RT6253A/B
capacitor should be 0402 or 0603 in size.
Output Capacitor Selection
V
CIN
C
IN
Ripple Voltage
The RT6253A/B are optimized for ceramic output
capacitors and best performance will be obtained using
them. The total output capacitance value is usually
determined by the desired output voltage ripple level and
transient response requirements for sag (undershoot on
load apply) and soar (overshoot on load release).
V
ESR
= I
x ESR
OUT
(1-D) x I
OUT
C
IN
Ripple Current
D x I
OUT
D x tSW
(1-D) x tSW
Output Ripple
Figure 1. CIN Ripple Voltage and Ripple Current
The output voltage ripple at the switching frequency is
a function of the inductor current ripple going through
the output capacitor’s impedance. To derive the output
voltage ripple, the output capacitor with Capacitance
In addition, the input capacitor needs to have a very
low ESR and must be rated to handle the worst-case
RMS input current of :
VOUT
V
IN
IRMS IOUT_MAX
1
(COUT) and its equivalent series resistance (RESR
)
V
IN
VOUT
must be taken into consideration. The output
peak-to-peak ripple voltage (VRIPPLE) caused by the
inductor current ripple (IL) is characterized by two
components, which are ESR ripple (VRIPPLE(ESR)) and
capacitive ripple (VRIPPLE(C)) and can be expressed as
below :
It is common to use the worse IRMS IOUT/2 at VIN
=
2VOUT for design. Note that ripple current ratings from
capacitor manufacturers are often based on only 2000
hours of life which makes it advisable to further de-rate
the capacitor, or choose a capacitor rated at a higher
temperature than required.
V
= V
V
RIPPLE
RIPPLE(ESR) RIPPLE(C)
Several capacitors may also be paralleled to meet size,
height and thermal requirements in the design. For low
input voltage applications, sufficient bulk input
capacitance is needed to minimize transient effects
during output load changes.
V
= I R
RIPPLE(ESR)
L
ESR
I
L
V
=
RIPPLE(C)
8C
f
OUT SW
As ceramic capacitors are used, both parameters
should be estimated due to the extremely low ESR and
relatively small capacitance. Refer to the RT6253A/B's
typical application circuit of 1.2V application, the actual
inductor current ripple (IL) is 0.85A, and the output
capacitors are 2 x 22F (Murata ceramic capacitor :
GRM219R60J226ME47), VRIPPLE can be obtained as
below.
Ceramic capacitors are ideal for switching regulator
applications because of its small size, robustness, and
very low ESR. However, care must be taken when
these capacitors are used at the input. A ceramic input
capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
RT6253A/B circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value,
possibly exceeding the device’s rating. This situation is
easily avoided by placing the low ESR ceramic input
capacitor in parallel with a bulk capacitor with higher
ESR to damp the voltage ringing.
The ripple caused by ESR (2m) can be calculated
as :
VRIPPLE ESR = 0.85A2m = 1.7mV
Considering the capacitance derating, the effective
capacitance is approximately 18F as the output
voltage is 1.2V, and another parameter is :
The input capacitor should be placed as close as
possible to the VIN pins, with a low inductance
connection to the GND of the IC. In addition to a larger
bulk capacitor, a small ceramic capacitors of 0.1F
should be placed close to the VIN and GND pin. This
0.85A
8218μF580kHz
VRIPPLE C
=
= 5.1mV
VRIPPLE = 1.7mV + 5.1mV = 6.8mV
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6253A/B-01 September 2019
www.richtek.com
21
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