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參數(shù)資料
| 型號： | 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)： | 10/15頁 |
| 文件大?。?/td> | 549K |
| 代理商： | A8450KLB |
10
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
Component Selection
Output Inductor (L1).
This inductor must be rated to handle
the total load current, I
LOAD
. In addition, the value chosen
must keep the ripple current to a reasonable level. A typical
selection is a power inductor rated at 100 μH and 1.3 A.
The worse case ripple current, I
RIPPLE(max)
(mA), can be
calculated as
I
RIPPLE(max)
=
V
L1OFF
×
t
OFF
L
L1
where L
L1
(μH) is the inductance for the selected component,
and V
L1OFF
is the voltage (V) through the inductor when the
A8450 is in the quiescent state
V
L1OFF
=
V
REG(max)
+
V
D1
+ (
I
LOAD
×
R
L1
)
where V
D1
(V) is the voltage drop on diode D1, I
LOAD
(mA)
is the total load current, and R
L1
is the specified dc resistence
(
) for the selected inductor at its rated temperature.
The frequency, f
PWM
(Hz), of the switching regulator in the
buck converter can then be estimated by
f
PWM
= 1/(
t
ON
+
t
OFF
)
where t
ON
(μs) is calculated as
t
ON
=
I
RIPPLE(max)
×
L
L1
V
L1ON
and V
L1ON
(V) as
V
L1ON
=
V
BB
– (
I
LOAD
×
R
DSON(max)
)
– (
I
LOAD
×
R
L1
) –
V
REG(max)
Example
Given a typical application with V
BB
= 14 V, t
OFF
= 4.75 μs,
and I
LOAD
= 550 mA. (Note that the value for t
OFF
is con-
stant for V
BB
> 12 V, as shown in figure 3.)
Given also a 100 μH power inductor rated at 400 m
for
125oC. (Note that temperature ratings for inductors may
include self-heating effects. If a 125oC rating includes a self-
heating temperature rise of 20oC at maximum current, then
the actual ambient temperature, T
A
, cannot exceed 105oC.)
V
L1OFF
= 5.8 + 0.8 + (0.550
×
0.400) = 6.821 V
I
RIPPLE(max)
= 6.821
×
4.75
100 = 0.324 A
V
L1ON
= 14 – (0.550
×
0.750)
–
(0.550
×
0.400)
– 5.8 = 7.56 V
t
ON
= 0.324
×
100
7.56 = 4.3 μs
f
PWM
= 1/(4.3 + 4.75) = 111 kHz
In the case of a shorted output, the buck converter could
reach its internal current limit, I
DSLIM
, of 1.2 A typical. To
ensure safe operation, the I
SAT
rating for the selected induc-
tor should be greater than 1.4 A. However, if the external
current limit resistors, R3 and R4, selected for the 3.3 V and
adjustable (1.2 V to 3.3 V) regulators, are rated such that the
total inductor current, I
LOAD
, could never reach that inter-
nal current limit, then an inductor can be selected that has
an I
SAT
rating closer to the calculated output current of the
device, I
LOAD
, plus the maximum ripple current, I
RIPPLE(max)
.
Higher inductor values can be chosen to lower I
RIPPLE
. This
may be an option if it is desired to increase the total maxi-
mum current that is drawn from the switching regulator. The
maximum total current available, I
LOAD
(mA), is calculated as
I
LOAD
=
I
DSLIM
– (
I
RIPPLE(max)
2)
Catch Diode (D1)
. The Schottky catch diode should be
rated to handle 1.2 times the maximum load current, I
LOAD
,
because the duty cycle at low input voltages, V
BB
, can be
very close to 100%. The voltage rating should be higher than
the maximum input voltage, V
BB(max)
, expected during any
operating condition.
VREG Output Capacitor (COUT)
. Voltage ripple in the
VREG output is the main consideration when selecting the
VREG output capacitor, COUT. The peak-to-peak output
voltage ripple, V
RIPPLE(p-p)
(mV), is calculated as
V
RIPPLE(p-p)
=
I
RIPPLE
×
ESR
COUT
with ESR in ohms. It is recommended that the maximum
level of V
RIPPLE(p-p)
be less than 200 mV.
Application Information
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