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LTC3808 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3808 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  9 LTC3808 3808f OPERATIO Main Control Loop The LTC3808 uses a constant frequency, current mode architecture. During normal operation, the top external P-channel power MOSFET is turned on when the clock sets the RS latch, and is turned off when the current comparator (ICMP) resets the latch. The peak inductor current at which ICMP resets the RS latch is determined by the voltage on the ITH pin, which is driven by the output of the error amplifier (EAMP). The VFB pin receives the output voltage feedback signal from an external resistor divider. This feedback signal is compared to the internal 0.6V reference voltage by the EAMP. When the load current increases, it causes a slight decrease in VFB relative to the 0.6V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. While the top P-channel MOSFET is off, the bottom N-channel MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next cycle. Shutdown, Soft-Start and Tracking Start-Up (RUN and TRACK/SS Pins) The LTC3808 is shut down by pulling the RUN pin low. In shutdown, all controller functions are disabled and the chip draws only 9µA. The TG output is held high (off) and the BG output low (off) in shutdown. Releasing the RUN pin allows an internal 0.7µA current source to pull up the RUN pin to VIN. The controller is enabled when the RUN pin reaches 1.1V. The start-up of VOUT is based on the three different connections on the TRACK/SS pin. The start-up of VOUT is controlled by the LTC3808’s internal soft-start when TRACK/SS is connected to VIN. During soft-start, the error amplifier EAMP compares the feedback signal VFB to the internal soft-start ramp (instead of the 0.6V reference), which rises linearly from 0V to 0.6V in about 1ms. This allows the output voltage to rise smoothly from 0V to its final value while maintaining control of the inductor current. The 1ms soft-start time can be changed by connecting the optional external soft-start capacitor CSS between the TRACK/SS and GND pins. When the controller is enabled by releasing the RUN pin, the TRACK/SS pin is charged up by an internal 1µA current source and rises linearly from 0V to above 0.6V. The error amplifier EAMP compares the feedback signal VFB to this ramp instead, and regulates VFB linearly from 0V to 0.6V. When the voltage on the TRACK/SS pin is less than the 0.6V internal reference, the LTC3808 regulates the VFB voltage to the TRACK/SS pin instead of the 0.6V reference. Therefore VOUT of the LTC3808 can track an external voltage VX during start-up. Typically, a resistor divider on VX is connected to the TRACK/SS pin to allow the start-up of VOUT to “track” that of VX. For coincident tracking during start-up, the regulated final value of VX should be larger than that of VOUT, and the resistor divider on VX has the same ratio as the divider on VOUT that is connected to VFB. See detailed discussions in the Run and Soft-Start/ Tracking Functions in the Applications Information Section. Light Load Operation (Burst Mode Operation, Continuous Conduction or Pulse Skipping Mode) (SYNC/MODE Pin) The LTC3808 can be programmed for either high effi- ciency Burst Mode operation, forced continuous conduc- tion mode or pulse skipping mode at low load currents. To select Burst Mode operation, tie the SYNC/MODE pin to VIN. To select forced continuous operation, tie the SYNC/ MODE pin to a DC voltage below 0.4V (e.g., GND). Tying the SYNC/MODE to a DC voltage above 0.4V and below 1.2V (e.g., VFB) enables pulse skipping mode. The 0.4V threshold between forced continuous operation and pulse skipping mode can be used in secondary winding regula- tion as described in the Auxiliary Winding Control Using SYNC/MODE Pin discussion in the Applications Informa- tion section. When the LTC3808 is in Burst Mode operation, the peak current in the inductor is set to approximate one-fourth of the maximum sense voltage even though the voltage on the ITH pin indicates a lower value. If the average inductor current is higher than the load current, the EAMP will decrease the voltage on the ITH pin. When the ITH voltage drops below 0.85V, the internal SLEEP signal goes high and the external MOSFET is turned off. (Refer to Functional Diagram) |
Similar Part No. - LTC3808_15 |
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Similar Description - LTC3808_15 |
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