A8450
Automotive Multioutput Voltage Regulator
Application Information
Component Selection
VL1OFF = 5.8 + 0.8 + (0.550 0.400) = 6.821 V
×
Output Inductor (L1). This inductor must be rated to handle
the total load current, ILOAD. 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.
IRIPPLE(max) = 6.821 4.75 ⁄ 100 = 0.324 A
×
VL1ON =14 – (0.550 0.750) – (0.550 0.400)
×
×
– 5.8 = 7.56 V
tON
= 0.324 100 ⁄ 7.56 = 4.3 µs
×
The worse case ripple current, IRIPPLE(max) (mA), can be
calculated as
fPWM = 1/(4.3 + 4.75) = 111 kHz
IRIPPLE(max) = VL1OFF
t
⁄ LL1
OFF
×
In the case of a shorted output, the buck converter could
reach its internal current limit, IDSLIM, of 1.2 A typical. To
ensure safe operation, the ISAT 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, ILOAD, could never reach that inter-
nal current limit, then an inductor can be selected that has
an ISAT rating closer to the calculated output current of the
device, ILOAD, plus the maximum ripple current, IRIPPLE(max)
where LL1 (µH) is the inductance for the selected component,
and VL1OFF is the voltage (V) through the inductor when the
A8450 is in the quiescent state
VL1OFF = VREG(max) + VD1 + (ILOAD R )
×
L1
where VD1 (V) is the voltage drop on diode D1, ILOAD (mA)
is the total load current, and RL1 is the specified dc resistence
(Ω) for the selected inductor at its rated temperature.
.
The frequency, fPWM (Hz), of the switching regulator in the
buck converter can then be estimated by
Higher inductor values can be chosen to lower IRIPPLE. This
may be an option if it is desired to increase the total maxi-
mum current that is drawn from the switching regulator. The
fPWM = 1/(tON + tOFF
where tON (µs) is calculated as
tON = IRIPPLE(max)
)
maximum total current available, ILOAD (mA), is calculated as
ILOAD = IDSLIM – (IRIPPLE(max) ⁄ 2)
L
⁄ VL1ON
×
L1
Catch Diode (D1). The Schottky catch diode should be
and VL1ON (V) as
VL1ON =VBB –(ILOAD
rated to handle 1.2 times the maximum load current, ILOAD
because the duty cycle at low input voltages, VBB, can be
,
R
)
×
DSON(max)
very close to 100%. The voltage rating should be higher than
the maximum input voltage, VBB(max), expected during any
operating condition.
– (ILOAD R )–V
×
L1
REG(max)
Example
Given a typical application with VBB = 14 V, tOFF = 4.75 µs,
and ILOAD = 550 mA. (Note that the value for tOFF is con-
stant for VBB > 12 V, as shown in figure 3.)
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, VRIPPLE(p-p) (mV), is calculated as
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, TA, cannot exceed 105ºC.)
VRIPPLE(p-p) = IRIPPLE ESR
×
COUT
with ESR in ohms. It is recommended that the maximum
level of VRIPPLE(p-p) be less than 200 mV.
Allegro MicroSystems, Inc.
10
115 Northeast Cutoff, Box 15036
A8450KLB-DS, Rev. 1
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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