Datasheet LTC3604 (Analog Devices) - 9

LTC3604
OPERATION
The LTC3604 is a current mode, monolithic, step-down the part into a low quiescent current sleep state resulting regulator capable of providing up to 2.5A of output current. in discontinuous operation and increased efficiency at low Its unique controlled on-time architecture allows extremely load currents. Both power MOSFETs will remain off with low step-down ratios while maintaining a constant switch- the part in sleep and the output capacitor supplying the ing frequency. Part operation is enabled by raising the load current until the ITH voltage rises sufficiently to initiate voltage on the RUN pin above 1.25V nominally. another cycle. Discontinuous operation is disabled by tying the MODE/SYNC pin to ground placing the LTC3604 into
Main Control Loop
forced continuous mode. During forced continuous mode, In normal operation the internal top power MOSFET is continuous synchronous operation occurs regardless of turned on for a fixed interval determined by an internal the output load current. one-shot timer (“ON” signal in the Block Diagram). When the top power MOSFET turns off, the bottom power MOS-
“Power Good” Status Output
FET turns on until the current comparator, ICMP , trips, The PGOOD open-drain output will be pulled low if the thus restarting the one-shot timer and initiating the next regulator output exits a ±8% window around the regulation cycle. The inductor current is monitored by sensing the point. This condition is released once regulation within a voltage drop across the SW and PGND nodes of the bot- ±5% window is achieved. To prevent unwanted PGOOD tom power MOSFET. The voltage at the ITH pin sets the glitches during transients or dynamic VOUT changes, the ICMP comparator threshold corresponding to the induc- LTC3604 PGOOD falling edge includes a filter time of tor valley current. The error amplifier EA adjusts this ITH approximately 40µs. voltage by comparing an internal 0.6V reference to the feedback signal, VFB, derived from the output voltage. If, for
VIN Overvoltage Protection
example, the load current increases, the feedback voltage In order to protect the internal power MOSFET devices will decrease relative to the internal 0.6V reference. The against transient voltage spikes, the LTC3604 constantly ITH voltage then rises until the average inductor current monitors the V matches that of the load current. IN pin for an overvoltage condition. When VIN rises above 17.5V, the regulator suspends operation The operating frequency is determined by the value of the by shutting off both power MOSFETs. Once VIN drops RT resistor, which programs the current for the internal below 16.5V, the regulator immediately resumes normal oscillator. An internal phase-locked loop servos the switch- operation. The regulator does not execute its soft-start ing regulator on-time to track the internal oscillator edge function when exiting an overvoltage condition. and force a constant switching frequency. A clock signal can be applied to the MODE/SYNC pin to synchronize the
Short-Circuit Protection
switching frequency to an external source. The regulator Foldback current limiting is provided in the event the defaults to forced continuous operation once the clock output is inadvertently shorted to ground. During this signal is applied. condition the internal current limit (ILIM) will be lowered At low load currents the inductor current can drop to zero to approximately one-third its normal value. This feature or become negative. If the LTC3604 is configured for reduces the heat dissipation in the LTC3604 during short- Burst Mode operation, this inductor current condition is circuit conditions and protects both the IC and the input detected by the current reversal comparator, I supply from any potential damage. REV , which in turn shuts off the bottom power MOSFET and places 3604fa For more information www.linear.com/LTC3604 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts