LTC3822-1 OPERATION (Refer to Functional Diagram) signal goes low and the controller resumes normal opera- to normal when VFB rises above ground. This feature is tion by turning on the top N-channel MOSFET on the next disabled during start-up. cycle of the internal oscillator. Output Overvoltage Protection When the controller is enabled for Burst Mode or pulse skipping operation, the inductor current is not allowed to As further protection, the overvoltage comparator (OVP) reverse. Hence, the controller operates discontinuously. guards against transient overshoots, as well as other more The reverse current comparator RICMP senses the drain- serious conditions that may overvoltage the output. When to-source voltage of the bottom N-channel MOSFET. This the feedback voltage on the VFB pin has risen 13.33% MOSFET is turned off just before the inductor current above the reference voltage of 0.6V, the top MOSFET is reaches zero, preventing it from going negative. turned off and the bottom MOSFET is turned on until the overvoltage is cleared. In forced continuous operation, the inductor current is allowed to reverse at light loads or under large transient Frequency Selection and Phase-Locked Loop conditions. The peak inductor current is determined by (PLLLPF and SYNC/MODE Pins) the voltage on the ITH pin. The top MOSFET is turned on every cycle (constant frequency) regardless of the I The selection of switching frequency is a tradeoff between TH pin voltage. In this mode, the effi ciency at light loads is lower effi ciency and component size. Low frequency opera- than in Burst Mode operation. However, continuous mode tion increases effi ciency by reducing MOSFET switching has the advantages of lower output ripple and no noise at losses, but requires larger inductance and/or capacitance audio frequencies. to maintain low output ripple voltage. When the SYNC/MODE pin is clocked by an external clock The switching frequency of the LTC3822-1’s controllers source to use the phase-locked loop (see Frequency Selec- can be selected using the PLLLPF pin. If the SYNC/MODE tion and Phase-Locked Loop), or is set to a DC voltage is not being driven by an external clock source, the PLLLPF between 0.4V and several hundred mV below V can be fl oated, tied to VIN or tied to GND to select 550kHz, IN, the LTC3822-1 operates in PWM pulse skipping mode at light 750kHz or 300kHz, respectively. loads. In this mode, the current comparator ICMP may A phase-locked loop (PLL) is available on the LTC3822-1 remain tripped for several cycles and force the external top to synchronize the internal oscillator to an external clock MOSFET to stay off for the same number of cycles. The source that connects to the SYNC/MODE pin. In this case, inductor current is not allowed to reverse (discontinuous a series RC should be connected between the PLLLPF pin operation). This mode, like forced continuous operation, and GND to serve as the PLL’s loop fi lter. The LTC3822-1 exhibits low output ripple as well as low audio noise and phase detector adjusts the voltage on the PLLLPF pin to reduced RF interference as compared to Burst Mode op- align the turn-on of the top MOSFET to the rising edge of eration. However, it provides low current effi ciency higher the synchronizing signal. than forced continuous mode, but not nearly as high as The typical capture range of the LTC3822-1’s phase-locked Burst Mode operation. During start-up or an undervoltage loop is from approximately 200kHz to 1MHz. condition (VFB ≤ 0.54V), the LTC3822-1 operates in pulse skipping mode (no current reversal allowed), regardless Boost Capacitor Refresh Timeout of the state of the SYNC/MODE pin. In order to maintain suffi cient charge on CB, the converter Short-Circuit Protection will briefl y turn off the top MOSFET and turn on the bottom MOSFET if at any time the bottom MOSFET has remained The LTC3822-1 monitors VFB to detect a short-circuit off for 10 cycles. This most commonly occurs in a dropout on VOUT. When VFB is near ground, switching frequency situation where V is reduced to prevent the inductor current from running IN is close to VOUT. away. The oscillator frequency will progressively return 38221f 10
