voltage less than the maximum operating volt-
age of 5.5V, allowing for level shifting between circuits.
Thermal Considerations
Since the AAT4618 has internal current limit and over-
temperature protection, junction temperature is rarely a
concern. However, if the application requires large cur-
rents in a hot environment, it is possible that tempera-
ture, rather than current limit, will be the dominant
regulating condition. In these applications, the maxi-
mum current available without risk of an over-tempera-
ture condition must be calculated. The maximum inter-
nal 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
AAT4618 at high temperature. RJA is the thermal resis-
tance between the AAT4618 die and the board onto
which it is mounted. TA(MAX) is the maximum temperature
that the PCB under the AAT4618 would be if the AAT4618
were not dissipating power. Equation 1 can be rear-
ranged to solve for IMAX, as shown in Equation 2.
Eq. 2: =
IMAX
TSD(MIN) - TA(MAX)
RDS(ON)(MAX) · RΘJA
TSD(MIN) is the minimum temperature required to activate
the AAT4618 over-temperature protection. With the
typical specification of 125°C, 115°C is a safe minimum
value to use.
For example, if an application is specified to operate in
50°C environments, the PCB operates at temperatures
as high as 85°C. The application is sealed and its PCB is
small, causing RJA to be approximately 150 °C/W. Using
Equation 2,
Eq. 3: =
IMAX = 950m
115 - 85
220 · 150
The AAT4618 is safe to operate at the minimum current
limit level of 500mA at 85°C.