RT6207A/B
Application information
Inductor Selection
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.
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 (ΔIL) 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
ΔIL as some percentage of that current.
Considering the Typical Operating Circuit for 1.2V output
at 5Aand an input voltage of 12V, using an inductor ripple
of 1A (20%), the calculated inductance value is :
1.2 121.2
12650kHz1A
L =
= 1.66μH
The ripple current was selected at 1A and, as long as we
use the calculated 1.8μH inductance, that should be the
actual ripple current amount. The ripple current and required
peak current as below :
1.2 121.2
12650kHz1.8μH
IL=
= 0.923A
0.923A
and IL(PEAK) = 5A
= 5.4615A
2
For the 1.8μH value, the inductor's saturation and thermal
rating should exceed at least 5.4615A. 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.
V
V V
IN OUT
OUT
L =
V f
I
L
IN SW
Once an inductor value is chosen, the ripple current (ΔIL)
is calculated to determine the required peak inductor
current.
Input Capacitor Selection
V
V V
IN OUT
and I
OUT
I
2
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 :
L
I =
L
= I
L(PEAK)
OUT(MAX)
V f
L
IN SW
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
V
V
IN
V
OUT
OUT
I
= I
1
RMS
OUT(MAX)
IN
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
capacitors are used at the input of circuits supplied by a
wall adapter or other supply connected through long, thin
wires. Current surges through the inductive wires can
induce ringing at the RT6207A/B input which could
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 inductor ripple
current (ΔIL) to increase the average inductor current (and
the output current) while ensuring that IL(PEAK) does not
exceed the upper current limit level.
Copyright 2015 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6207A/B-02 December 2015
www.richtek.com
15
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
