BD906xxEFJ-C
(2) Set of output Capacitor CO constant
The output capacitor is selected on the basis of ESR that is required from the expression (a). ΔVPP can be reduced by
using a capacitor with a small ESR. The ceramic capacitor is the best option that meets this requirement. The ceramic
capacitor contributes to the size reduction of the set because it has small ESR. Please confirm frequency characteristic
of ESR from the datasheet of the manufacturer, and consider ESR value is low in the switching frequency being used. It
is necessary to consider the ceramic capacitor because the DC biasing characteristic is remarkable. For the voltage
rating of the ceramic capacitor, twice or more than the maximum output voltage is usually required. By selecting these
high voltages rating, it is possible to reduce the influence of DC bias characteristics. Moreover, in order to maintain good
temperature characteristics, the one with the characteristic of X7R and X5R or more is recommended. Because the
voltage rating of a mass ceramic capacitor is low, the selection becomes difficult in the application with high output
voltage. In that case, please select electrolytic capacitor. Please consider having a voltage rating of 1.2 times or more of
the output voltage when using electrolytic capacitor. Electrolytic capacitors have a high voltage rating, large capacity,
small amount of DC biasing characteristic, and are generally cheap. Because main failure mode is OPEN, it is effective
to use electrolytic capacitor for applications when reliability is required such as in-vehicle. But there are disadvantages
such as, ESR is relatively high, and decreases capacitance value at low temperatures. In this case, please take note
that ΔVPP may increase at low temperature conditions. Moreover, consider the lifetime characteristic of this capacitor
because there is a possibility for it to dry up.
These capacitors are rated in ripple current. The RMS values of the ripple electric current obtained in the next
expression must not exceed the ratings ripple electric current.
∆ꢀ
퐿
ꢆ퐶푂(ꢓꢃꢋ)
=
[A]
12
√
Where:
ICO (RMS) is the value of the ripple electric current
In addition, with respect to CO, choose capacitance value less than the value obtained by the following equation.
×(ꢀ −ꢀ
푇
)
)
ꢈ푆ꢔ퐴ꢕꢔ 푀퐴푋
푆푆(푀퐼푁)
ꢈ퐿퐼푀퐼ꢔ(푀퐼푁)
(
ꢊ푂(ꢃꢄꢅ)
=
[F]
푉
ꢈ
Where:
SWLIMIT (MIN) is the OCP operation switch current (Min)
SS (MIN) is the Soft Start Time (Min)
ISWSTART (MAX) is the maximum output current of boot
I
T
There is a possibility that boot failure happens when the limits from the above-mentioned are exceeded. Especially if the
capacitance value is extremely large, over-current protection may be activated by the inrush current at startup, and the
output does not start. Please confirm this on the actual circuit. For stable transient response, the loop is dependent on
the CO. Please select after confirming the setting of the phase compensation circuit.
(3) Setting constant of capacitor CIN / Cbulk input
The input capacitor is usually required for two types of decoupling: capacitors CIN and bulk capacitors Cbulk. Ceramic
capacitors with values 1 µF to 10 µF are necessary for the decoupling capacitor. Ceramic capacitors are effective by being
placed as close as possible to the VIN pin. Voltage rating is recommended to more than 1.2 times the maximum input
voltage, or twice the normal input voltage. The bulk capacitor prevents the decrease in the line voltage and serves a
backup power supply to keep the input potential constant. The low ESR electrolytic capacitor with large capacity is suitable
for the bulk capacitor. It is necessary to select the best capacitance value as per set of application. When impedance on
the input side is high because of wiring from the power supply to VIN is long, etc., and then high capacitance is needed. In
actual conditions, it is necessary to verify that there is no problem when IC operation turns off the output due to the
decrease in VIN at transient response. In that case, please consider not to exceed the rated ripple current of the capacitor.
The RMS value of the input ripple electric current is obtained in the next expression.
푉 ×(푉 −푉 )
ꢖ
ꢈ
퐼푁
ꢈ
ꢆ퐶ꢀꢂ(ꢓꢃꢋ) = ꢆ푂(ꢃꢄꢅ)
∙
[A]
푉
퐼푁
Where:
CIN (RMS) is the RMS value of the input ripple electric current
I
In addition, in automotive and other applications requiring high reliability, it is recommended that capacitors are connected
in parallel to accommodate a multiple of electrolytic capacitors to minimize the chances of drying up. It is recommended by
making it into two series + two parallel structures to decrease the risk of ceramic capacitor destruction due to short circuit
conditions. The line has been improved to the summary respectively by 1pack in each capacitor manufacturer and
confirms two series and two parallel structures to each manufacturer.
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