A8450KLB Allegro Microsystems Inc, A8450KLB Datasheet - Page 12

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

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: -

A8450
mended, with a minimum voltage rating of 10 V. However,
because ESR decreases with voltage, the most cost-effective
choice may be a capacitor with a higher voltage rating.
Regulator Output Capacitors (C3 and C4). The output
capacitors used with the 3.3 V regulator (C3) and the 1.2 V to 3.3
V adjustable regulator (C4), should be 1 μF or greater X7R (5%
tolerance) ceramic or equivalent capacitors, with a maximum
capacitance change of ±15% over a temperature range of –55ºC
to 125ºC.
The ESR of these capacitors does not affect the outputs of the
corresponding regulators. If a greater capacitance is used, the
regulators have improved ripple rejection at frequencies greater
than 100 kHz.
Pass Transistors (Q1 and Q2). The pass transistors used to
implement the 3.3 V regulator and the 1.2 V to 3.3 V adjustable
regulator must ensure the following:
• Stable operation. The cutoff frequency for the control loops of
• Adequate base drive. It is acceptable to use a lower level of
• Packaged for sufficient power dissipation. In order to ensure
the regulators is 100 kHz. Transistors must be selected that have
gain bandwidth product, f
such that
current gain, h
limit for I
A8450, I
such that
appropriate thermal handling, the design of the application must
take into consideration the thermal characteristics of the PCB
where the A8450 and pass transistors are mounted, the ambient
temperature, and the power dissipation characteristics of the
transistor packages. In general, the power dissipation, P
is estimated by
Note that h
temperatures decrease h
transistor.
For a typical application where V
and I
LOAD
BD(min)
LOAD
= 190 mA
FE
FE
is dependant on operating temperature. Lower
P
is limited by the minimum base current for the
, and the minimum h
D
, for lower total load currents, I
P
I
= (5.8 – 2.5)
D
LOAD
= (V
f
T
= I
FE
REG
⁄ h
T
, affecting the current capacity of the
(kHz), and beta, h
FE
BD(min)
– V
> 100 kHz
×
OUT
190 = 627 mW
REG
×
FE
)
h
×
FE(min)
= 5.8 V, V
of the pass transistor,
I
LOAD
FE
Automotive Multioutput Voltage Regulator
LOAD
(A), ratings
OUT
. The lower
= 2.5 V,
D
(mW),
Adjusting Pass Transistor Power Dissipation
Transistors are manufactured in a wide variety of package types,
and the thermal dissipation efficiencies of the packages can vary
greatly. In general, increasing thermal efficiency can also increase
cost substantially. Selecting the package to closely match operat-
ing conditions is important to optimizing application design and
cost.
Even when using a thermally-enhanced package, it remains dif-
ficult to provide high current to a load at high ambient operating
temperatures. Depending on the load requirements, using drop
resistors, as shown in figure 5, may be necessary to protect the
pass transistor from overheating.
The output current-limiting resistors, RCL (corresponding to R3
and R4), will drop between 175 mV and 225 mV at the highest
current output, I
voltage dropped, V
This can be substituted into the power dissipation formula
Given a typical application where V
V
814 mW.
Figure 5. Placement of drop resistors for thermal protection; example
shown is for the 3.3 V regulator.
OUT
= 3.3 V, and I
A8450
LOAD
V
DROP
DROP
LOAD
. Assuming no additional resistance, the
P
VREG
CL33
V33BD
V33
D
(mV), on each pass transistor is
= V
= 350 mA, then P
= V
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
REG
DROP
– V
×
REG
V
RCL
RCL
CE
I
LOAD
= 5.8 V, V
– V
V
DROP
D
OUT
is approximately
V
OUTV33
I
LOAD
RCL
= 0.175 V,
12