English
Language : 

AAT4670 Datasheet, PDF (7/12 Pages) Advanced Analogic Technologies – Dual-Input, Dual-Output Load Switches
◁
AAT4670
Dual-Input, Dual-Output Load Switches
Applications Information
Input Capacitor
The input capacitors, CINA and CINB, protect the input
power supplies from current transients generated 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 circuitry activates.
(See characteristic curve "Short Circuit Through
0.3Ω.") In this event, a properly sized input capaci-
tor can dramatically reduce the voltage transient
seen by the power supply and other circuitry
upstream from the AAT4670. CIN should be located
as close to the device VIN pin as practically possible.
Ceramic, tantalum or aluminum electrolytic capaci-
tors may be selected for CIN. There is no specific
capacitor ESR requirement for CIN. However, for
higher current operation, ceramic capacitors are rec-
ommended for CIN due to their inherent capability
over tantalum capacitors to withstand input current
surges from low impedance sources such as batter-
ies in portable devices.
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 down stream from their
controlling power switch. The current limiting circuit
will allow an output capacitance of 1000µF or more
without disturbing the upstream power supply.
Attaching Loads
Capacitive loads attached to the AAT4670 will charge
at a rate no greater than the current limit setting.
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
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
4670.2002.1.0.92
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.
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 appli-
cations, the maximum current available without 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.
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 maxi-
mum rated RDS(ON) of the AAT4670 at high temper-
ature. RθJA is the thermal resistance between the
AAT4670 die and the board onto which it is mount-
ed. TA(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 IMAX; Equation 2.
IMAX=
TSD(MIN) - TA(MAX)
RDS(ON)(MAX) × RθJA
TSD(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.
For example, if an application is specified to oper-
ate in 50°C environments, the PCB operates at
temperatures as high as 85°C. The application is
sealed and its PCB is small, causing RθJA to be
approximately 120°C/W. Using Equation 2,
IMAX=
115
160m
- 85
× 120
=
1.25
A
To prevent thermal limiting, the operating load cur-
rent in the application must be less than 1.25A
which lies in the current limiting range, so in this
application, any operating current below the cur-
rent limit threshold is allowed.
7
 ▷