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LTC3827-1 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3827-1 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 32 page  LTC3827-1 10 38271fe OPERATION INTVCC/EXTVCC Power Power for the top and bottom MOSFET drivers and most other internal circuitry is derived from the INTVCC pin. When the EXTVCC pin is left open or tied to a voltage less than 4.7V, an internal 5.25V low dropout linear regulator supplies INTVCC power from VIN. If EXTVCC is taken above 4.7V, the 5.25V regulator is turned off and a 7.5V low dropout linear regulator is enabled that supplies INTVCC power from EXTVCC. If EXTVCC is less than 7.5V (but greater than 4.7V), the 7.5V regulator is in dropout and INTVCC is approximately equal to EXTVCC. When EXTVCC is greater than 7.5V (up to an absolute maximum rating of 10V), INTVCC is regulated to 7.5V. Using the EXTVCC pin allows the INTVCC power to be derived from a high efficiency external source such as one of the LTC3827-1 switching regulator outputs. Each top MOSFET driver is biased from the floating boot- strap capacitor CB, which normally recharges during each off cycle through an external diode when the top MOSFET turns off. If the input voltage VIN decreases to a voltage close to VOUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout detector detects this and forces the top MOSFET off for about one twelfth of the clock period every tenth cycle to allow CB to recharge. Shutdown and Start-Up (RUN1, RUN2 and TRACK/SS1, TRACK/SS2 Pins) The two channels of the LTC3827-1 can be independently shut down using the RUN1 and RUN2 pins. Pulling either of these pins below 0.7V shuts down the main control loop for that controller. Pulling both pins low disables both controllers and most internal circuits, including the INTVCC regulator, and the LTC3827-1 draws only 8μA of quiescent current. Releasing either RUN pin allows an internal 0.5μA current to pull up the pin and enable that controller. Alternatively, the RUN pin may be externally pulled up or driven directly by logic. Be careful not to exceed the Absolute Maximum rating of 7V on this pin. The start-up of each controller’s output voltage VOUTiscon- trolled by the voltage on the TRACK/SS1 and TRACK/SS2 pin. When the voltage on the TRACK/SS pin is less than the 0.8V internal reference, the LTC3827-1 regulates the VFB voltage to the TRACK/SS pin voltage instead of the 0.8V reference. This allows the TRACK/SS pin to be used to program a soft-start 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 linearly from 0V to 0.8V (and beyond), the output voltage VOUT rises smoothly from zero to its final value. Alternatively the TRACK/SS pin can be used to cause the startup of VOUT to “track” that of another supply. Typically, this requires connecting to the TRACK/SS pin an external resistor divider from the other supply to ground (see Ap- plications Information section). When the corresponding RUN pin is pulled low to disable a controller, or when VIN drops below its undervoltage lockout threshold of 3.5V, the TRACK/SS pin is pulled low by an internal MOSFET. When in undervoltage lockout, both controllers are disabled and the external MOSFETs are held off. Light Load Current Operation (Burst Mode Operation, Pulse-Skipping or Continuous Conduction) (PLLIN/MODE Pin) The LTC3827-1 can be enabled to enter high efficiency Burst Mode operation, constant frequency pulse-skipping mode, or forced continuous conduction mode at low load currents. To select Burst Mode operation, tie the PLLIN/ MODE pin to a DC voltage below 0.7V (e.g., SGND). To select forced continuous operation, tie the PLLIN/MODE pin to INTVCC. To select pulse-skipping mode, tie the PLLIN/MODE pin to a DC voltage greater than 0.9V and less than INTVCC – 1.2V. When a controller is enabled for Burst Mode operation, the peak current in the inductor is set to approximately one-tenth of the maximum sense voltage even though the voltage on the ITH pin indicates a lower value. If the aver- age inductor current is lower than the load current, the error amplifier EA will decrease the voltage on the ITH pin. (Refer to Functional Diagram) |
Similar Part No. - LTC3827-1 |
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Similar Description - LTC3827-1 |
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