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LT8331 Datasheet(HTML) 14 Page - Analog Devices

Part No. LT8331
Description  Low IQ Boost/SEPIC/Flyback/Inverting Converter with 0.5A, 140V Switch
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LT8331 Datasheet(HTML) 14 Page - Analog Devices

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LT8331
14
Rev. C
For more information www.analog.com
inductance forms a high quality (under damped) tank cir-
cuit. If the LT8331 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8331’s voltage rating. This situation is
easily avoided (see Application Note 88).
BOOST CONVERTER: OUTPUT CAPACITOR SELECTION
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple volt-
age. Multilayer ceramic capacitors are an excellent choice,
as they are small and have extremely low ESR. Use X5R or
X7R types. This choice will provide low output ripple and
good transient response. A 10µF to 47µF output capacitor
is sufficient for most applications, but systems with very
low output currents may need only a 1µF or 2.2µF out-
put capacitor. Solid tantalum or OS-CON capacitor can be
used, but they will occupy more board area than a ceramic
and will have a higher ESR. Always use a capacitor with a
sufficient voltage rating.
Contributions of ESR (equivalent series resistance), ESL
(equivalent series inductance) and the bulk capacitance
must be considered when choosing the correct output
capacitors for a given output ripple voltage. The effect of
these three parameters (ESR, ESL and bulk C) on the out-
put voltage ripple waveform for a typical boost converter
is illustrated in Figure 4.
between ∆VESR and ∆VCOUT. This percentage ripple will
change, depending on the requirements of the applica-
tion, and the following equations can easily be modi-
fied. For a 1% contribution to the total ripple voltage, the
ESR of the output capacitor can be determined using the
following equation:
ESRCOUT
0.01 • VOUT
ID(PEAK)
For the bulk C component, which also contributes 1% to
the total ripple:
COUT
IO(MAX)
0.01 • VOUT • fOSC
The output capacitor in a boost regulator experiences
high RMS ripple currents, as shown in Figure 4. The RMS
ripple current rating of the output capacitor can be deter-
mined using the following equation:
IRMS(COUT) ≥ IO(MAX)
DMAX
1 − DMAX
Multiple capacitors are often paralleled to meet ESR
requirements. Typically, once the ESR requirement is sat-
isfied, the capacitance is adequate for filtering and has the
required RMS current rating. Additional ceramic capaci-
tors in parallel are commonly used to reduce the effect of
parasitic inductance in the output capacitor, which reduces
high frequency switching noise on the converter output.
CERAMIC CAPACITORS
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8331 due to their piezoelectric
nature. When in Burst Mode operation, the LT8331’s
switching frequency depends on the load current, and
at very light loads the LT8331 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT8331 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet to a
casual ear. If this is unacceptable, use a high performance
Figure 4. The Output Ripple Waveform of a Boost Converter
VOUT
(AC)
tON
∆VESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
∆VCOUT
8331 F04
tOFF
The choice of component(s) begins with the maximum
acceptable ripple voltage (expressed as a percentage of
the output voltage), and how this ripple should be divided
between the ESR step ∆VESR and the charging/discharg-
ing ∆VCOUT. For the purpose of simplicity, we will choose
2% for the maximum output ripple, to be divided equally
APPLICATIONS INFORMATION


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