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supposed to be the value of dc offset. This value may
influence the performance (e.g. regulation or peak value
of VOUT) of converter slightly, the suggested CFF is to select
a pair of pole and zero to provide the maximum phase
lead at switching frequency.
V
(V V
)
OUT
IN
OUT
L
V f
I
IN SW
L
Once an inductor value is chosen, the ripple current (ΔIL)
is calculated to determine the required peak inductor
current.
V
(V V
)
R1
R2
I
L
2
OUT
IN
OUT
VOUT,valley = 1+
VREF + Vdc,offset
I
L
and I
I
OUT(MAX)
L(PEAK)
V f
L
IN SW
VOUT,valley is the valley of output voltage, and Vdc,offset is
used for describing the additional dc offset on VOUT, the
value is related to the output voltage ripple and CFF.
To guarantee the required output current, the inductor
needs a saturation current rating and a thermal rating that
exceeds IL(PEAK). These are minimum requirements. To
maintain control of inductor current in overload and short-
circuit conditions, some applications may desire current
ratings up to the current limit value. However, the IC's
output under-voltage shutdown feature make this
unnecessary for most applications.
V
OUT
R1
R2
C
(opt.)
FF
FB
For best efficiency, choose an inductor with a low DC
resistance that meets the cost and size requirements.
For low inductor core losses some type of ferrite core is
usually best and a shielded core type, although possibly
larger or more expensive, will probably give fewer EMI
and other noise problems.
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GND
Figure 5. The Equivalent Circuit of Feedback Loop
Inductor Selection
Selecting an inductor involves specifying its inductance
and also its required peak current. The exact inductor value
is generally flexible and is ultimately chosen to obtain the
best mix of cost, physical size, and circuit efficiency.
Lower inductor values benefit from reduced size and cost
and they can improve the circuit's transient response.
However, they increase the inductor ripple current and
output voltage ripple and reduce the efficiency due to the
resulting higher peak currents. Conversely, higher inductor
values increase efficiency, but the inductor will either be
physically larger or have higher resistance since more
turns of wire are required and transient response will be
slower since more time is required to change current (up
or down) in the inductor. A good compromise between
size, efficiency, and transient response is to use a ripple
current (ΔIL) about 20-50% of the desired full output load
current. Calculate the approximate inductor value by
selecting the input and output voltages, the switching
Input Capacitor Selection
High quality ceramic input decoupling capacitor, such as
X5R or X7R, with values greater than 20μF are
recommended for the input capacitor. The X5R and X7R
ceramic capacitors are usually selected for power regulator
capacitors because the dielectric material has less
capacitance variation and more temperature stability.
Voltage rating and current rating are the key parameters
when selecting an input capacitor. Generally, selecting an
input capacitor with voltage rating 1.5 times greater than
the maximum input voltage is a conservatively safe design.
The input capacitor is used to supply the input RMS
current, which can be calculated using the following
equation :
2
VOUT
V
IN
VOUT
V
IN
IL
12
IRMS
(1
)IOUT2
frequency (fSW), the maximum output current (IOUT(MAX)
)
and estimating a ΔIL as some percentage of that current.
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6219-06 January 2018
www.richtek.com
13
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