Datasheet LTC3605A (Analog Devices) - 8

LTC3605A
operaTion Main Control Loop
condition except during start-up when the TRACK pin is The LTC3605A is a current mode monolithic step-down ramping up to 0.6V. regulator. In normal operation, the internal top power Foldback current limiting is provided if the output is MOSFET is turned on for a fixed interval determined by shorted to ground. As VFB drops to zero, the maximum a one-shot timer, OST. When the top power MOSFET sense voltage allowed across the bottom power MOSFET turns off, the bottom power MOSFET turns on until the is lowered to approximately 40% of the original value to current comparator, ICMP , trips, restarting the one-shot reduce the inductor valley current. timer and initiating the next cycle. Inductor current is de- termined by sensing the voltage drop across the bottom
RUN Threshold
power MOSFET ’s VDS. The voltage on the ITH pin sets Pulling the RUN pin to ground forces the LTC3605A into the comparator threshold corresponding to the inductor its shutdown state, turning off both power MOSFETs and valley current. The error amplifier, EA, adjusts this ITH most of its internal control circuitry. Bringing the RUN pin voltage by comparing the feedback signal, VFB, from the above 0.6V turns on the internal reference only, while still output voltage with that of an internal 0.6V reference. If keeping the power MOSFETs off. Further increasing the the load current increases, it causes a drop in the feedback RUN voltage above 1.2V turns on the entire chip. voltage relative to the internal reference. The ITH voltage then rises until the average inductor current matches that
INTVCC Regulator
of the load current. An internal low dropout (LDO) regulator produces the At low load current, the inductor current can drop to zero 3.3V supply that powers the drivers and the internal bias and become negative. This is detected by current reversal circuitry. The INTVCC can supply up to 100mA RMS and comparator, IREV , which then shuts off the bottom power must be bypassed to ground with a minimum of 1µF MOSFET, resulting in discontinuous operation. Both power ceramic capacitor. Good bypassing is necessary to supply MOSFETs will remain off with the output capacitor supplying the high transient currents required by the power MOSFET the load current until the ITH voltage rises above the zero gate drivers. Applications with high input voltage and high current level (0.6V) to initiate another cycle. Discontinu- switching frequency will increase die temperature because ous mode operation is disabled by tying the MODE pin to of the higher power dissipation across the LDO. Connect- INTVCC, which forces continuous synchronous operation ing a load to the INTVCC pin is not recommended since regardless of output load. it will further push the LDO into its RMS current rating The operating frequency is determined by the value of the while increasing power dissipation and die temperature. RT resistor, which programs the current for the internal oscillator. An internal phase-lock loop servos the oscillator
VIN Overvoltage Protection
frequency to an external clock signal if one is present on In order to protect the internal power MOSFET devices the CLKIN pin. Another internal phase-lock loop servos the against transient voltage spikes, the LTC3605A constantly switching regulator on-time to track the internal oscillator monitors the VIN pin for an overvoltage condition. When to force constant switching frequency. VIN rises above 23.5V, the regulator suspends operation Overvoltage and undervoltage comparators OV and UV by shutting off both power MOSFETs. Once VIN drops pull the PGOOD output low if the output feedback volt- below 21.5V, the regulator immediately resumes normal age, V operation. The regulator does not execute its soft-start FB, exits a ±10% window around the regulation point. Continuous operation is forced during OV and UV function when exiting an overvoltage condition. 3605afg 8 For more information www.linear.com/LTC3605A Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Description Revision History Typical Application Related Parts