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LT8361 Datasheet(PDF) 11 Page - Linear Technology |
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LT8361 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  LT8361 11 Rev 0 For more information www.analog.com APPLICATIONS INFORMATION INTVCC REGULATOR A low dropout (LDO) linear regulator, supplied from VIN, produces a 3.2V supply at the INTVCC pin. A minimum 1µF low ESR ceramic capacitor must be used to bypass the INTVCC pin to ground to supply the high transient cur- rents required by the internal power MOSFET gate driver. No additional components or loading is allowed on this pin. The INTVCC rising threshold (to allow soft-start and switching) is typically 2.65V. 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) instead of the VIN pin. For SEPIC applications with VIN often greater than VOUT, the BIAS pin can be di- rectly 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 LT8361 uses a constant frequency PWM architecture that can be programmed to switch from 300kHz to 2MHz 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 = 51.2 fOSC – 5.6 where RT is in kΩ and fOSC is the desired switching fre- quency in MHz. Table 1. SW Frequency vs RT Value fOSC (MHz) RT (kΩ) 0.3 165 0.45 107 0.75 63.4 1 45.3 1.5 28.7 2 20 Synchronization and Mode Selection To select low ripple Burst Mode operation, for high effi- ciency at light loads, tie the SYNC/MODE pin below 0.14V (this can be ground or a logic low output). To synchronize the LT8361 oscillator to an external fre- quencyconnectasquarewave(with20%to80%dutycycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.4V and peaks above 1.7V (up to 6V).TheLT8361willnotenterBurstModeoperationatlow output loads while synchronized to an external clock, but instead will pulse skip to maintain regulation. The LT8361 may be synchronized over a 300kHz to 2MHz range. The RT resistor should be chosen to set the LT8361 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 LT8361 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 main- tained at lower output load than in Burst Mode operation. These two differences come at the expense of increased quiescent current. To enable pulse-skipping mode, float the SYNC pin. To improve EMI/EMC, the LT8361 can provide spread spectrumfrequencymodulation(SSFM).Thisfeaturevaries the clock with a triangle frequency modulation of 20%. For example, if the LT8361's frequency was programmed to switch at 2MHz, spread spectrum mode will modulate the oscillator between 2MHz and 2.4MHz. The 20% modula- tion will occur at a frequency: fOSC/256 where fOSC is the switching frequency programmed using the RT pin. The LT8361 can also be configured to operate in pulse- skipping/SSFM mode by tying the SYNC/MODE pin above 1.7V. The LT8361 can also be configured for Burst Mode operation at light loads (for improved efficiency) and SSFM at heavy loads (for low EMI) by tying a 100k from the SYNC/MODE pin to GND. |
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