RT7255 Datasheet, PDF (11/17 Pages) Richtek Technology Corporation – The RT7255 is a synchronous step-down

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 (IL)
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 (fSW), the maximum output current
(IOUT(MAX)) and estimating a IL as some percentage of
that current.
L = ^{VOUT  VIN  VOUT }
V_IN  f_SW  I_L
Once an inductor value is chosen, the ripple current
(IL) is calculated to determine the required peak
inductor current.
I_L
=
V_OUT 
V_IN
 VIN  VOUT
 f_SW L

I_L(PEAK)
=
I_OUT(MAX)

I_L
2
I_L(VALLY)
=
I_OUT(MAX) 
I_L
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 = _{12500kHz 0.6} = 3.6μH
RT7255
The ripple current was selected at 0.6A and, as long as
we use the calculated 3.6H inductance, that should be
the actual ripple current amount. The ripple current and
required peak current as below :
1.2 12 1.2
I_L = _{12 500kHz 3.6μH} = 0.6A
and I_L(PEAK)
=
1.5A

0.6
2
= 1.8A
Inductor's 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 (IRMS) is a function of the input
voltage, output voltage, and load current :
I_RMS
=
I_OUT(MAX)

V_OUT
V_IN
V_IN
V_OUT
1
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 RT7255 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F and one 0.1F low ESR ceramic capacitors on
the input.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS7255-00 June 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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