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LTC3809EDD-PBF Datasheet(PDF) 9 Page - Linear Technology |
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LTC3809EDD-PBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LTC3809 9 3809fc OPERATION (Refer to Functional Diagram) The reverse current comparator RICMP senses the drain-to- source voltage of the bottom external N-channel MOSFET. This MOSFET is turned off just before the inductor current reaches zero, preventing it from going negative. In forced continuous operation, the inductor current is allowed to reverse at light loads or under large transient conditions. The peak inductor current is determined by the voltage on the ITH pin. The P-channel MOSFET is turned on every cycle (constant frequency) regardless of the ITH pin voltage. In this mode, the efficiency at light loads is lower than in Burst Mode operation. However, continuous mode has the advantages of lower output ripple and no noise at audio frequencies. When the SYNC/MODE pin is clocked by an external clock source to use the phase-locked loop (see Frequency Selection and Phase-Locked Loop), or is set to a DC voltage between 0.4V and several hundred mV below VIN, the LTC3809 operates in PWM pulse-skipping mode at light loads. In this mode, the current comparator ICMP may remain tripped for several cycles and force the external P-channel MOSFET to stay off for the same number of cycles. The inductor current is not allowed to reverse (discontinuous operation). This mode, like forced continuous operation, exhibits low output ripple as well as low audio noise and reduced RF interference as compared to Burst Mode operation. However, it provides low current efficiency higher than forced continuous mode, but not nearly as high as Burst Mode operation. During start-up or an undervoltage condition (VFB ≤ 0.54V), the LTC3809 operates in pulse-skipping mode (no current reversal allowed), regardless of the state of the SYNC/MODE pin. Short-Circuit and Current Limit Protection The LTC3809 monitors the voltage drop ΔVSC (between the GND and SW pins) across the external N-channel MOSFET with the short-circuit current limit comparator. The allowed voltage is determined by: ΔVSC(MAX) = A • 90mV 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 inductor current limit for short-circuit protection is determined by ΔVSC(MAX) and the on-resistance of the external N-channel MOSFET: I V R SC SC MAX DS ON = Δ () () Once the inductor current exceeds ISC, the short current comparator will shut off the external P-channel MOSFET until the inductor current drops below ISC. Output Overvoltage Protection As further protection, the overvoltage comparator (OVP) guards against transient overshoots, as well as other more serious conditions that may overvoltage the output. When the feedback voltage on the VFB pin has risen 13.33% above the reference voltage of 0.6V, the external P-chan- nel MOSFET is turned off and the N-channel MOSFET is turned on until the overvoltage is cleared. |
Similar Part No. - LTC3809EDD-PBF |
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Similar Description - LTC3809EDD-PBF |
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