ACT81460
Rev 1.0, 18-Dec-2018
loads. Choose the input capacitor value to keep the in-
put voltage ripple less than 50mV.
1% of the output voltage. The following equation calcu-
lates the output voltage ripple as a function of output
capacitance when the IC in in PWM mode.
ꢅꢂꢃꢄ
ꢅꢆꢇ
ꢅꢂꢃꢄ
ꢅꢆꢇ
∗ ꢈ1 ꢉ
ꢊ
∆ꢁꢏ
Vripple ꢀ ꢁꢂꢃꢄ ∗
VRIPPLE
ꢀ
ꢋꢌꢍ ∗ ꢎꢆꢇ
8 ∗ ꢋ ∗ ꢎꢐꢑꢒ
ꢕꢖ
Where Vripple is the input voltage AC voltage ripple, Iout is
the output current, Vout is the output voltage, Vin is the
input voltage, Fsw is the switching frequency, and Cin is
the input capacitance after taking DC bias into account.
Where ΔIL is the inductor ripple current, FSW is the
switching frequency, and COUT is the output capacitance
after taking DC bias into account.
Be sure to consider the capacitor’s DC bias effects and
maximum ripple current rating when using capacitors
smaller than 0805.
Be sure to consider the capacitor’s DC bias effects and
maximum ripple current rating when using capacitors
smaller than 0805.
A capacitor’s actual capacitance is strongly affected by
its DC bias characteristics. The output capacitor is typi-
cally an X5R, X7R, or similar dielectric. Use of Y5U, Z5U,
or similar dielectrics are not recommended due to their
wide variation in capacitance over temperature and volt-
age ranges.
A capacitor’s actual capacitance is strongly affected by
its DC bias characteristics. The input capacitor is typi-
cally an X5R, X7R, or similar dielectric. Use of Y5U, Z5U,
or similar dielectrics is not recommended. Input capaci-
tor placement is critical for proper operation. Each
buck’s input capacitor must be placed as close to the IC
as possible. The traces from VINBx to the capacitor and
from the capacitor to PGNDx should as short and wide
as possible.
The output capacitance also affects output ripple in
PFM mode as well as the output voltage discharge time
when the converter is disabled.
Inductor Selection
The Buck converters utilize a Constant ON-Time and a
hysteretic mode hybrid topology and a proprietary inter-
nal compensation scheme to simultaneously simplify
external component selection and to optimize transient
performance over the entire operating range. The
ACT81460 is designed to operate with 1.0μH Choose
an inductor with a low DC-resistance, and avoid induc-
tor saturation by choosing inductors with DC ratings that
exceed the maximum output current by at least 30%.
The following equation calculates the inductor ripple
current.
ꢅ
ꢈ1 ꢉ ꢐꢑꢒꢊ ∗ ꢅꢐꢑꢒ
ꢅ
ꢓꢔ
∆ꢁꢏ
ꢀ
ꢋ
∗ ꢗ
ꢕꢖ
Where VOUT is the output voltage, VIN is the input voltage,
FSW is the switching frequency, and L is the inductor
value.
Output Capacitor Selection
The BUCK regulators were designed to take advantage
of the benefits of ceramic capacitors, namely small size
and very-low ESR capacitors. They are designed to op-
erate with 10µF to 44µF output capacitors over most of
their output voltage ranges. In order to ensure stability,
the Buck effective capacitance must be greater than
4.7uF. The output capacitance can be increased to re-
duce output voltage ripple and improve load transients
if needed. Design for an output ripple voltage less than
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