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2014.02 - Rev. C
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VIN
CO D
L
RL
Q1
VOUT
IO
ON
OFF
CIN
ICIN
IIN IDD
IL
ID
ICO
tON
tOFF
tON
tOFF
tON
tOFF
t
t
t
t
t
t
Q1
IDD
IO
IO
ID
IIN
IL
IO
Δ
IL
ICO
ICIN
Current which flows at tOFFCurrent which flows at tON
Switching Regulator IC series
Capacitor Calculation for Buck converter IC
This application note explains the calculation of external capacitor value for buck converter IC circuit.
●Buck converter
Figure 1 is the basic circuit of buck
converter. When switching element Q
1 is
ON, current flows from VIN through the
coil L and charges the output smoothing
capacitor C
o, and the output current Io is
supplied. The current which flows into
the coil L at this time induces a magnetic
field, and electric energy is transformed
into magnetic energy and accumulated
for storage.
When switching element Q
1 is OFF,
free-wheeling diode D turns ON and
energy stored in L is then released to the
output side.
●Calculation of Input capacitor
Rated voltage of input capacitor must be higher than the
maximum input voltage.
Also rated ripple-current of the capacitor must be higher than
the maximum input ripple-current of the IC.
Although the average value of an input current becomes
smaller in proportion to the transformation ratio, momentarily
the same current equal to output current flows through the buck
converter as shown as I
DD in Figure 2.
This will be averaged by the input capacitor, but as it is clearly
shown as ICIN of Figure 2, the alternating ripple-current flowing in
the input capacitor, is higher than ICO of the output.
Effective value of I
CIN can be calculated by following equation:
1
∆
[ARMS] (1)
Figure 3 shows the ripple heat generation characteristics of a
ceramic capacitor (by Murata Manufacturing Co.). Whether it
can be used as input capacitor or not is decided by this graph
and the absolute maximum rating of ripple-current.
Be well aware of the temperature and DC bias impressed to
the capacitor when using ceramic capacitor.
No.14027ECY02
Figure 1 Basic buck converter circuit
Figure 2 Current waveform of each part
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