參數(shù)資料
型號(hào)： A8450KLBTR-T
廠商： Allegro Microsystems Inc
文件頁(yè)數(shù)： 13/17頁(yè)
文件大小： 546K
描述： IC REG QD BUCK/LINEAR 24-SOIC
標(biāo)準(zhǔn)包裝： 3,000
拓?fù)洌?/td> 降壓（降壓）（1），線性（LDO）（4）
功能： 車載
輸出數(shù)： 4
電壓/電流 - 輸出 1： 5.7V，1.2A
電壓/電流 - 輸出 2： 5V，200mA
電壓/電流 - 輸出 3： 5V，200mA
帶 LED 驅(qū)動(dòng)器： 無
帶監(jiān)控器： 無
帶序列發(fā)生器： 是
電源電壓： 6 V ~ 45 V
工作溫度： -40°C ~ 135°C
安裝類型： 表面貼裝
封裝/外殼： 24-SOIC（0.295"，7.50mm 寬）
供應(yīng)商設(shè)備封裝： 24-SOIC W
包裝： 帶卷 (TR)
Automotive Multioutput Voltage Regulator
A8450
13
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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
窲A
 (癈/W). This
includes the significant effect of dissipation 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贑/W. The expected self-induced temperature rise in the pack-
age, T
J
 (癈), given P
D
= 0.814 W, is approximated as
T
J
 = P
D
 
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
R
窲A
= 26癈
In automotive applications, where under-the-hood ambient tem-
peratures can exceed 125贑, the pass transistor would have to be
rated to provide the required beta at e 151癈, plus a safe operat-
ing margin.
For a selected transistor, V
CE
 can change depending on current,
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 maxi-
mum 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
  e 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癈, and V
CE
 = 1V (as specified for the
MPSW06 npn transistor, beta = 300 at 125贑).
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)
 
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
R
CL
)
V
RDROP
 = (I
LOAD(lim)
 
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
R
RDROP
)
Assume a typical I
LOAD
= 350 mA. However, under normal oper-
ating 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 determine V
RCL
V
RCL
 = 0.400
 
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
0.44 = 0.176 V
We can now solve for R
RDROP
 and then V
DROP
V
REG
 V
RCL
 (I
LOAD
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
R
RDROP
)  V
CE
e V
OUT
  5.8  0.176  (0.4
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
R
RDROP
)  1
 
e 3.30 V
therefore
R
RDROP
e 3.31 ?/DIV>
and
V
RDROP
 = 0.4
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
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 dissipa-
tion in the transistor, P
D
 (W) is reduced to
P
D
 = I
LOAD(lim)
 
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
(V
REG
 V
RCL
 V
RDROP
 V
OUT
)
  = 0.400
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
(5.8  0.176  1.3  3.3) = 0.410 W
and
T
J
 = P
D
 
 
?SPAN class="pst A8450KLBTR-T_2464169_2"> 
R
窲A
= 13癈
The power dissipated in the transistor is significantly reduced. A
transistor in a power package with an R
窲A
 of 32贑/W at 400 mA
(a 50 mA margin) undergoes a temperature rise of 13贑 with the
drop resistors, as opposed to a similar transistor at 350 mA rising
26贑 without drop resistors. At high output currents, properly
selected drop resistors can protect the external pass transitor from
overheating.
A8450 Power Dissipation. The A8450 is designed to operate 
in applications with high ambient temperatures. The total power
dissipated in the device must be considered in conjunction with
the thermal dissipation capabilities of the PCB where the A8450
is mounted, as well as the capabilities of the device package
itself.
The ability of a package to dissipate heat is approximated by
the thermal resistance from the die (junction) to the ambient
environment, R
窲A
 (癈/W). This includes the significant effect
of dissipation through the package leads and the PCB on which
the package is mounted, and the temperature of the ambient air.
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