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LTM4603HV Datasheet(PDF) 11 Page - Linear Technology |
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LTM4603HV Datasheet(HTML) 11 Page - Linear Technology |
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11 / 36 page  LTC3613 11 3613fa OPERATION (Refer to Functional Diagram) and turns the bottom MOSFET off immediately and the top MOSFET on. Again in order to avoid shoot-through current there is a small dead time delay before the top MOSFET turns on. The voltage on the ITH pin sets the ICMP valley threshold point. The error amplifier, EA, adjusts this ITH voltage by comparing the differential feedback signal, VOSNS+ − VOSNS–, to a 0.6V internal reference voltage. Consequently, the LTC3613 regulates the output voltage by forcing the differential feedback voltage to be equal to the 0.6V internal reference. The difference amplifier, DA, converts the dif- ferential feedback signal to a single-ended input for the EA. If the load current increases, it causes a drop in the differential feedback voltage relative to the reference. The EA forces ITH voltage to rise until the average inductor current again matches the load current. Differential Output Sensing The output voltage is resistively divided externally to create a feedback voltage for the controller. The internal difference amplifier, DA, senses this feedback voltage along with the output’s remote ground reference to create a differential feedback voltage. This scheme overcomes any ground offsets between local ground and remote output ground, resulting in a more accurate output voltage. The LTC3613 allows for remote output ground deviations as much as ±500mV with respect to local ground. INTVCC/EXTVCC Power Power for the top and bottom MOSFET drivers and most other internal circuitry is derived from the INTVCCpin.Power on the INTVCC pin is derived in two ways: if the EXTVCC pin is below 4.6V, then an internal 5.3V low dropout linear regulator, LDO, supplies INTVCC power from PVIN; if the EXTVCC pin is tied to an external source larger than 4.6V, then the LDO is shut down and an internal switch shorts the EXTVCC pin to the INTVCC pin, thereby powering the INTVCC pin with the external source and helping to increase overall efficiency and decrease internal self heating through power dissipated in the LDO. This external power source could be the output of the step-down switching regulator itself if the output is programmed to higher than 4.6V. The top MOSFET driver is biased from the floating boot- strap capacitor, CB, which normally recharges during each off cycle through an external Schottky diode when the top MOSFET turns off. If the VIN voltage is low and INTVCC drops below 3.65V, undervoltage lockout circuitry disables the external MOSFET driver and prevents the power switches from turning on. Shutdown and Start-Up The LTC3613 can be shut down using the RUN pin. Pull- ing this pin below 1.2V prevents switching, and less than 0.75V disables most of the internal bias circuitry, including the INTVCC regulator. When RUN is less than 0.75V, the shutdown IQ is about 15μA. Pulling the RUN pin between 0.75V and 1.2V enables the controller into a standby mode where all internal circuitry is powered-up except for the MOSFET driver. The standby IQ is about 2mA. Releasing the RUN pin from ground allows an internal 1.3μA current to pull the pin above 1.2V and fully enable the controller including the MOSFET driver. Alternatively, the RUN pin may be externally pulled up or driven directly by logic. Be careful not to exceed the absolute maximum rating of 6V on this pin. When pulled up by a resistor to an external voltage, the RUN pin will sink about 35μA of current before reaching 6V. If the external voltage is above 6V (e.g., VIN), select a large enough resistor value so that the voltage on RUN will not exceed 6V. The start-up of the output voltage, VOUT, is controlled by the voltage on the TRACK/SS pin. When the voltage on the TRACK/SS pin is less than the 0.6V internal reference, the LTC3613 regulates the differential feedback voltage to the TRACK/SS voltage instead of the 0.6V reference. This allows the TRACK/SS pin to be used for programming a ramp-up time for VOUT by connecting an external capacitor from the TRACK/SS pin to SGND. An internal 1μA pull-up current charges this capacitor, creating a voltage ramp on the TRACK/SS pin. As the TRACK/SS voltage rises from 0V to 0.6V (and beyond), the LTC3613 forces the output voltage, VOUT, to ramp up smoothly to its final value. Alternatively, the TRACK/SS pin can be used to track the start-up of VOUT to another external supply as in a master slave configuration. Typically, this requires connecting a resistor divider from the master supply to the TRACK/SS pin (see Soft-Start and Tracking). |
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Similar Description - LTM4603HV |
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