AAT4651 Datasheet, PDF (9/14 Pages) Advanced Analogic Technologies – 5V/3V PC Card Power Switch

(T_A), the maximum package power dissipation can
be determined by the following equation:
P_D(MAX) =
T_J(MAX) -T_A
θ_JA
Constants for the AAT4651 are maximum junction
temperature, T_J(MAX) = 125°C, and package thermal
resistance, Θ_JA = 120°C/W. Worst case conditions
are calculated at the maximum operating tempera-
ture where T_A = 85°C. Typical conditions are cal-
culated under normal ambient conditions where T_A
= 25°C. At T_A = 85°C, P_D(MAX) = 333mW. At T_A =
25°C, P_D(MAX) = 833mW.
Maximum current is given by the following equation:
I_OUT(MAX) ⁼
P_D(MAX)
R_DS(ON)
For the AAT4651 at 85°C, I_OUT(MAX) = 1.65A, a
value greater than the internal minimum current
limit specification.
Over-Current and Over-Temperature
Protection
Because many AAT4651 applications provide power
to external devices, it is designed to protect its host
device from malfunctions in those peripherals
through slew rate control, current limiting, and ther-
mal limiting. The AAT4651 current limit and thermal
limit serve as an immediate and reliable electronic
fuse without any increase in R_DS(ON) for this function.
Other solutions, such as a poly fuse, do not protect
the host power supply and system from mishandling
or short circuiting peripherals; they will only prevent
a fire. The AAT4651 high-speed current limit and
thermal limit not only prevent fires, they also isolate
the power supply and entire system from any activi-
ty at the external port and report a mishap by means
of a FAULT signal.
Over-current and over-temperature go hand in hand.
Once an over-current condition exists, the current
supplied to the load by the AAT4651 is limited to the
over-current threshold. This results in a voltage drop
across the AAT4651 which causes excess power
4651.2006.05.1.2
AAT4651
5V/3V PC Card Power Switch
dissipation and a package temperature increase. As
the die begins to heat up, the over-temperature cir-
cuit is activated. If the temperature reaches the
maximum level, the AAT4651 automatically switches
off the P-channel MOSFETs. While they are off, the
over-temperature circuit remains active. Once the
temperature has cooled by approximately 10°C, the
P-channel MOSFETs are switched back on. In this
manner, the AAT4651 is thermally cycled on and off
until the short circuit is removed. Once the short is
removed, normal operation automatically resumes.
To save power, the full high-speed over-current cir-
cuit is not activated until a lower threshold of cur-
rent (approximately 500mA) is exceeded in the
power device. When the load current exceeds this
crude threshold, the AAT4651 quiescent current
increases from 15µA to 200µA. The high-speed
over-current circuit works by linearly limiting the
current when the current limit is reached. As the
voltage begins to drop on V_CC due to current limit-
ing, the current limit magnitude varies and general-
ly decreases as the V_CC voltage drops to 0V.
Switching V_CC Voltage
The AAT4651 meets PC card standards for switch-
ing the V_CC output by providing a ground path for
V_CC as well as “off” state. The PC card protocol for
determining low voltage operations is to first power
the peripheral with 5V and poll for 3.3V operation.
When transitioning from 5V to 3.3V, V_CC must be
discharged to less than 0.8V to provide a hard
reset. The resistive ground state (VCCD0 =
VCCD1) will accommodate this. The ground state
will also guarantee the V_CC voltage to be discharged
within the specified 100ms amount of time.
Printed Circuit Board Layout
Recommendations
For proper thermal management, to minimize PCB
trace resistance, and to take advantage of the low
R_DS(ON) of the AAT4651, a few circuit board layout
rules should be followed: V_CC3, V_CC5, and V_CC
should be routed using wider than normal traces;
the two V_CC pins (Pins 6 and 7) should be con-
nected to the same wide PCB trace; and GND
should be connected to a ground plane. For best
performance, C_IN and C_OUT should be placed close
to the package pins.
9