Diodes AP65503 - Manual (Page 11)

AP65503
Document number: DS37127 Rev. 1 - 2
11 of 15
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April 2014
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AP65503
Application Information
(cont.)
Inductor (cont.)
Choose the inductor ripple current to be 30% of the maximum load current. The maximum inductor peak current is calculated from:
2
∆I
I I L
LOAD L(MAX)
+ =
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 100mΩ. 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 44µF 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 * ∆I Vout
inductor capacitor
=
An output capacitor with ample capacitance and low ESR is the best option. For most applications, a 72µF ceramic capacitor will be sufficient.
2
out
2
out
2 inductor
out
o
V ) V V (∆
)
2
∆I
L(I
C
− +
+
=
Where ∆V is the maximum output voltage overshoot.