RT7285C
9
DS7285C-03 July 2014
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Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application information
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 (
Δ
I
L
) about
20% to 40% 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
Δ
I
L
as some percentage of that current.
OUT
IN
OUT
IN
SW
L
V
V
V
L =
V
f
I
Once an inductor value is chosen, the ripple current (
Δ
I
L
)
is calculated to determine the required peak inductor
current.
OUT
IN
OUT
L
IN
SW
L
L(PEAK)
OUT(MAX)
L
L(VALLY)
OUT(MAX)
V
V
V
I =
V
f
L
I
I
= I
2
I
I
= I
2
Considering the Typical Operating Circuit for 1.2V output
at 1.5A and an input voltage of 12V, using an inductor
ripple of 0.6A (40%), the calculated inductance value is :
1.2 12 1.2
L =
= 3.6
μ
H
12 500kHz 0.6
The ripple current was selected at 0.6A and, as long as
we use the calculated 3.6
μ
H inductance, that should be
the actual ripple current amount. The ripple current and
required peak current as below :
L
1.2 12 1.2
I =
= 0.6A
12 500kHz 3.6
μ
H
L(PEAK)
0.6
and I
= 1.5A
= 1.8A
2
Inductor saturation current should be chosen over IC's
current limit.
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
IN
RMS
OUT(MAX)
IN
OUT
V
V
I
= I
1
V
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
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
RT7285C
input which could
potentially cause large, damaging voltage spikes at VIN.
If this phenomenon is observed, some bulk input
capacitance may be required. Ceramic capacitors (to meet
the RMS current requirement) can be placed in parallel
with other types such as tantalum, electrolytic, or polymer
(to reduce ringing and overshoot).
Choose capacitors rated at higher temperatures than
required. Several ceramic capacitors may be paralleled to
meet the RMS current, size, and height requirements of
the application. The typical operating circuit use 10
μ
F and
one 0.1
μ
F low ESR ceramic capacitors on the input.
