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LT8331 Datasheet(HTML) 8 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) 8 Page - Analog Devices

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Rev. C
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The LT8331 uses a fixed frequency, current mode con-
trol scheme to provide excellent line and load regula-
tion. Operation can be best understood by referring to
the Block Diagram. An oscillator (with frequency pro-
grammed by a resistor at the RT pin) turns on the inter-
nal power switch at the beginning of each clock cycle.
Current in the inductor then increases until the current
comparator trips and turns off the power switch. The
peak inductor current at which the switch turns off is
controlled by the voltage on the internal VC node. The
error amplifier servos the VC node by comparing the
voltage on the FBX pin with an internal reference voltage
(1.60V or –0.80V, depending on the chosen topology).
When the load current increases it causes a reduction
in the FBX pin voltage relative to the internal reference.
This causes the error amplifier to increase the VC voltage
until the new load current is satisfied. In this manner,
the error amplifier sets the correct peak switch current
level to keep the output in regulation.
The LT8331 is capable of generating either a positive or
negative output voltage with a single FBX pin. It can be
configured as a boost, SEPIC or flyback converter to gen-
erate a positive output voltage, or as an inverting converter
to generate a negative output voltage. When configured
as a SEPIC converter, as shown in the Block Diagram, the
FBX pin is pulled up to the internal bias voltage of 1.60V
by a voltage divider (R1 and R2) connected from VOUT
to GND. Amplifier A2 becomes inactive and amplifier A1
performs (inverting) amplification from FBX to VC. When
the LT8331 is in an inverting configuration, the FBX pin
is pulled down to –0.80V by a voltage divider from VOUT
to GND. Amplifier A1 becomes inactive and amplifier A2
performs (non-inverting) amplification from FBX to VC.
If the EN/UVLO pin voltage is below 1.6V, the LT8331
enters undervoltage lockout (UVLO), and stops switch-
ing. When the EN/UVLO pin voltage is above 1.74V (typi-
cal), the LT8331 resumes switching. If the EN/UVLO pin
voltage is below 0.2V, the LT8331 draws less than 1µA
from VIN.
For the SYNC/MODE pin tied to ground, the LT8331 pro-
vides low output ripple Burst Mode operation with ultra
low quiescent current at light loads. For the SYNC/MODE
pin tied to INTVCC, the LT8331 uses pulse-skipping mode,
at the expense of hundreds of microamps, to maintain
output voltage regulation at light loads by skipping switch
pulses. For the SYNC/MODE pin driven by an external
clock, the converter switching frequency is synchronized
to that clock and pulse-skipping mode is also enabled.
The LT8331 includes a BIAS pin to improve efficiency
across all loads. The INTVCC supply current can be
drawn from the BIAS pin instead of the VIN pin for 4.4V
Protection features ensure the immediate disable of
switching and reset of the SS pin for any of the following
faults: internal reference UVLO, INTVCC UVLO, switch cur-
rent > 1.9× maximum limit, EN/UVLO < 1.6V or junction
temperature > 170°C.

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