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LM21215AMHX-1/NOPB Datasheet(PDF) 20 Page - Texas Instruments

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Part No. LM21215AMHX-1/NOPB
Description  15-A Ultra High-Efficiency Synchronous Buck Converter
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM21215AMHX-1/NOPB Datasheet(HTML) 20 Page - Texas Instruments

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IN
OUT
RMS CIN
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DROOP
OUT STEP
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LM21215A
SNOSB87C – MARCH 2011 – REVISED JANUARY 2016
www.ti.com
8.2.1.2.4
Output Capacitor Selection
The output capacitor, designated COUT, filters the inductor ripple current and provides a source of charge for
transient load conditions. A wide range of output capacitors may be used with the LM21215A that provide various
advantages. The best performance is typically obtained using ceramic, SP or OSCON type chemistries. Typical
trade-offs are that the ceramic capacitor provides extremely low ESR to reduce the output ripple voltage and
noise spikes, while the SP and OSCON capacitors provide a large bulk capacitance in a small volume for
transient loading conditions.
When selecting an output capacitor, the two performance characteristics to consider are the output voltage ripple
and load transient response. Approximate the output voltage ripple by using Equation 7.
where
ΔVOUT is the peak-to-peak output voltage ripple
RESR is the effective series resistance (ESR) of the output capacitor
FSW is the switching frequency
COUT is the effective output capacitance
(7)
The amount of output voltage ripple is application specific. A general recommendation is to keep the output ripple
less than 1% of the rated output voltage. Keep in mind that ceramic capacitors are sometimes preferred because
they have very low ESR. However, depending on package and voltage rating of the capacitor, the effective in-
circuit capacitance can drop significantly with applied voltage. The output capacitor selection also affects the
output voltage droop during a load transient. The peak deviation of the output voltage during a load transient is
dependent on many factors. An approximation of the transient dip ignoring loop bandwidth is obtained using
Equation 8:
where
COUT is the minimum required output capacitance
LF is the filter inductance
VDROOP is the output voltage deviation ignoring loop bandwidth considerations
ΔIOUT-STEP is the load step change
RESR is the output capacitor ESR
VIN is the input voltage
VOUT is the output voltage setpoint
(8)
Three 100-µF, 6.3-V ceramic capacitors with X5R dielectric and 1210 footprint are selected here based on a
review of the capacitor's tolerance and voltage coefficient to meet output ripple specification.
8.2.1.2.5
Input Capacitor Selection
High quality input capacitors are necessary to limit the input voltage ripple while supplying switching-frequency
AC current to the buck power stage. It is generally recommended to use X5R or X7R dielectric ceramic
capacitors, thus providing low impedance and high RMS current rating over a wide temperature range. To
minimize the parasitic inductance in the switching loop, position the input capacitors as close as possible to the
PVIN and PGND pins. A good approximation for the required ripple current rating is given by Equation 9.
(9)
20
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Product Folder Links: LM21215A


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