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參數(shù)資料
| 型號： | AAT4250 |
| 廠商： | Advanced Analogic Technologies, Inc. |
| 英文描述： | Slew Rate Controlled Load Switch |
| 中文描述： | 擺率控制的負(fù)荷開關(guān) |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 193K |
| 代理商： | AAT4250 |
Applications Information
Input Capacitor
Typically a 1μF or larger capacitor is recommend-
ed for C
IN
in most applications. A C
IN
capacitor is
not required for basic operation, however, it is use-
ful in preventing load transients from affecting up
stream circuits. C
IN
should be located as close to
the device V
IN
pin as practically possible. Ceramic,
tantalum or aluminum electrolytic capacitors may
be selected for C
IN
. There is no specific capacitor
ESR requirement for C
IN
. However, for higher cur-
rent operation, ceramic capacitors are recom-
mended for C
IN
due to their inherent capability over
tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries in portable devices.
Output Capacitor
For proper slew operation, a 0.1μF capacitor or
greater between VOUT and GND is required.
Likewise, with the output capacitor, there is no spe-
cific capacitor ESR requirement. If desired, C
OUT
maybe increased without limit to accommodate any
load transient condition without adversely affecting
the slew rate.
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
2.0 volts. The load switch will go into shutdown
mode when the voltage on the ON pin falls below
0.8 volts. When the load switch is in shutdown
mode, the OUT pin is tristated, and quiescent cur-
rent drops to leakage levels below 1μA.
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 main-
taining 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 and possibly damage
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 schot-
tky diode from V
IN
to V
OUT
(connecting the cathode
to V
IN
and anode to V
OUT
). The Schottky diode for-
ward voltage should be less then 0.45 volts.
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 need to be taken into account.
The following discussions will assume the load
switch is mounted on a printed circuit board utiliz-
ing the minimum recommended footprint as stated
in the layout considerations section.
At any given ambient temperature (T
A
) the maxi-
mum package power dissipation can be deter-
mined by the following equation:
P
D(MAX)
= [T
J(MAX)
- T
A
] /
Θ
JA
Constants for the AAT4250 are maximum junction
temperature, T
J(MAX)
= 125°C, and package thermal
resistance,
Θ
JA
= 150°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)
= 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)
= 175m
×
(1 + .002800
×
(125°C
-
25°C))
R
DS(MAX)
= 224m
AAT4250
Slew Rate Controlled Load Switch
8
4250.2001.12.0.94
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