Datasheet LTC3809-1 (Analog Devices) - 10

LTC3809-1
OPERATION (Refer to Functional Diagram)
In sleep mode, much of the internal circuitry is turned off, high as Burst Mode operation. During start-up or an reducing the quiescent current that the LTC3809-1 draws. undervoltage condition (VFB ≤ 0.54V), the LTC3809-1 The load current is supplied by the output capacitor. As operates in pulse-skipping mode (no current reversal the output voltage decreases, the EAMP increases the allowed), regardless of the state of the MODE pin. ITH voltage. When the ITH voltage reaches 0.925V, the SLEEP signal goes low and the controller resumes normal
Short-Circuit and Current Limit Protection
operation by turning on the external P-channel MOSFET The LTC3809-1 monitors the voltage drop ΔVSC (between on the next cycle of the internal oscillator. the GND and SW pins) across the external N-channel When the controller is enabled for Burst Mode or pulse- MOSFET with the short-circuit current limit comparator. skipping operation, the inductor current is not allowed to The allowed voltage is determined by: reverse. Hence, the controller operates discontinuously. ΔVSC(MAX) = A • 90mV The reverse current comparator RICMP senses the drain-to-source voltage of the bottom external N-channel where A is a constant determined by the state of the IPRG MOSFET. This MOSFET is turned off just before the inductor pin. Floating the IPRG pin selects A = 1; tying IPRG to VIN current reaches zero, preventing it from going negative. selects A = 5/3; tying IPRG to GND selects A = 2/3. In forced continuous operation, the inductor current is The inductor current limit for short-circuit protection is allowed to reverse at light loads or under large transient determined by ΔVSC(MAX) and the on-resistance of the conditions. The peak inductor current is determined by the external N-channel MOSFET: voltage on the ITH pin. The P-channel MOSFET is turned V Δ SC M ( AX) on every cycle (constant frequency) regardless of the I I = TH SC R pin voltage. In this mode, the effi ciency at light loads is DS ON ( ) lower than in Burst Mode operation. However, continuous Once the inductor current exceeds I mode has the advantages of lower output ripple and no SC, the short current comparator will shut off the external P-channel MOSFET noise at audio frequencies. until the inductor current drops below ISC. When the MODE pin is set to the VFB Pin, the LTC3809-1 operates in PWM pulse-skipping mode at light loads. In
Output Overvoltage Protection
this mode, the current comparator ICMP may remain As further protection, the overvoltage comparator (OVP) tripped for several cycles and force the external P-channel guards against transient overshoots, as well as other more MOSFET to stay off for the same number of cycles. The serious conditions that may overvoltage the output. When inductor current is not allowed to reverse (discontinuous the feedback voltage on the V operation). This mode, like forced continuous operation, FB pin has risen 13.33% above the reference voltage of 0.6V, the external P-channel exhibits low output ripple as well as low audible noise MOSFET is turned off and the N-channel MOSFET is turned and reduced RF interference as compared to Burst Mode on until the overvoltage is cleared. operation. However, it provides low current effi ciency higher than forced continuous mode, but not nearly as 38091fc 10