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LT8331 Datasheet(HTML) 12 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) 12 Page - Analog Devices

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LT8331
12
Rev. C
For more information www.analog.com
APPLICATIONS INFORMATION
frequency foldback provides a larger switch-off time,
allowing inductor current to fall enough each cycle (see
Normalized Switching Frequency vs FBX Voltage in the
Typical Performance Characteristics section).
THERMAL LOCKOUT
If the LT8331 die temperature reaches 170°C (typical),
the part will stop switching and go into thermal lockout.
When the die temperature has dropped by 5°C (nominal),
the part will resume switching with a soft-started inductor
peak current.
COMPENSATION
The LT8331 is internally compensated. The decision to
use either low ESR (ceramic) capacitors or the higher ESR
(tantalum or OS-CON) capacitors, for the output capacitor,
can affect the stability of the overall system. The ESR of
any capacitor, along with the capacitance itself, contrib-
utes a zero to the system. For the tantalum and OS-CON
capacitors, this zero is located at a lower frequency due
to the higher value of the ESR, while the zero of a ceramic
capacitor is at a much higher frequency and can generally
be ignored.
A phase lead zero can be intentionally introduced by plac-
ing a capacitor in parallel with the resistor between VOUT
and FBX. By choosing the appropriate values for the resis-
tor and capacitor, the zero frequency can be designed to
improve the phase margin of the overall converter. The
typical target value for the zero frequency is between 5kHz
to 20kHz.
A practical approach to compensation is to start with one
of the circuits in this data sheet that is similar to your
application. Optimize performance by adjusting the output
capacitor and/or the feed forward capacitor (connected
across the feedback resistor from output to FBX pin).
THERMAL CONSIDERATIONS
Care should be taken in the layout of the PCB to ensure
good heat sinking of the LT8331. The package has an
exposed pad (Pin 17) underneath the IC which is the best
path for heat out of the package. Pin 17 should be sol-
dered to a continuous copper ground plane under the
device to reduce die temperature and increase the power
capability of the LT8331. The ground plane should be
connected to large copper layers to spread heat dissi-
pated by the LT8331. Power dissipation within the LT8331
(PDISS_LT8331) can be estimated by subtracting the induc-
tor and Schottky diode power losses from the total power
losses calculated in an efficiency measurement. The junc-
tion temperature of LT8331 can then be estimated by:
TJ(LT8331) = TA + θJA • PDISS_LT8331
APPLICATION CIRCUITS
The LT8331 can be configured for different topologies. The
first topology to be analyzed will be the boost converter,
followed by the flyback, SEPIC and inverting converters.
Boost Converter: Switch Duty Cycle
The LT8331 can be configured as a boost converter for
the applications where the converter output voltage is
higher than the input voltage. Remember that boost con-
verters are not short-circuit protected. Under a shorted
output condition, the inductor current is limited only by
the input supply capability. For applications requiring a
step-up converter that is short-circuit protected, please
refer to the Applications Information section covering
SEPIC converters.
The conversion ratio as a function of duty cycle is:
VOUT
VIN
=
1
1 − D
in continuous conduction mode (CCM).
For a boost converter operating in CCM, the duty cycle
of the main switch can be calculated based on the output
voltage (VOUT) and the input voltage (VIN). The maximum
duty cycle (DMAX) occurs when the converter has the
minimum input voltage:
DMAX =
VOUT − VIN(MIN)
VOUT


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