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LT8331 Datasheet(HTML) 10 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) 10 Page - Analog Devices

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LT8331
10
Rev. C
For more information www.analog.com
APPLICATIONS INFORMATION
No additional components or loading is allowed on this
pin. The INTVCC rising threshold (to allow soft-start and
switching) is typically 2.6V. The INTVCC falling threshold
(to stop switching and reset soft-start) is typically 2.5V.
To improve efficiency across all loads, the majority of
INTVCC current can be drawn from the BIAS pin (4.4V ≤
BIAS ≤ VIN – 0.4V) instead of the VIN pin. For flyback or
SEPIC applications with VIN often greater than VOUT, the
BIAS pin can be directly connected to VOUT. If the BIAS
pin is connected to a supply other than VOUT, be sure to
bypass the pin with a local ceramic capacitor.
Programming Switching Frequency
The LT8331 uses a constant frequency PWM architecture
that can be programmed to switch from 100kHz to 500kHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary RT value for a desired switching
frequency is in Table 1.
The RT resistor required for a desired switching frequency
can be calculated using:
RT =
32.85
fSW
– 9.5
where RT is in kΩ and fSW is the desired switching fre-
quency in MHz.
Table 1. SW Frequency vs RT Value
fSW (MHz)
RT (kΩ)
0.1
324
0.2
154
0.3
100
0.4
73.2
0.45
63.4
0.5
56.2
Synchronization and Mode Selection
To select low ripple Burst Mode operation, tie the SYNC/
MODE pin below 0.6V (this can be ground or a logic low
output). To synchronize the LT8331 oscillator to an exter-
nal frequency connect a square wave (with 20% to 80%
duty cycle) to the SYNC pin. The square wave amplitude
should have valleys that are below 0.6V and peaks above
2.4V (up to 6V).
The LT8331 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation. The LT8331
may be synchronized over a 100kHz to 625kHz range. The
RT resistor should be chosen to set the LT8331 switching
frequency equal to or below the lowest synchronization
input. For example, if the synchronization signal will be
500kHz and higher, the RT should be selected for 500kHz.
For some applications it is desirable for the LT8331 to
operate in pulse-skipping mode, offering two major differ-
ences from Burst Mode operation. Firstly, the clock stays
awake at all times and all switching cycles are aligned to
the clock. Secondly, the full switching frequency is reached
at lower output load than in Burst Mode operation. These
two differences come at the expense of increased quies-
cent current. To enable pulse-skipping mode, tie the SYNC
pin above 2.4V (this can be INTVCC or a logic high output).
DUTY CYCLE CONSIDERATION
The LT8331 minimum on-time, minimum off-time and
switching frequency (fOSC) define the allowable minimum
and maximum duty cycles of the converter (see Minimum
On-Time, Minimum Off-Time, and Switching Frequency
in the Electrical Characteristics table).
Minimum Allowable Duty Cycle =
Minimum On-Time(MAX) • fOSC(MAX)
Maximum Allowable Duty Cycle =
1 – Minimum Off-Time(MAX) • fOSC(MAX)
The required switch duty cycle range for a Boost converter
operating in continuous conduction mode (CCM) can be
calculated as:
DMIN = 1 –
VIN(MAX)
(VOUT + VD)
DMAX = 1 –
VIN(MIN)
(VOUT + VD)
where VD is the diode forward voltage drop. If the above
duty cycle calculations for a given application violate


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