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LT8331 Datasheet(HTML) 11 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) 11 Page - Analog Devices

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LT8331
11
Rev. C
For more information www.analog.com
the minimum and/or maximum allowed duty cycles
for the LT8331, operation in discontinuous conduction
mode (DCM) might provide a solution. For the same VIN
and VOUT levels, operation in DCM does not demand as
low a duty cycle as in CCM. DCM also allows higher duty
cycle operation than CCM. The additional advantage of
DCM is the removal of the limitations to inductor value
and duty cycle required to avoid sub-harmonic oscillations
and the right half plane zero (RHPZ). While DCM provides
these benefits, the trade-off is higher inductor peak cur-
rent, lower available output power and reduced efficiency.
SETTING THE OUTPUT VOLTAGE
The output voltage is programmed with a resistor divider
from the output to the FBX pin. Choose the resistor values
for a positive output voltage according to:
R1 = R2 •
VOUT
1.60V
– 1
Choose the resistor values for a negative output voltage
according to:
R1 = R2 •
|VOUT|
0.80V
– 1
The locations of R1 and R2 are shown in the Block
Diagram. 1% resistors are recommended to maintain
output voltage accuracy.
Higher-value FBX divider resistors result in the lowest
input quiescent current and highest light-load efficiency.
FBX divider resistors R1 and R2 are usually in the range
from 25k to 1M.
SOFT-START
The LT8331 contains several features to limit peak switch
currents and output voltage (VOUT) overshoot during
start-up or recovery from a fault condition. The primary
purpose of these features is to prevent damage to external
components or the load.
High peak switch currents during start-up may occur
in switching regulators. Since VOUT is far from its final
value, the feedback loop is saturated and the regulator
tries to charge the output capacitor as quickly as possible,
resulting in large peak currents. A large surge current may
cause inductor saturation or power switch failure.
The LT8331 addresses this mechanism with a programma-
ble soft-start function. As shown in the Block Diagram, the
soft-start function controls the ramp of the power switch
current by controlling the ramp of VC through Q1. This
allows the output capacitor to be charged gradually toward
its final value while limiting the start-up peak currents.
Figure 3 shows the output voltage and supply current for
the first page Typical Applications. It can be seen that both
the output voltage and supply current come up gradually.
Figure 3. Soft-Start Waveforms
FAULT PROTECTION
An inductor overcurrent fault (> 1.15A) and/or INTVCC
undervoltage (INTVCC < 2.5V) and/or thermal lockout
(TJ > 170°C) will immediately prevent switching, will
reset the SS pin and will pull down VC. Once all faults
are removed, the LT8331 will soft-start VC and hence
inductor peak current.
FREQUENCY FOLDBACK
During start-up or fault conditions in which VOUT is very
low, extremely small duty cycles may be required to main-
tain control of inductor peak current. The minimum on-
time limitation of the power switch might prevent these
low duty cycles from being achievable. In this scenario
inductor current rise will exceed inductor current fall
during each cycle, causing inductor current to “walk up”
beyond the switch current limit. The LT8331 provides
protection from this by folding back switching frequency
whenever FBX pin is close to GND (low VOUT levels). This
APPLICATIONS INFORMATION
2ms/DIV
VOUT
20V/DIV
IL1+IL2
100mA/DIV
8331 F03


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