A8450KLB Allegro Microsystems Inc, A8450KLB Datasheet - Page 13

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A8450KLB

Manufacturer Part Number
A8450KLB
Description
IC REG MULTI-OUT AUTO 24-SOIC
Manufacturer
Allegro Microsystems Inc
Type
Step-Down (Buck)r
Datasheet

Specifications of A8450KLB

Topology
Step-Down (Buck) (1), Linear (LDO) (4)
Function
Automotive
Number Of Outputs
4
Voltage/current - Output 1
5.7V, 1.2A
Voltage/current - Output 2
5V, 200mA
Voltage/current - Output 3
5V, 200mA
W/led Driver
No
W/supervisor
No
W/sequencer
Yes
Voltage - Supply
6 V ~ 45 V
Operating Temperature
-40°C ~ 135°C
Mounting Type
Surface Mount
Package / Case
24-SOIC (7.5mm Width)
Current - Output
200mA, 550mA
Voltage - Output
3.3V, 5V, 5.7V, Adj
Voltage - Input
6 ~ 45 V
Internal Switch(s)
Yes
Synchronous Rectifier
No
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Power - Output
-
Frequency - Switching
-

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Part Number
Manufacturer
Quantity
Price
Part Number:
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Manufacturer:
ALLEGRO/雅丽高
Quantity:
20 000
Part Number:
A8450KLBTR-T
Manufacturer:
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A8450
P
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
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˚C/W. The expected self-induced temperature rise in the pack-
age, ∆T
In automotive applications, where under-the-hood ambient tem-
peratures can exceed 125˚C, the pass transistor would have to be
rated to provide the required beta at ≥ 151°C, plus a safe operat-
ing margin.
For a selected transistor, V
temperature, and transistor beta. Typically, transistors are rated
at a minimum beta at a defined V
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
The level of V
also the voltage available for V
mum and minimum voltage drops is useful in determining the
values of the drop resistors.
The required drop resistor value, R
terms of the voltage drops across each component of the circuit,
as shown in the following formula
where
Assume that V
also that T
MPSW06 npn transistor, beta = 300 at 125˚C).
In order to determine the resistance values for the current-limiting
and drop resistors, V
I
LOAD(lim)
D
can be used to estimate the minimum required operating
J
(°C), given P
A
= 125°C, and V
REG(min)
V
REG(max)
DROP
V
V
RDROP
RCL
RCL
∆T
= V
D
is 5.6 V, at which level P
= (I
= 5.8 V and V
= 0.814 W, is approximated as
J
REG
and V
= (I
= P
V
CE
LOAD(lim)
DROP
CE
– V
D
LOAD(lim)
can change depending on current,
DROP
×
= 1V (as specified for the
CE
RCL
R
CE
≥ V
is reduced. Calculating maxi-
θJA
RDROP
. However, V
×
– V
can be expressed in terms of
OUT
OUT(max)
= 26°C
×
R
RDROP
CL
R
, can be determined in
RDROP
)
– V
= 3.3 V. Assume
D
θJA
)
CE
Automotive Multioutput Voltage Regulator
is reduced, but
CE
should be
(°C/W). This
REG
.
Assume a typical I
ating conditions, the current limit set by RCL would be higher
than the expected normal current, so assume I
and R
We can now solve for R
therefore
and
Using four 0.25 W resistors valued at 14.7 Ω in parallel will drop
1.3 volts.
Using the drop resistors as calculated above, the power dissipa-
tion in the transistor, P
and
The power dissipated in the transistor is significantly reduced. A
transistor in a power package with an R
(a 50 mA margin) undergoes a temperature rise of 13˚C with the
drop resistors, as opposed to a similar transistor at 350 mA rising
26˚C 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
of dissipation through the package leads and the PCB on which
the package is mounted, and the temperature of the ambient air.
CL
P
= 44 Ω. Substituting to determine V
D
V
REG
= 0.400
= I
LOAD(lim)
– V
θJA
5.8 – 0.176 – (0.4
V
V
LOAD
RCL
RCL
×
(°C/W). This includes the significant effect
RDROP
∆T
(5.8 – 0.176 – 1.3 – 3.3) = 0.410 W
– (I
D
= 0.400
×
J
RDROP
R
= 350 mA. However, under normal oper-
(W) is reduced to
= P
RDROP
(V
LOAD
= 0.4
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
REG
D
×
and then V
×
×
×
– V
≥ 3.31 Ω
R
0.44 = 0.176 V
R
×
θJA
3.31 = 1.3 V
RDROP
RCL
R
= 13°C
RDROP
θJA
– V
DROP
) – V
of 32˚C/W at 400 mA
RDROP
) – 1 ≥ 3.30 V
RCL
LOAD(lim)
CE
≥ V
– V
OUT
OUT
= 0.400 A
)
13

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