Dual-Input, Dual-Output Load Switch
Applications Information
Input Capacitor
The input capacitors, C
INA
and C
INB
, protect the
input power supplies from current transients gener-
ated by loads attached to the AAT4670. If a short
circuit is suddenly applied to an output of the
AAT4670, there is a 750 nanosecond period during
which a large current flows before current limit cir-
cuitry activates. (See characteristic curve "Short
Circuit Through 0.3Ω.") In this event, a properly
sized input capacitor can dramatically reduce the
voltage transient seen by the power supply and
other circuitry upstream from the AAT4670. C
IN
should be located as close to the device VIN pin as
practically possible. Ceramic, tantalum, or alu-
minum electrolytic capacitors may be selected for
C
IN
. There is no specific capacitor equivalent series
resistance (ESR) requirement for C
IN
. However, for
higher current operation, ceramic capacitors are
recommended for C
IN
due to their inherent capabil-
ity over tantalum capacitors to withstand input cur-
rent surges from low impedance sources such as
batteries in portable devices.
than approximately three milliseconds, the associ-
ated FAULT flag is pulled to ground through
approximately 100Ω. Removal of voltage or cur-
rent transients of less than three milliseconds pre-
vents capacitive loads connected to either
AAT4670 output from activating the associated
FAULT flag when they are initially attached. Pull-up
resistances of 1kΩ to 100kΩ are recommended.
Since FAULT is an open drain terminal, it may be
pulled up to any unrelated voltage less than the
maximum operating voltage of 5.5V, allowing for
level shifting between circuits.
AAT4670
Thermal Considerations
Since the AAT4670 has internal current limit and
over-temperature protection, junction temperature
is rarely a concern. However, if the application
requires large currents in a hot environment, it is
possible that temperature, rather than current limit,
will be the dominant regulating condition. In these
applications, the maximum current available with-
out risk of an over-temperature condition must be
calculated. The maximum internal temperature
while current limit is not active can be calculated
using Equation 1.
Output Capacitor
In order to insure stability while the current limit is
active, a small capacitance of approximately 1µF is
required on each output. No matter how big the
output capacitor, output current is limited to the
value set by the AAT4670 current limiting circuitry,
allowing very large output capacitors to 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.
Eq. 1:
T
J(MAX)
= I
MAX2
· R
DS(ON)(MAX)
· R
ΘJA
+ T
A(MAX)
Attaching Loads
Capacitive loads attached to the AAT4670 will charge
at a rate no greater than the current limit setting.
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 AAT4670 at high temperature.
R
θJA
is the thermal resistance between the
AAT4670 die and the board onto which it is mount-
ed. T
A(MAX)
is the maximum temperature that the
PCB under the AAT4670 would be if the AAT4670
were not dissipating power. Equation 1 can be
rearranged to solve for I
MAX
; Equation 2.
Eq. 2:
I
MAX
=
T
SD(MIN)
- T
A(MAX)
R
DS(ON)(MAX)
· R
ΘJA
FAULT Output
FAULT flags are provided to alert the system if an
AAT4670 load is not receiving sufficient voltage to
operate properly. If current limit or over-tempera-
ture circuits in any combination are active for more
T
SD(MIN)
is the minimum temperature required to
activate the AAT4670 over-temperature protection.
With typical specification of 125°C, 115°C is a safe
minimum value to use.
8
4670.2006.09.1.4
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
