AP6503
340kHz 23V 3A SYNCHRONOUS DC/DC BUCK CONVERTER
Applications Information
(cont.)
Compensation Components
(cont.)
2. Choose the compensation capacitor (C3) to achieve the
desired phase margin set the compensation zero, f
Z1
, to
below one forth of the crossover frequency to provide
sufficient phase margin. Determine the C3 value by the
following equation:
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
C3
>
2
π ×
R3
×
fc
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.
L1
(µH)
Where R3 is the compensation resistor value.
V
OUT
(V)
Cin/C1
(µF)
Cout/C2
(µF)
Rc/R3
(kΩ)
Cc/C3
(nF)
1.2
1.8
2.5
3.3
5
12
22
22
22
22
22
22
47
47
47
47
47
47
3.24
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
3.3
3.3
10
10
10
15
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.
Table 2—Recommended
Component Selection
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;
L
=
VOUT
⋅
(VIN
−
VOUT )
VIN
⋅
ΔI
L
⋅
fSW
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:
Where
ΔI
L
is the inductor ripple current.
And
f
SW
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:
ΔI
IL(MAX)
=
ILOAD
+
L
2
Vout capacitor
=
ΔI
inductor * ESR
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.
An output capacitor with ample capacitance and low ESR
is the best option. For most applications, a 22µF ceramic
capacitor will be sufficient.
ΔI
inductor 2
)
2
Co
=
(Δ V
+
Vout ) 2
−
Vout 2
L(Iout
+
Where
ΔV
is the maximum output voltage overshoot.
AP6503
Document number: DS35077 Rev. 1 - 2
9 of 13
September 2011
© Diodes Incorporated
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