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LT8331 Datasheet(HTML) 21 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) 21 Page - Analog Devices

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LT8331
21
Rev. C
For more information www.analog.com
CDC has nearly a rectangular current waveform. During
the switch off-time, the current through CDC is IIN, while
approximately –IO flows during the on-time. The RMS
rating of the coupling capacitor is determined by the fol-
lowing equation:
IRMS(CDC) > IO(MAX)
VOUT + VD
VIN(MIN)
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for CDC.
INVERTING CONVERTER APPLICATIONS
The LT8331 can be configured as a dual-inductor invert-
ing topology, as shown in Figure 10. The VOUT to VIN
ratio is:
VOUT+ VD
VIN
=
D
1– D
Conversely, the minimum duty cycle (DMIN) occurs when
the converter operates at the maximum input voltage :
DMIN =
VOUT + VD
VOUT + VD+ VIN(MAX)
Be sure to check that DMAX and DMIN obey :
DMAX < 1 – Minimum Off-Time(MAX) • fOSC(MAX)
and
DMIN > Minimum On-Time(MAX) • fOSC(MAX)
where Minimum Off-Time, Minimum On-Time and fOSC
are specified in the Electrical Characteristics table.
Inverting Converter: Inductor, Output Diode and Input
Capacitor Selections
The selections of the inductor, output diode and input
capacitor of an inverting converter are similar to those
of the SEPIC converter. Please refer to the corresponding
SEPIC converter sections.
Inverting Converter: Output Capacitor Selection
The inverting converter requires much smaller output
capacitors than those of the boost, flyback and SEPIC
converters for similar output ripples. This is due to the
fact that, in the inverting converter, the inductor L2 is
in series with the output, and the ripple current flowing
through the output capacitors are continuous. The output
ripple voltage is produced by the ripple current of L2 flow-
ing through the ESR and bulk capacitance of the output
capacitor:
ΔVOUT(P–P) = ΔIL2 • ESRCOUT +
1
8 • fOSC • COUT
After specifying the maximum output ripple, the user
can select the output capacitors according to the
preceding equation.
Figure 10. A Simplified Inverting Converter
CDC
VIN
CIN
L1
D1
COUT
VOUT
8331 F10
+
GND
LT8331
SW
L2
+
+ –
+
APPLICATIONS INFORMATION
in continuous conduction mode (CCM).
Inverting Converter: Switch Duty Cycle and Frequency
For an inverting converter operating in CCM, the duty
cycle of the main switch can be calculated based on the
negative output voltage (VOUT) and the input voltage (VIN).
The maximum duty cycle (DMAX) occurs when the con-
verter has the minimum input voltage:
DMAX =
VOUT + VD
VOUT + VD + VIN(MIN)


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