Datasheet LTC3809 (Analog Devices) - 10

LTC3809
OPERATION (Refer to Functional Diagram) Frequency Selection and Phase-Locked Loop
frequency fOSC (= 550kHz) over a wider range (460kHz to
(PLLLPF and SYNC/MODE Pins)
635kHz), reducing the peaks of the harmonic output on a The selection of switching frequency is a tradeoff between spectral analysis of the output noise. In this case, a 2.2nF effi ciency and component size. Low frequency operation fi lter cap should be connected between the PLLLPF pin increases effi ciency by reducing MOSFET switching and GND and another 1000pF cap should be connected losses, but requires larger inductance and/or capacitance between PLLLPF and the SYNC/MODE pin. The controller to maintain low output ripple voltage. operates in PWM pulse-skipping mode at light loads when spread spectrum modulation is selected. See the discussion The switching frequency of the LTC3809’s controllers can of Spread Spectrum Modulation with SYNC/MODE and be selected using the PLLLPF pin. If the SYNC/MODE is PLLLPF Pins in the Applications Information section. not being driven by an external clock source, the PLLLPF can be fl oated, tied to VIN or tied to GND to select 550kHz,
Dropout Operation
750kHz or 300kHz, respectively. When the input supply voltage (VIN) approaches the output A phase-locked loop (PLL) is available on the LTC3809 voltage, the rate of change of the inductor current while the to synchronize the internal oscillator to an external clock external P-channel MOSFET is on (ON cycle) decreases. source that connects to the SYNC/MODE pin. In this case, This reduction means that the P-channel MOSFET will a series RC should be connected between the PLLLPF pin remain on for more than one oscillator cycle if the inductor and GND to serve as the PLL’s loop fi lter. The LTC3809 current has not ramped up to the threshold set by the phase detector adjusts the voltage on the PLLLPF pin to EAMP on the ITH pin. Further reduction in the input supply align the turn-on of the external P-channel MOSFET to voltage will eventually cause the P-channel MOSFET to be the rising edge of the synchronizing signal. turned on 100%; i.e., DC. The output voltage will then be The typical capture range of the LTC3809’s phase-locked determined by the input voltage minus the voltage drop loop is from approximately 200kHz to 1MHz. across the P-channel MOSFET and the inductor.
Spread Spectrum Modulation (SYNC/MODE and Undervoltage Lockout PLLLPF Pins)
To prevent operation of the P-channel MOSFET below Connecting the SYNC/MODE pin to a DC voltage above safe input voltage levels, an undervoltage lockout is 1.35V and several hundred mV below V incorporated in the LTC3809. When the input supply IN enables spread spectrum modulation (SSM) operation. An internal 2.6μA voltage (VIN) drops below 2.25V, the external P- and pull-down current source at SYNC/MODE helps to set the N-channel MOSFETs and all internal circuits are turned voltage at the SYNC/MODE pin for this operation by tying a off except for the undervoltage block, which draws only resistor with appropriate value between SYNC/MODE and a few microamperes. VIN. This mode of operation spreads the internal oscillator 3809fc 10