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DS8099/A-01 February 2015 www.richtek.com
RT8099/A
©
Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Applications Information
The basic RT8099/A 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
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔI
L
increases with higher VIN and decreases
with higher inductance.
OUT OUT
L(MAX) IN(MAX)
VV
L = 1
fI V
Having a lower ripple current reduces the ESR losses in
the output capacitors and the output voltage ripple. Highest
efficiency operation is achieved at low frequency with small
ripple current. This, however, requires a large inductor.
A reasonable starting point for selecting the ripple current
is ΔI
L
= 0.4 (IMAX). The largest ripple current occurs at
the highest VIN. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation :
OUT OUT
L
IN
VV
I = 1
fL V
C
IN
and C
OUT
Selection
The input capacitance, C
IN
, 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 :
OUT
IN
RMS OUT(MAX)
IN OUT
V
V
I = I 1
VV
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/ 2. This simple worst-case condition is commonly
used for design because even significant deviations do
not offer much relief. Note that ripple current ratings from
capacitor manufacturers are often based on only 2000
hours of life which makes it advisable to further derate the
capacitor, or choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be paralleled to meet size or height requirements in the
design.
The selection of C
OUT
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 that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later section.
The output ripple, ΔV
OUT
, is determined by :
A 2.2μH inductor is recommended for L.
Model Vendor
Dimensions
L x W x H (mm)
NR4018T2R2M Taiyo 4.0 x 4.0 x 1.8
VLS3010ET-2R2M TDK 3.0 x 3.0 x 1.0
NR3010T2R2M Taiyo 3.0 x 3.0 x 1.0
SWPA3010S2R2NT Sunlord 3.0 x 3.0 x 1.0
Table 1. Suggested Inductors and Suppliers
OUT L
OUT
1
VIESR
8fC
The output ripple is highest at maximum input voltage
since
ΔI
L
increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and
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 that consideration is given to ripple current
ratings and long term reliability. Ceramic capacitors have
excellent low ESR characteristics but have a high voltage
coefficient and audible piezoelectric effects. The high Q
of ceramic capacitors with trace inductance can also lead
to significant ringing.
Using Ceramic Input and Output Capacitors
Higher values, 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
