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LTC3890-1 数据表(PDF) 26 Page - Linear Technology |
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LTC3890-1 数据表(HTML) 26 Page - Linear Technology |
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26 / 38 page  LTC3892/ LTC3892-1/LTC3892-2 26 38921fc For more information www.linear.com/LTC3892 APPLICATIONS INFORMATION above the nominal output voltage. When this condition is sensed, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. The bottom MOSFET remains on continuously for aslongastheovervoltageconditionpersists;ifVOUTreturns to a safe level, normal operation automatically resumes. AshortedtopMOSFETwillresultinahighcurrentcondition which will open the system fuse. The switching regulator will regulate properly with a leaky top MOSFET by altering the duty cycle to accommodate the leakage. Fault Conditions: Overtemperature Protection Athighertemperatures,orincaseswheretheinternalpower dissipation causes excessive self heating on chip (such as DRVCCshorttoground),theovertemperatureshutdowncir- cuitrywillshutdowntheLTC3892/LTC3892-1/LTC3892-2. When the junction temperature exceeds approximately 175°C, the overtemperature circuitry disables the DRVCC LDO, causing the DRVCC supply to collapse and effectively shuttingdowntheentireLTC3892/LTC3892-1/LTC3892-2 chip. Once the junction temperature drops back to the ap- proximately 155°C, the DRVCC LDO turns back on. Long- term overstress (TJ > 125°C) should be avoided as it can degrade the performance or shorten the life of the part. Phase-Locked Loop and Frequency Synchronization The LTC3892/LTC3892-1/LTC3892-2 has an internal phase-locked loop (PLL) comprised of a phase frequency detector, a lowpass filter, and a voltage-controlled oscilla- tor (VCO). This allows the turn-on of the top MOSFET of controller 1 to be locked to the rising edge of an external clock signal applied to the PLLIN/MODE pin. The turn-on of controller 2’s top MOSFET is thus 180° out of phase with the external clock. The phase detector is an edge sensitivedigitaltypethatprovideszerodegreesphaseshift between the external and internal oscillators. This type of phase detector does not exhibit false lock to harmonics of the external clock. If the external clock frequency is greater than the internal oscillator’sfrequency,fOSC,thencurrentissourcedcontinu- ously from the phase detector output, pulling up the VCO input. When the external clock frequency is less than fOSC, current is sunk continuously, pulling down the VCO input. If the external and internal frequencies are the same but exhibit a phase difference, the current sources turn on for an amount of time corresponding to the phase difference. The voltage at the VCO input is adjusted until the phase and frequency of the internal and external oscillators are identical. At the stable operating point, the phase detector output is high impedance and the internal filter capacitor, CLP, holds the voltage at the VCO input. Note that the LTC3892/LTC3892-1/LTC3892-2 can only be synchronized to an external clock whose frequency is within range of the LTC3892/LTC3892-1/LTC3892-2’s internal VCO, which is nominally 55kHz to 1MHz. This is guaranteed to be between 75kHz and 850kHz. Typically, the external clock (on the PLLIN/MODE pin) input high threshold is 1.6V, while the input low threshold is 1.1V. The LTC3892/LTC3892-1/LTC3892-2 is guaranteed to synchronize to an external clock that swings up to at least 2.5V and down to 0.5V or less. Rapid phase locking can be achieved by using the FREQ pin to set a free-running frequency near the desired synchronization frequency. The VCO’s input voltage is prebiased at a frequency corresponding to the frequency set by the FREQ pin. Once prebiased, the PLL only needs to adjust the frequency slightly to achieve phase lock and synchronization. Although it is not required that the free- running frequency be near the external clock frequency, doingsowillpreventtheoperatingfrequencyfrompassing through a large range of frequencies as the PLL locks. FREQ PIN RESISTOR (k) 15 600 800 1000 35 45 55 25 38921 F10 400 200 500 700 900 300 100 0 65 75 85 95 105 115 125 Figure 9. Relationship Between Oscillator Frequency and Resistor Value at the FREQ Pin |
类似零件编号 - LTC3890-1 |
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类似说明 - LTC3890-1 |
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