RT7275/76
15
DS7275/76-04 April 2017 www.richtek.com
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Copyright 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Design Procedure
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, but
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 (ΔI
L
) about
20-50% of the desired full output load current. Calculate
the approximate inductor value by selecting the input and
output voltages, the switching frequency (f
SW
), the
maximum output current (I
OUT(MAX)
) and estimating a ΔI
L
as some percentage of that current.
OUT IN OUT
IN SW L
VVV
L =
Vf I
Once an inductor value is chosen, the ripple current (ΔI
L
)
is calculated to determine the required peak inductor
current.
OUT IN OUT
L
L L(PEAK) OUT(MAX)
IN SW
VVV
I
I = and I = I
Vf L 2
To guarantee the required output current, the inductor
needs a saturation current rating and a thermal rating that
exceeds I
L(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.
I
L(PEAK)
should not exceed the minimum value of IC's upper
current limit level or the IC may not be able to meet the
desired output current. If needed, reduce the inductor ripple
current (ΔI
L
) to increase the average inductor current (and
the output current) while ensuring that I
L(PEAK)
does not
exceed the upper current limit level.
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.
Considering the Typical Operating Circuit for 1.05V output
at 3A and an input voltage of 12V, using an inductor ripple
of 1A (33%), the calculated inductance value is :
1.05V 12V 1.05V
L = = 1.4μH
12V 700kHz 1A
The ripple current was selected at 1A and, as long as we
use the calculated 1.4μH inductance, that should be the
actual ripple current amount. Typically the exact calculated
inductance is not readily available and a nearby value is
chosen. In this case 1.4μH was available and actually used
in the typical circuit. To illustrate the next calculation,
assume that for some reason is was necessary to select
a 1.8μH inductor (for example). We would then calculate
the ripple current and required peak current as below :
L
1.05V 12V 1.05V
I = = 0.76A
12V 700kHz 1.8μH
L(PEAK)
0.76
and I = 3A = 3.38A
2
For the 1.8
μH value, the inductor's saturation and thermal
rating should exceed 3.38A. Since the actual value used
was 1.4μH and the ripple current exactly 1A, the required
peak current is 3.5A.
Input Capacitor Selection
The input filter capacitors are needed to smooth out the
switched current drawn from the input power source and
to reduce voltage ripple on the input. The actual
capacitance value is less important than the RMS current
rating (and voltage rating, of course). The RMS input ripple
current (I
RMS
) is a function of the input voltage, output
voltage, and load current :
OUT VIN OUT
RMS OUT
VIN
VVV
I = I
V
Ceramic capacitors are most often used because of their
low cost, small size, high RMS current ratings, and robust
surge current capabilities. However, take care when these
