a8450 Allegro MicroSystems, Inc., a8450 Datasheet - Page 10

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

Component Selection
Output Inductor (L1).
the total load current, I
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
calculated as
where L
and V
A8450 is in the quiescent state
where V
is the total load current, and R
(Ω) for the selected inductor at its rated temperature.
The frequency, f
buck converter can then be estimated by
where t
and V
Example
Given a typical application with V
and I
stant for V
Given also a 100 μH power inductor rated at 400 mΩ 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
A8450KLB-DS, Rev. 2
LOAD
L1ON
L1OFF
ON
L1
D1
V
V
L1ON
BB
(μs) is calculated as
L1OFF
= 550 mA. (Note that the value for t
(μH) is the inductance for the selected component,
(V) as
(V) is the voltage drop on diode D1, I
is the voltage (V) through the inductor when the
I
> 12 V, as shown in figure 3.)
RIPPLE(max)
t
ON
= V
PWM
= V
= I
BB
f
REG(max)
PWM
RIPPLE(max)
(Hz), of the switching regulator in the
– (I
LOAD
This inductor must be rated to handle
LOAD
= V
= 1/(t
. In addition, the value chosen
L1OFF
+ V
L1
×
ON
RIPPLE(max)
×
D1
is the specified dc resistence
R
A
– (I
BB
+ t
DSON(max)
, cannot exceed 105ºC.)
×
L
+ (I
L1
OFF
LOAD
t
= 14 V, t
OFF
LOAD
⁄ V
)
L1ON
⁄ L
×
(mA), can be
)
×
Application Information
R
L1
OFF
OFF
L1
R
L1
) – V
LOAD
= 4.75 μs,
is con-
)
REG(max)
Automotive Multioutput Voltage Regulator
(mA)
In the case of a shorted output, the buck converter could
reach its internal current limit, I
ensure safe operation, the I
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
nal current limit, then an inductor can be selected that has
an I
device, I
Higher inductor values can be chosen to lower I
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
Catch Diode (D1). The Schottky catch diode should be
rated to handle 1.2 times the maximum load current, I
because the duty cycle at low input voltages, V
very close to 100%. The voltage rating should be higher than
the maximum input voltage, V
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
with ESR in ohms. It is recommended that the maximum
level of V
V
V
I
t
f
ON
SAT
RIPPLE(max)
L1ON
PWM
L1OFF
rating closer to the calculated output current of the
LOAD
RIPPLE(p-p)
= 0.324
= 14 – (0.550
= 1/(4.3 + 4.75) = 111 kHz
= 5.8 + 0.8 + (0.550
V
I
, plus the maximum ripple current, I
LOAD
RIPPLE(p-p)
= 6.821
RIPPLE(p-p)
×
= I
be less than 200 mV.
100 ⁄ 7.56 = 4.3 μs
LOAD
DSLIM
×
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
×
= I
4.75 ⁄ 100 = 0.324 A
0.750) – (0.550
SAT
(mV), is calculated as
, could never reach that inter-
RIPPLE
– (I
BB(max)
×
rating for the selected induc-
DSLIM
RIPPLE(max)
0.400) = 6.821 V
LOAD
×
, expected during any
, of 1.2 A typical. To
ESR
(mA), is calculated as
×
COUT
0.400)
⁄ 2)
A8450
– 5.8 = 7.56 V
BB
RIPPLE
, can be
RIPPLE(max)
. This
LOAD
10
,
.