a8450 Allegro MicroSystems, Inc., a8450 Datasheet - Page 11

• Manufacturer Part Number: a8450
• Description: Automotive Multioutput Voltage Regulator
• Manufacturer: Allegro MicroSystems, Inc.

For electrolytic output capacitors, a low-ESR type is recom-
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
• Adequate base drive. It is acceptable to use a lower level
• Packaged for sufficient power dissipation. In order to
A8450KLB-DS, Rev. 2
loops of the regulators is 100 kHz. Transistors must be
selected that have gain bandwidth product, f
beta, h
of current gain, h
The lower limit for I
current for the A8450, I
the pass transistor, such that
Note that h
Lower temperatures decrease h
capacity of the transistor.
ensure appropriate thermal handling, the design of the ap-
plication must take into consideration the thermal charac-
teristics 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
FE
(A), ratings such that
FE
is dependant on operating temperature.
P
I
D
LOAD
FE
= (V
f
, for lower total load currents, I
T
LOAD
= I
REG
⁄ h
BD(min)
FE
BD(min)
– V
is limited by the minimum base
> 100 kHz
OUT
, and the minimum h
FE
×
D
)
, affecting the current
h
(mW), is estimated by
×
FE(min)
I
LOAD
T
(kHz), and
LOAD
FE
Automotive Multioutput Voltage Regulator
of
.
Adjusting Pass Transistor Power Dissipation
Transistors are manufactured in a wide variety of pack-
age 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 operating conditions is important to
optimizing application design and cost.
Even when using a thermally-enhanced package, it remains
difficult 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
resistance, the voltage dropped, V
Figure 5. Placement of drop resistors for thermal protection; example
shown is for the 3.3 V regulator.
For a typical application where V
2.5 V, and I
A8450
LOAD
P
D
= (5.8 – 2.5)
= 190 mA
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
VREG
CL33
V33BD
V33
LOAD
×
190 = 627 mW
. Assuming no additional
DROP
REG
V
RCL
CE
(mV), on each pass
= 5.8 V, V
V
DROP
A8450
V
OUTV33
I
LOAD
OUT
=
11