RT2657BQ
RMS current handling requirements.Dry tantalum, special
polymer, aluminum electrolytic and ceramic capacitors are
all available in surface mount packages. Special polymer
capacitors offer very low ESR, but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density, but it is important to only
use types that have been surge tested for use in switching
power supplies. Aluminum electrolytic capacitors have
significantly higher ESR, but can be used in cost-sensitive
applications provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capacitors
have excellent low ESR characteristics, but can have a
high voltage coefficient and audible piezoelectric effects.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the Source of the high-side MOSFET.
To prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used. RMS
current is given by :
VOUT
V
IN
IRMS IOUT(MAX)
1
V
VOUT
IN
This formula has a maximum at VIN = 2VOUT, where IRMS
=
IOUT/2. This simple worst case condition is commonly used
for design because even significant deviations do not result
in much difference. Choose a capacitor rated at a higher
temperature than required.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing.
Several capacitors may also be paralleled to meet size or
height requirements in the design.
Using Ceramic Input and Output Capacitors
The selection of COUT is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
and load step transients, as well as the amount of bulk
capacitance that is necessary to ensure the control loop
is stable. Loop stability can be checked by viewing the
load transient response. The output ripple, ΔVOUT, is
determined by :
Higher value, lower 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. However, care must
be taken when these capacitors are used at the input and
output. 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 current through the long wires can potentially cause
a voltage spike at VIN large enough to damage the part.
1
VOUT I ESR
L
8fCOUT
The output ripple is highest at maximum input voltage
since ΔIL increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and
Table 2. Capacitors for CIN and COUT
Part No. Capacitance (F)
Component Supplier
MuRata
Case Size
1206
GRM31CR71A475KA01
GRM31CR71A106KA01
4.7F
10F
MuRata
1206
Copyright 2014 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS2657BQ-00 July 2014
www.richtek.com
11
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
