AAT4651 Datasheet, PDF (8/14 Pages) Advanced Analogic Technologies – 5V/3V PC Card Power Switch

Applications Information
Input Capacitor
A 1µF or larger capacitor is typically recommended
for C_IN. A C_IN capacitor is not required for basic
operation; however, it is useful in preventing load
transients from affecting upstream circuits. 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 rec-
ommended for C_IN 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
A 0.1µF or greater capacitor is generally required
between V_CC and GND. Likewise, with the output
capacitor, there is no specific capacitor ESR
requirement. If desired, C_OUT may be increased to
accommodate any load transient condition.
Parallel Interface / Break Before Make
A 2-bit parallel interface determines the state of the
V_CC output. The logic levels are compatible with
CMOS or TTL logic. A logic low value must be less
than 0.8V, and a logic high value must be greater
than 2.4V. In cases where the interface pins rapidly
change state directly from 3V to 5V (or vice versa),
internal break-before-make circuitry prevents any
backflow of current from one input power supply to
the other. In addition, the body connections of the
internal P-channel MOSFET switches are always
set to the highest potential of V_CC3, V_CC5, or V_CC,
which prevents any body diode conduction, power
supply backflow, or possible device damage.
FAULT Output
The FAULT output is pulled to ground by an open
drain N-channel MOSFET during an over-current
or output slew condition. It should be pulled up to
the reference power supply of the controller IC via
a nominal 100kΩ resistor.
8
AAT4651
5V/3V PC Card Power Switch
Voltage Regulation
The PC card specification calls for a regulated 5V
supply tolerance of +/-5%. Of this, a typical power
supply will drop less than 2%, and the PCB traces
will drop another 1%. This leaves 2% for the
AAT4651 as the PC card switch. In the PC card
application, the maximum allowable current for the
AAT4651 is dominated by voltage regulation rather
than by thermal considerations, and is set by either
the current limit or the maximum R_DS(ON) of the P-
channel MOSFET. The maximum R_DS(ON) at 85°C
is calculated by applying the R_DS(ON) temperature
coefficient to the maximum room temperature
R_DS(ON):
R_DS(ON)(MAX) = R_DS(ON)25 ⋅ (1 + [TC ⋅ ΔT])
-or-
R_DS(ON)(MAX) = 100mΩ ⋅ (1 + [0.0028 ⋅ 60]) = 116.8mΩ
The maximum current is equal to the 2% tolerance
of the 5V supply (100mV) across the AAT4651
divided by R_DS(ON)(MAX). Or:
I_MAX5
=
100mV
116.8mΩ
= 856.2mA
For the 3.3V supply in the PC card application, the
conditions are a bit relaxed, with the allowable volt-
age regulation drop equal to 300mV. With a 2%
supply and 1% PCB trace regulation, the PC card
switch can have a 200mV drop. So:
I_MAX3 =
200mV
134mΩ
= 1.5A
Since 1.5A is the nominal current limit value, the
AAT4651 will current limit before I_MAX3 is reached.
Thermal issues are not a problem in the SOP-8
package since Θ_JA, the package thermal resistance,
is only 120°C/W. At any given ambient temperature
4651.2006.05.1.2