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LTC3826IUH-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3826IUH-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 36 page  LTC3826 11 3826fc OPERATION (Refer to Functional Diagram) Main Control Loop The LTC3826 uses a constant frequency, current mode step-down architecture with the two controller channels operating 180 degrees out of phase. During normal opera- tion, each external top MOSFET is turned on when the clock for that channel sets the RS latch, and is turned off when the main current comparator, ICMP, resets the RS latch. The peak inductor current at which ICMP trips and resets the latch is controlled by the voltage on the ITH pin, which is the output of the error amplifier EA. The error amplifier compares the output voltage feedback signal at the VFB pin, (which is generated with an external resistor divider connected across the output voltage, VOUT, to ground) to the internal 0.800V reference voltage. When the load cur- rent increases, it causes a slight decrease in VFB relative to the reference, which causes the EA to increase the ITH voltage until the average inductor current matches the new load current. After the top MOSFET is turned off each cycle, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current comparator IR, or the beginning of the next clock cycle. 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 LTC3826 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 LTC3826 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 LTC3826 draws only 4μA 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 VOUT is controlled 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 LTC3826 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 start-up of VOUT to “track” that of another supply. Typi- cally, this requires connecting to the TRACK/SS pin an |
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