RT5735
Application Information
The basic RT5735 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
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT(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, VIN. 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
VIN large enough to damage the part. Thus, care must
be taken to select a suitable input capacitor.
frequency followed by CIN and COUT
.
Inductor Selection
The inductor value and operating frequency determine
the ripple current according to a specific input and
output voltage. The ripple current, IL, increases with
higher VIN and decreases with higher inductance, as
shown in equation below :
V
V
OUT
OUT
I
L
1
f L
V
IN
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 VIN. A reasonable starting point for
selecting the ripple current is IL = 0.3 × IMAX to
0.4×IMAX. To guarantee that the ripple current stays
below a specified maximum, the inductor value should
be chosen according to the following equation :
The selection of COUT is determined by the required
ESR to minimize output voltage ripple. Moreover, the
amount of bulk capacitance is also a key for COUT
selection to ensure that the control loop is stable. Loop
stability can be checked by viewing the load transient
response. The output voltage ripple, VOUT, is
determined by :
VOUT
VOUT
L
1
1
f IL(MAX)
V
V
I ESR
IN(MAX)
OUT
L
8f
C
OSC OUT
The inductor's current rating (defined by a temperature
rise from 25C 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.
where fOSC is the switching frequency and IL is the
inductor ripple current. The output voltage ripple will be
the highest at the maximum input voltage since ΔIL
increases with input voltage. Multiple capacitors placed
in parallel may be needed to meet the ESR and RMS
current handling requirement. Ceramic capacitors have
excellent low ESR characteristics, but can have a high
voltage coefficient and audible piezoelectric effects. The
high Q of ceramic capacitors with trace inductance can
also lead to significant ringing. Nevertheless, high value,
low cost ceramic capacitors are now becoming
available in smaller case sizes. Their high ripple current,
high voltage rating and low ESR make them ideal for
switching regulator applications.
Input and Output Capacitor Selection
An input capacitor, CIN, 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 :
VOUT
V
IN
IRMS IOUT(MAX)
1
V
IN
VOUT
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is a registered trademark of Richtek Technology Corporation
DS5735-00 August 2014
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11
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