AAT4250
Slew Rate Controlled Load Switch
Enable Function
The AAT4250 features an enable / disable function.
This pin (ON) is active high and is compatible with
TTL or CMOS logic. To assure the load switch will
turn on, the ON control level must be greater than
2.0V. The load switch will go into shutdown mode
when the voltage on the ON pin falls below 0.8V.
When the load switch is in shutdown mode, the
OUT pin is tri-stated, and quiescent current drops
to leakage levels below 1µA.
At any given ambient temperature (T
A
), the maxi-
mum package power dissipation can be deter-
mined by the following equation:
T
J(MAX)
- T
A
θ
JA
P
D(MAX)
=
Reverse Output-to-Input Voltage
Conditions and Protection
Under normal operating conditions, a parasitic diode
exists between the output and input of the load
switch. The input voltage should always remain
greater than the output load voltage, maintaining a
reverse bias on the internal parasitic diode.
Conditions where V
OUT
might exceed V
IN
should be
avoided since this would forward bias the internal
parasitic diode and allow excessive current flow into
the V
OUT
pin, possibly damaging the load switch.
In applications where there is a possibility of V
OUT
exceeding V
IN
for brief periods of time during nor-
mal operation, the use of a larger value C
IN
capaci-
tor is highly recommended. A larger value of C
IN
with respect to C
OUT
will effect a slower C
IN
decay
rate during shutdown, thus preventing V
OUT
from
exceeding V
IN
. In applications where there is a
greater danger of V
OUT
exceeding V
IN
for extended
periods of time, it is recommended to place a
Schottky diode from V
IN
to V
OUT
(connecting the
cathode to V
IN
and anode to V
OUT
). The Schottky
diode forward voltage should be less than 0.45V.
Constants for the AAT4250 are maximum junction
temperature (T
J(MAX)
= 125°C) and package ther-
mal resistance (Θ
JA
= 150°C/W). Worst case con-
ditions are calculated at the maximum operating
temperature, 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)
= 267mW. At T
A
=
25°C, P
D(MAX)
= 667mW.
The maximum continuous output current for the
AAT4250 is a function of the package power dissi-
pation and the R
DS
of the MOSFET at T
J(MAX)
. The
maximum R
DS
of the MOSFET at T
J(MAX)
is calcu-
lated by increasing the maximum room tempera-
ture R
DS
by the R
DS
temperature coefficient. The
temperature coefficient (TC) is 2800ppm/°C.
Therefore, at 125°C:
R
DS(MAX)
= R
DS(25°C)
×
(1 + TC
× ∆T)
R
DS(MAX)
= 175mΩ
×
(1 + 0.002800
×
(125°C - 25°C))
R
DS(MAX)
= 224mΩ
For maximum current, refer to the following equation:
1
2
⎛
P
D(MAX)
⎞
I
OUT(MAX)
<
⎝
R
⎠
DS
Thermal Considerations and High
Output Current Applications
The AAT4250 is designed to deliver a continuous
output load current. The limiting characteristic for
maximum safe operating output load current is
package power dissipation. In order to obtain high
operating currents, careful device layout and circuit
operating conditions must be taken into account.
The following discussions will assume the load
switch is mounted on a printed circuit board utilizing
the minimum recommended footprint as stated in
the Printed Circuit Board Layout Recommendations
section of this datasheet.
4250.2006.03.1.3
For example, if V
IN
= 5V, R
DS(MAX)
= 224mΩ, and T
A
= 25°C, I
OUT(MAX)
= 1.7A. If the output load current
were to exceed 1.7A or if the ambient temperature
were to increase, the internal die temperature
would increase and the device would be damaged.
Higher peak currents can be obtained with the
AAT4250. To accomplish this, the device thermal
resistance must be reduced by increasing the heat
sink area or by operating the load switch in a duty-
cycle manner. Duty cycles with peaks less than
2ms in duration can be considered using the
method below.
9
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
