參數(shù)資料
型號(hào)： A8450KLB
廠商： Allegro MicroSystems, Inc.
英文描述： Automotive Multioutput Voltage Regulator
中文描述： 汽車Multioutput穩(wěn)壓器
文件頁數(shù)： 12/15頁
文件大小： 549K
代理商： A8450KLB
12
A8450KLB-DS, Rev. 1
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
A8450
Automotive Multioutput Voltage Regulator
transistor is
V
DROP
=
V
REG
– V
RCL
–
V
OUT
This can be substituted into the power dissipation formula
P
D
=
V
DROP
×
I
LOAD
Given a typical application where V
REG
= 5.8 V, V
RCL
=
0.175 V, V
OUT
= 3.3 V, and I
LOAD
= 350 mA, then P
D
is
approximately 814 mW.
P
D
can be used to estimate the minimum required operating
temperature rating for the transistor. The ability of a package
to dissipate heat is approximated by the thermal resistance
from the die (junction) to the ambient environment, R
θ
JA
(°C/W). This includes the significant effect of dissipa-
tion through the package leads and the PCB on which the
transistor is mounted, and the state of the ambient air. The
typical rating for a DPAK package is 32
C/W. The expected
self-induced temperature rise in the package,
T
J
(°C), given
P
D
= 0.814 W, is approximated as
T
J
=
P
D
×
R
θ
JA
= 26°C
In automotive applications, where under-the-hood ambient
temperatures can exceed 125
C, the pass transistor would
have to be rated to provide the required beta at
≥
151°C, plus
a safe operating margin.
For a selected transistor, V
CE
can change depending on cur-
rent, temperature, and transistor beta. Typically, transistors
are rated at a minimum beta at a defined V
CE
. However, V
CE
should be calculated with some margin so there is always
enough headroom to drive the device at the desired load.
To provide an operating margin, or if a lower-value RCL is
required, voltage drop resistors, RDROP, can be added to
the circuit, between the RCL and the transistor (figure 5). It
is also important to consider tolerances in resistance values
and V
REG
. The level of V
REG(min)
is 5.6 V, at which level P
D
is reduced, but also the voltage available for V
CE
is reduced.
Calculating maximum and minimum voltage drops is useful
in determining the values of the drop resistors.
The required drop resistor value, R
RDROP
, can be determined
in terms of the voltage drops across each component of the
circuit, as shown in the following formula
V
DROP
≥
V
OUT
where
V
DROP
=
V
REG
– V
RCL
– V
RDROP
– V
CE
Assume that V
REG(max)
= 5.8 V and V
OUT(max)
= 3.3 V.
Assume also that T
A
= 125°C, and V
CE
= 1V (as specified for
the MPSW06 npn transistor, beta = 300 at 125
C).
In order to determine the resistance values for the current-
limiting and drop resistors, V
RCL
and V
DROP
can be expressed
in terms of I
LOAD(lim)
V
RCL
= (I
LOAD(lim)
×
R
CL
)
V
RDROP
= (I
LOAD(lim)
×
R
RDROP
)
Assume a typical I
LOAD
= 350 mA. However, under normal
operating conditions, the current limit set by RCL would
be higher than the expected normal current, so assume
I
LOAD(lim)
= 0.400 A and R
CL
= 44
. Substituting to deter-
mine V
RCL
V
RCL
= 0.400
×
0.44 = 0.176 V
We can now solve for R
RDROP
and then V
DROP
V
REG
– V
RCL
– (
I
LOAD
×
R
RDROP
) – V
CE
≥
V
OUT
5.8 – 0.176 – (0.4
×
R
RDROP
) – 1
≥
3.30 V
therefore
R
RDROP
≥
3.31
and
V
RDROP
= 0.4
×
3.31 = 1.3 V
Using four 0.25 W resistors valued at 14.7
in parallel will
drop 1.3 volts.
Using the drop resistors as calculated above, the power dis-
sipation in the transistor, P
D
(W) is reduced to
P
D
= I
LOAD(lim)
×
(V
REG
– V
RCL
– V
RDROP
– V
OUT
)
= 0.400
×
(5.8 – 0.176 – 1.3 – 3.3) = 0.410 W
and
T
J
=
P
D
×
R
θ
JA
= 13°C
The power dissipated in the transistor is significantly
reduced. A transistor in a power package with an R
θ
JA
of
32
C/W at 400 mA (a 50 mA margin) undergoes a tempera-
ture rise of 13
C with the drop resistors, as opposed to a
similar transistor at 350 mA rising 26
C without drop resis-
tors. At high output currents, properly selected drop resistors
can protect the external pass transitor from overheating.
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