Diodes AP6502 - Manual (Page 11)

AP6502
Document Number: DS35423 Rev. 9 - 2
11 of 15
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January 2013
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AP6502
Applications Information
(cont.)
Inductor (cont.)
Peak current determines the required saturation current rating, which influences the size of the inductor. Saturating the inductor decreases
the converter efficiency while increasing the temperatures of the inductor and the internal MOSFETs. Hence choosing an inductor with
appropriate saturation current rating is important.
A 1µH to 10µH inductor with a DC current rating of at least 25% percent higher than the maximum load current is recommended for most
applications.
For highest efficiency, the inductor’s DC resistance should be less than 200mΩ. Use a larger inductance for improved efficiency under light
load conditions.
Input Capacitor
The input capacitor reduces the surge current drawn from the input supply and the switching noise from the device. The input capacitor has
to sustain the ripple current produced during the on time on the upper MOSFET. It must hence have a low ESR to minimize the losses.
The RMS current rating of the input capacitor is a critical parameter that must be higher than the RMS input current. As a rule of thumb,
select an input capacitor which has RMs rating that is greater than half of the maximum load current.
Due to large dI/dt through the input capacitors, electrolytic or ceramics should be used. If a tantalum must be used, it must be surge
protected. Otherwise, capacitor failure could occur. For most applications, a 4.7µF ceramic capacitor is sufficient.
Output Capacitor
The output capacitor keeps the output voltage ripple small, ensures feedback loop stability and reduces the overshoot of the output voltage.
The output capacitor is a basic component for the fast response of the power supply. In fact, during load transient, for the first few
microseconds it supplies the current to the load. The converter recognizes the load transient and sets the duty cycle to maximum, but the
current slope is limited by the inductor value.
Maximum capacitance required can be calculated from the following equation:
ESR of the output capacitor dominates the output voltage ripple. The amount of ripple can be calculated from the equation below:
ESR * inductor ΔI capacitor Vout =
An output capacitor with ample capacitance and low ESR is the best option. For most applications, a 22µF ceramic capacitor will be
sufficient.
2
out
2
out
2 inductor
out
o
V ) V V (Δ
)
2
ΔI
L(I
C
− +
+
=
Where
ΔV is the maximum output voltage overshoot.