Datasheet LT8331 (Analog Devices) - 10

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