Ap6503, Application information – Diodes AP6503 User Manual
Page 11
AP6503
Document number: DS35077 Rev. 5 - 2
11 of 15
January 2013
© Diodes Incorporated
AP6503
Application Information
(cont.)
Inductor
Calculating the inductor value is a critical factor in designing a buck converter. For most designs, the following equation can be used to calculate
the inductor value;
SW
f
L
ΔI
IN
V
)
OUT
V
IN
(V
OUT
V
L
⋅
⋅
−
⋅
=
Where
L
ΔI
is the inductor ripple current.
And
SW
f
is the buck converter switching frequency.
Choose the inductor ripple current to be 30% of the maximum load current. The maximum inductor peak current is calculated from:
2
L
ΔI
LOAD
I
L(MAX)
I
+
=
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 an 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
V
2
)
out
V
V
(Δ
2
)
2
inductor
ΔI
out
L(I
o
C
−
+
+
=
Where
ΔV
is the maximum output voltage overshoot.