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LTC3788 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3788 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 38 page  LTC3784 11 3784fb For more information www.linear.com/LTC3784 operaTion Main Control Loop The LTC3784 uses a constant-frequency, current mode step-up architecture with the two controller channels operating out of phase. During normal operation, each external bottom 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 1.200V reference voltage. In a boost converter, the required inductor current is determined by the load current, VIN and VOUT. When the 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 in each channel matches the new requirement based on the new load current. After the bottom MOSFET is turned off each cycle, the top MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current comparator, IREV, 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 4.8V, the VBIAS LDO (low dropout linear regulator) supplies 5.4V from VBIAS to INTVCC. If EXTVCC is taken above 4.8V, the VBIAS LDO is turned off and an EXTVCC LDO is turned on. Once enabled, the EXTVCC LDO supplies 5.4V from EXTVCC to INTVCC. Using the EXTVCC pin allows the INTVCC power to be derived from an external source, thus removing the power dissipation of the VBIAS LDO. Shutdown and Start-Up (RUN and SS Pins) The two internal controllers of the LTC3784 can be shut down using the RUN pin. Pulling this pin below 1.28V shuts down the main control loops for both phases. Pull- ing this pin below 0.7V disables both channels and most internal circuits, including the INTVCC LDOs. In this state, the LTC3784 draws only 4μA of quiescent current. NOTE:Donotapplyaheavyloadforanextendedtimewhile the chip is in shutdown. The top MOSFETs are turned off during shutdown and the output load may cause excessive dissipation in the body diodes. 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), as long as the maxi- mum current into the RUN pin does not exceed 100μA. An external resistor divider connected to VIN can set the threshold for converter operation. Once running, a 4.5μA current is sourced from the RUN pin allowing the user to program hysteresis using the resistor values. The start-up of the controller’s output voltage VOUT is controlled by the voltage on the SS pin. When the voltage on the SS pin is less than the 1.2V internal reference, the LTC3784 regulates the VFB voltage to the SS pin voltage instead of the 1.2V reference. This allows the SS pin to be used to program a soft-start by connecting an external capacitor from the SS pin to SGND. An internal 10μA pull-up current charges this capacitor creating a voltage ramp on the SS pin. As the SS voltage rises linearly from 0V to 1.2V (and beyond up to INTVCC), the output voltage rises smoothly to its final value. Light Load Current Operation—Burst Mode Operation, Pulse-Skipping or Continuous Conduction (PLLIN/MODE Pin) TheLTC3784canbeenabledtoenterhighefficiencyBurst 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 ground (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 1.2V and less than INTVCC – 1.3V. |
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Similar Description - LTC3788 |
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