Datasheet LTC3809 (Analog Devices) - 9

LTC3809
OPERATION (Refer to Functional Diagram)
The reverse current comparator RICMP senses the drain-to-
Short-Circuit and Current Limit Protection
source voltage of the bottom external N-channel MOSFET. The LTC3809 monitors the voltage drop ΔV This MOSFET is turned off just before the inductor current SC (between the GND and SW pins) across the external N-channel reaches zero, preventing it from going negative. MOSFET with the short-circuit current limit comparator. In forced continuous operation, the inductor current is The allowed voltage is determined by: allowed to reverse at light loads or under large transient ΔV conditions. The peak inductor current is determined by the SC(MAX) = A • 90mV voltage on the ITH pin. The P-channel MOSFET is turned where A is a constant determined by the state of the IPRG on every cycle (constant frequency) regardless of the ITH pin. Floating the IPRG pin selects A = 1; tying IPRG to VIN pin voltage. In this mode, the effi ciency at light loads is selects A = 5/3; tying IPRG to GND selects A = 2/3. lower than in Burst Mode operation. However, continuous The inductor current limit for short-circuit protection is mode has the advantages of lower output ripple and no determined by ΔV noise at audio frequencies. SC(MAX) and the on-resistance of the external N-channel MOSFET: When the SYNC/MODE pin is clocked by an external V Δ SC MAX ( ) clock source to use the phase-locked loop (see Frequency ISC = Selection and Phase-Locked Loop), or is set to a DC R DS ON ( ) voltage between 0.4V and several hundred mV below Once the inductor current exceeds I V SC, the short current IN, the LTC3809 operates in PWM pulse-skipping mode comparator will shut off the external P-channel MOSFET at light loads. In this mode, the current comparator until the inductor current drops below I ICMP may remain tripped for several cycles and force SC. the external P-channel MOSFET to stay off for the same
Output Overvoltage Protection
number of cycles. The inductor current is not allowed to reverse (discontinuous operation). This mode, like As further protection, the overvoltage comparator (OVP) forced continuous operation, exhibits low output ripple guards against transient overshoots, as well as other more as well as low audio noise and reduced RF interference as serious conditions that may overvoltage the output. When compared to Burst Mode operation. However, it provides the feedback voltage on the VFB pin has risen 13.33% low current effi ciency higher than forced continuous above the reference voltage of 0.6V, the external P-chan- mode, but not nearly as high as Burst Mode operation. nel MOSFET is turned off and the N-channel MOSFET is During start-up or an undervoltage condition (V turned on until the overvoltage is cleared. FB ≤ 0.54V), the LTC3809 operates in pulse-skipping mode (no current reversal allowed), regardless of the state of the SYNC/MODE pin. 3809fc 9