RT6218A/B
input, caused by the long input power path and input
VOUT
V
IN
V
IN
VOUT
IRMS = IOUT
1
ceramic capacitor.
It is suggested to choose capacitors with higher
temperature ratings than required. Several ceramic
capacitors may be parallel to meet application
requirements, such as the RMS current, size, and
height. The Typical Application Circuit can use one
22F, or two 10F and one high-frequency-
noise-filtering 0.1uF low-ESR ceramic capacitors at the
input.
From the above, the maximum RMS input ripple
current occurs at maximum output load, which will be
used as the requirements to consider the current
capabilities of the input capacitors. Furthermore, for a
single phase buck converter, the duty cycle is
approximately the ratio of output voltage to input
voltage. The maximum ripple voltage usually occurs at
50% duty cycle, that is, VIN = 2 x VOUT. The maximum
IRMS, as IRMS (Max), can be approximated as 0.5 x
IOUT_MAX, where IOUT_MAX is the maximum rated
output current. Besides, the variation of the
capacitance value with temperature, DC bias voltage,
switching frequency, and allowable peal-to-peak ripple
voltage that reflects back to the input, also need to be
taken into consideration. For example, the capacitance
value of a capacitor decreases as the DC bias across
the capacitor increases; also, higher switching
frequency allows the use of input capacitors of smaller
capacitance values.
Output Capacitor Selection
Output capacitance affects the output voltage of the
converter, the response time of the output feedback
loop, and the requirements for output voltage sag and
soar. The sag occurs after a sudden load step current
applied, and the soar occurs after a sudden load
removal. Increasing the output capacitance reduces
the output voltage ripple and output sag and soar, while
it increases the response time that the output voltage
feedback loop takes to respond to step loads,
Therefore, there is
a
tradeoff between output
Ceramic capacitors are most commonly used to be
placed right at the input of the converter to reduce
ripple voltage amplitude because only ceramic
capacitors have extremely low ESR which is required
to reduce the ripple voltage. Note that the capacitors
need to be placed as close as to the input pins as
possible for highest effectiveness. Ceramic capacitors
are preferred also due to their low cost, small size, high
RMS current ratings, robust inrush surge current
capabilities, and low parasitic inductance, which helps
reduce the high-frequency ringing on the input supply.
capacitance and output response. It is recommended
to choose a minimum output capacitance to meet the
output voltage requirements of the converter, and have
a quick transient response to step loads.
The ESR of the output capacitor affects the damping of
the output filter and the transient response. In general,
low-ESR capacitors are good choices due to their
excellent capability in energy storage and transient
performance. The RT6218A/B, therefore, is specially
optimized for ceramic capacitors. Consider also DC
bias and aging effects while selecting the output
capacitor.
However, care must be taken when ceramic capacitors
are used at the input, and the input power is supplied
by a wall adapter, connected through a long and thin
wire. When a load step occurs at the output, a sudden
inrush current will surge through the long inductive wire,
which can induce ringing at the device’s power input
and potentially cause a very large voltage spike at the
VIN pin to damage the device. For applications where
the input power is located far from the device input, it
may be required that the low-ESR ceramic input
capacitors be placed in parallel with a bulk capacitor of
other types, such as tantalum, electrolytic, or polymer,
to dampen the voltage ringing and overshoot at the
Output Ripple
Output ripple at the switching frequency is caused by
the inductor current ripple and its effect on the output
capacitor's ESR and stored charge. These two ripple
components are called ESR ripple and capacitive ripple.
Since ceramic capacitors have extremely low ESR and
relatively little capacitance, both components are
similar in amplitude and both should be considered if
ripple is critical.
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6218A/B-01 October 2018
www.richtek.com
13
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