RT6232A/B
Application Information
inductor ripple current (IL) to increase the average
inductor current (and the output current) while ensuring
that IL(PEAK) does not exceed the upper current limit
level.
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 (IL) about
20% to 50% of the desired full output load current.
Calculate the approximate inductor value by selecting
the input and output voltages, the switching frequency
(fSW), the maximum output current (IOUT(MAX)) and
estimating a IL as some percentage of that current.
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 1.2V
output at 2A and an input voltage of 12V, using an
inductor ripple of 1A (50%), the calculated inductance
value is :
1.2 121.2
12500kHz1A
L =
= 2.16μH
The ripple current was selected at 1A and, as long as
we use the calculated 2.16H inductance, that should
be the actual ripple current amount. The ripple current
and required peak current as below :
1.2 121.2
12500kHz2.16μH
I =
L
= 1A
VOUT V V
IN
OUT
L =
V fSW IL
IN
1A
2
and I
= 2A
= 2.5A
L(PEAK)
Once an inductor value is chosen, the ripple current (IL)
is calculated to determine the required peak inductor
current.
For the 2.16H value, the inductor's saturation and
thermal rating should exceed at least 2.5A. For more
conservative, the rating for inductor saturation current
must be equal to or greater than switch current limit of
the device rather than the inductor peak current.
VOUT VIN VOUT
and IL(PEAK) = IOUT(MAX)
IL
2
IL=
V fSW L
IN
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.
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 (IRMS) is a function of the input
voltage, output voltage, and load current :
IL(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
VOUT
V
IN
V
IN
IRMS = IOUT(MAX)
1
VOUT
Ceramic capacitors are most often used because of
their low cost, small size, high RMS current ratings, and
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is a registered trademark of Richtek Technology Corporation
DS6232A/B-02 December 2018
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13
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