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LTC3808 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3808 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  11 LTC3808 3808f A phase-locked loop (PLL) is available on the LTC3808 to synchronize the internal oscillator to an external clock source that connects to the SYNC/MODE pin. In this case, a series RC should be connected between the PLLLPF pin and GND to serve as the PLL’s loop filter. The LTC3808 phase detector adjusts the voltage on the PLLLPF pin to align the turn-on of the external P-channel MOSFET to the rising edge of the synchronizing signal. The typical capture range of the LTC3808’s phase-locked loop is from approximately 200kHz to 1MHz. Spread Spectrum Modulation (SYNC/MODE and PLLLPF Pins) Connecting the SYNC/MODE pin to a DC voltage above 1.35V and several hundred mV below VIN enables spread spectrum modulation (SSM) operation. An internal 2.6µA pull-down current source at SYNC/MODE helps to set the voltage at the SYNC/MODE pin for this operation by tying a resistor with appropriate value between SYNC/MODE and VIN. This mode of operation spreads the internal oscillator frequency fOSC (= 550kHz) over a wider range (460kHz to 635kHz), reducing the peaks of the harmonic output on a spectral analysis of the output noise. In this case, a 2.2nF filter cap should be connected between the PLLLPF pin and GND and another 1000pF cap should be connected between PLLLPF and the SYNC/MODE pin. The controller operates in PWM pulse skipping mode at light loads when spread spectrum modulation is selected. See more discussions in the Spread Spectrum Modulation with SYNC/MODE and PLLLPF Pins in the Applications Information section. Dropout Operation When the input supply voltage (VIN) approaches the output voltage, the rate of change of the inductor current while the external P-channel MOSFET is on (ON cycle) decreases. This reduction means that the P-channel MOSFET will remain on for more than one oscillator cycle if the inductor current has not ramped up to the threshold set by the EAMP on the ITH pin. Further reduction in the input supply voltage will eventually cause the P-channel MOSFET to be turned on 100%; i.e., DC. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. Undervoltage Lockout To prevent operation of the P-channel MOSFET below safe input voltage levels, an undervoltage lockout is incorporated in the LTC3808. When the input supply voltage (VIN) drops below 2.25V, the external P- and N-channel MOSFETs and all internal circuits are turned off except for the undervoltage block, which draws only a few microamperes. Peak Current Sense Voltage Selection and Slope Compensation (IPRG Pin) When the LTC3808 controller is operating below 20% duty cycle, the peak current sense voltage (between the SENSE+ and SENSE–/SW pins) allowed across the external P- channel MOSFET is determined by: ∆ = VA VV SENSE MAX ITH () • –. 07 10 where A is a constant determined by the state of the IPRG pin. Floating the IPRG pin selects A = 1; tying IPRG to VIN selects A = 5/3; tying IPRG to GND selects A = 2/3. The OPERATIO (Refer to Functional Diagram) |
Similar Part No. - LTC3808_15 |
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Similar Description - LTC3808_15 |
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