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DS8088A-01 August 2017www.richtek.com
RT8088A
©
Copyright 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
The basic RT8088A application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
followed by C
IN
and C
OUT.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current, ΔI
L
, increases with higher V
IN
and decreases with higher inductance, as shown in
equation below :
where f is the operating frequency and L is the inductance.
Having a lower ripple current reduces not only the ESR
losses in the output capacitors, but also the output voltage
ripple. Higher operating frequency combined with smaller
ripple current is necessary to achieve high efficiency. Thus,
a large inductor is required to attain this goal. The largest
ripple current occurs at the highest V
IN
. To guarantee that
the ripple current stays below the specified ΔI
L(MAX)
, the
inductor value should be chosen according to the following
equation :
The inductor's current rating (defined by a temperature
rise from 25°C ambient to 40°C) should be greater than
the maximum load current and its saturation current should
be greater than the short-circuit peak current limit. Refer
to Table 1 for the suggested inductor selection.
Table 1. Suggested Inductors for Typical
Application Circuit
Component
Supplier
Part Number
Dimensions
(mm)
CYNTEC
PIFE20161B-
R33MS-39
2.0 X 1.6 X 1.2
Input and Output Capacitor Selection
An input capacitor, C
IN
, is needed to filter out the
trapezoidal current at the source of the high-side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
RMS current is given by :
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT(MAX)
/2. This simple worst-case condition is commonly
used for design. Choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be paralleled to meet the size or height requirements of
the design. Ceramic capacitors have high ripple current,
high voltage rating and low ESR, which makes them ideal
for switching regulator applications. However, they can
also have a high voltage coefficient and audible
piezoelectric effects. The high Q of ceramic capacitors
with trace inductance can lead to significant ringing. When
a ceramic capacitor is used at the input and the power is
supplied by a wall adapter through long wires, a load step
at the output can induce ringing at the input, V
IN
. At best,
this ringing can couple to the output and be mistaken as
loop instability. At worst, a sudden inrush of current
through the long wires can potentially cause a voltage
spike at V
IN
large enough to damage the part. Thus, care
must be taken to select a suitable input capacitor.
The selection of C
OUT
is determined by the required ESR
to minimize output voltage ripple. Moreover, the amount
of bulk capacitance is also a key for C
OUT
selection to
ensure that the control loop is stable. Loop stability can
be checked by viewing the load transient response. The
output voltage ripple, ΔV
OUT
, is determined by :
OUT OUT
L
IN
VV
I x 1
f x L V
OUT OUT
L(MAX) IN(MAX)
VV
L x 1
f x I V
OUT
IN
RMS OUT(MAX)
IN OUT
V
V
II 1
VV
OUT L
OSC OUT
1
V I ESR
8f C
