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LTC3834 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3834 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 40 page  LTC3890-3 11 38903f For more information www.linear.com/3890-3 operaTion (Refer to the Functional Diagram) Main Control Loop The LTC3890-3 uses a constant frequency, current mode step-down architecture with the two controller channels operating 180 degrees out-of-phase. During normal op- eration, 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)totheinternal0.800Vreferencevoltage.Whenthe load current 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 MOSFETisturnedonuntileithertheinductorcurrentstarts 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 tied to a voltage less than 4.7V, the VIN LDO (low dropout linear regulator) supplies 5.1V from VIN to INTVCC. If EXTVCC is taken above 4.7V, the VIN LDO is turned off and an EXTVCC LDO is turned on. Once enabled, the EXTVCC LDO supplies 5.1V from EXTVCC to INTVCC. Using the EXTVCC pin allows the INTVCC power to be derived from a high efficiency external source such as one of the LTC3890-3 switching regulator outputs. Each top MOSFET driver is biased from the floating boot- strap capacitor CB, which normally recharges during each 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 LTC3890-3 can be independently shutdownusingtheRUN1andRUN2pins.Pullingeitherof these pins below 1.15V shuts down the main control loop for that controller. Pulling both pins below 0.7V disables both controllers and most internal circuits, including the INTVCC LDOs. In this state, the LTC3890-3 draws only 14µA of quiescent current. Releasing either RUN pin allows a small internal current to pull up the pin to enable that controller. The RUN1 pin has a 7µApull-upcurrentwhiletheRUN2pinhasasmaller0.5µA. The 7µA current on RUN1 is designed to be large enough so that the RUN1 pin can be safely floated (to always en- able the controller) without worry of condensation or other small board leakage pulling the pin down. This is ideal for always-on applications where one or both controllers are enabled continuously and never shut down. The RUN pin may be externally pulled up or driven directly by logic. When driving the RUN pin with a low impedance source, do not exceed the absolute maximum rating of 8V. The RUN pin has an internal 11V voltage clamp that allows the RUN pin to be connected through a resistor to a higher voltage (for example, VIN),solongasthemaximum current into the RUN pin does not exceed 100µA. The start-up of each controller’s output voltage VOUT is controlled by the voltage on the TRACK/SS pin for that channel. When the voltage on the TRACK/SS pin is less than the 0.8V internal reference, the LTC3890-3 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 toprogramasoft-startbyconnectinganexternalcapacitor 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 up to 5V), 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. Typically, this requires connecting to the TRACK/SS pin an external resistor divider from the other supply to ground (see the Applications Information section). |
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