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LTC3601EMSETRPBF Datasheet(PDF) 9 Page - Linear Technology |
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LTC3601EMSETRPBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LTC3601 9 3601f OPERATION The LTC3601 is a current mode, monolithic, step-down regulator capable of providing up to 1.5A of output current. Its unique controlled on-time architecture allows extremely low step-down ratios while maintaining a constant switch- ing frequency. Part operation is enabled by raising the voltage on the RUN pin above 1.25V nominally. Main Control Loop In normal operation the internal top power MOSFET is turned on for a fixed interval determined by an internal one-shot timer (“ON” signal in the Block Diagram). When the top power MOSFET turns off, the bottom power MOSFET turns on until the current comparator, ICMP, trips, thus restarting the one-shot timer and initiating the next cycle. The inductor current is monitored by sensing the voltage drop across the SW and PGND nodes of the bottom power MOSFET. The voltage at the ITH pin sets the ICMP comparator threshold corresponding to the induc- tor valley current. The error amplifier EA adjusts this ITH voltage by comparing an internal 0.6V reference to the feedback signal, VFB,derivedfromtheoutputvoltage.If,for example, the load current increases, the feedback voltage will decrease relative to the internal 0.6V reference. The ITH voltage then rises until the average inductor current matches that of the load current. The operating frequency is determined by the value of the RT resistor, which programs the current for the internal oscillator. An internal phase-locked loop servos the switch- ing regulator on-time to track the internal oscillator edge and force a constant switching frequency. A clock signal can be applied to the SYNC/MODE pin to synchronize the switching frequency to an external source. The regulator defaults to forced continuous operation once the clock signal is applied. At low load currents the inductor current can drop to zero or become negative. If the LTC3601 is configured for Burst Mode operation, this inductor current condition is detected by the current reversal comparator, IREV, which in turn shuts off the bottom power MOSFET and places the part into a low quiescent current sleep state resulting in discontinuous operation and increased efficiency at low load currents. Both power MOSFETs will remain off with the part in sleep and the output capacitor supplying the load current until the ITH voltage rises sufficiently to initi- ate another cycle. Discontinuous operation is disabled by tying the MODE/SYNC pin to ground placing the LTC3601 into forced continuous mode. During forced continuous mode, continuous synchronous operation occurs regard- less of the output load current. “Power Good” Status Output The PGOOD open-drain output will be pulled low if the regulator output exits a ±8% window around the regulation point. This condition is released once regulation within a 5% window is achieved. To prevent unwanted PGOOD glitches during transients or dynamic VOUT changes, the LTC3601 PGOOD falling edge includes a filter time of ap- proximately 40μs. VIN Overvoltage Protection In order to protect the internal power MOSFET devices against transient voltage spikes, the LTC3601 constantly monitors the VIN pin for an overvoltage condition. When VIN rises above 17.5V, the regulator suspends operation by shutting off both power MOSFETs. Once VIN drops below 16.5V, the regulator immediately resumes normal operation. The regulator does not execute its soft-start function when exiting an overvoltage condition. Short-Circuit Protection Foldback current limiting is provided in the event the output is inadvertently shorted to ground. During this condition the internal current limit (ILIM) will be lowered to approximately one-third its normal value. This feature reduces the heat dissipation in the LTC3601 during short- circuit conditions and protects both the IC and the input supply from any potential damage. |
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