Datasheet LTC3809-1 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | No RSENSE , Low Input Voltage, Synchronous DC/DC Controller with Output Tracking |
| Pages / Page | 24 / 9 — OPERATION (Refer to Functional Diagram). Main Control Loop |
| File Format / Size | PDF / 267 Kb |
| Document Language | English |
OPERATION (Refer to Functional Diagram). Main Control Loop
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LTC3809-1
OPERATION (Refer to Functional Diagram) Main Control Loop
by releasing the RUN pin, the TRACK/SS pin is charged up by an internal 1μA current source and rises linearly from The LTC3809-1 uses a constant frequency, current mode 0V to above 0.6V. The error amplifi er EAMP compares the architecture. During normal operation, the top external feedback signal V P-channel power MOSFET is turned on when the clock sets FB to this ramp instead, and regulates V the RS latch, and is turned off when the current comparator FB linearly from 0V to 0.6V. (ICMP) resets the latch. The peak inductor current at which When the voltage on the TRACK/SS pin is less than the ICMP resets the RS latch is determined by the voltage on the 0.6V internal reference, the LTC3809-1 regulates the VFB ITH pin, which is driven by the output of the error amplifi er voltage to the TRACK/SS pin instead of the 0.6V reference. (EAMP). The VFB pin receives the output voltage feedback Therefore VOUT of the LTC3809-1 can track an external signal from an external resistor divider. This feedback voltage VX during start-up. Typically, a resistor divider on signal is compared to the internal 0.6V reference voltage VX is connected to the TRACK/SS pin to allow the start-up by the EAMP. When the load current increases, it causes a of VOUT to “track” that of VX. For coincident tracking during slight decrease in VFB relative to the 0.6V reference, which start-up, the regulated fi nal value of VX should be larger in turn causes the ITH voltage to increase until the average than that of VOUT, and the resistor divider on VX has the inductor current matches the new load current. While the top same ratio as the divider on VOUT that is connected to VFB. P-channel MOSFET is off, the bottom N-channel MOSFET is See detailed discussions in the Run and Soft-Start/Tracking turned on until either the inductor current starts to reverse, Functions in the Applications Information Section. as indicated by the current reversal comparator IRCMP, or the beginning of the next cycle.
Light Load Operation (Burst Mode Operation, Continuous Conduction or Pulse-Skipping Mode) Shutdown, Soft-Start and Tracking Start-Up (MODE Pin) (RUN and TRACK/SS Pins)
The LTC3809-1 can be programmed for either high effi ciency The LTC3809-1 is shut down by pulling the RUN pin low. Burst Mode operation, forced continuous conduction mode In shutdown, all controller functions are disabled and the or pulse-skipping mode at low load currents. To select chip draws only 9μA. The TG output is held high (off) and Burst Mode operation, tie the MODE pin to VIN. To select the BG output low (off) in shutdown. Releasing the RUN forced continuous operation, tie the MODE pin to a DC pin allows an internal 0.7μA current source to pull up the voltage below 0.4V (e.g., GND). Tying the MODE pin to a RUN pin to VIN. The controller is enabled when the RUN DC voltage above 0.4V and below 1.2V (e.g., VFB) enables pin reaches 1.1V. pulse-skipping mode. The 0.4V threshold between forced continuous operation and pulse-skipping mode can be The start-up of VOUT is based on the three different con- used in secondary winding regulation as described in the nections on the TRACK/SS pin. The start-up of VOUT is Auxiliary Winding Control Using the MODE Pin discussion controlled by the LTC3809-1’s internal soft-start when in the Applications Information section. TRACK/SS is connected to VIN. During soft-start, the error amplifi er EAMP compares the feedback signal VFB to the When the LTC3809-1 is in Burst Mode operation, the peak internal soft-start ramp (instead of the 0.6V reference), current in the inductor is set to approximately one-fourth which rises linearly from 0V to 0.6V in about 1ms. This al- of the maximum sense voltage even though the voltage on lows the output voltage to rise smoothly from 0V to its fi nal the ITH pin indicates a lower value. If the average induc- value while maintaining control of the inductor current. tor current is higher than the load current, the EAMP will decrease the voltage on the I The 1ms soft-start time can be changed by connecting TH pin. When the ITH voltage drops below 0.85V, the internal SLEEP signal goes high the optional external soft-start capacitor CSS between the and the external MOSFET is turned off. TRACK/SS and GND pins. When the controller is enabled 38091fc 9
OPERATION (Refer to Functional Diagram) Main Control Loop
by releasing the RUN pin, the TRACK/SS pin is charged up by an internal 1μA current source and rises linearly from The LTC3809-1 uses a constant frequency, current mode 0V to above 0.6V. The error amplifi er EAMP compares the architecture. During normal operation, the top external feedback signal V P-channel power MOSFET is turned on when the clock sets FB to this ramp instead, and regulates V the RS latch, and is turned off when the current comparator FB linearly from 0V to 0.6V. (ICMP) resets the latch. The peak inductor current at which When the voltage on the TRACK/SS pin is less than the ICMP resets the RS latch is determined by the voltage on the 0.6V internal reference, the LTC3809-1 regulates the VFB ITH pin, which is driven by the output of the error amplifi er voltage to the TRACK/SS pin instead of the 0.6V reference. (EAMP). The VFB pin receives the output voltage feedback Therefore VOUT of the LTC3809-1 can track an external signal from an external resistor divider. This feedback voltage VX during start-up. Typically, a resistor divider on signal is compared to the internal 0.6V reference voltage VX is connected to the TRACK/SS pin to allow the start-up by the EAMP. When the load current increases, it causes a of VOUT to “track” that of VX. For coincident tracking during slight decrease in VFB relative to the 0.6V reference, which start-up, the regulated fi nal value of VX should be larger in turn causes the ITH voltage to increase until the average than that of VOUT, and the resistor divider on VX has the inductor current matches the new load current. While the top same ratio as the divider on VOUT that is connected to VFB. P-channel MOSFET is off, the bottom N-channel MOSFET is See detailed discussions in the Run and Soft-Start/Tracking turned on until either the inductor current starts to reverse, Functions in the Applications Information Section. as indicated by the current reversal comparator IRCMP, or the beginning of the next cycle.
Light Load Operation (Burst Mode Operation, Continuous Conduction or Pulse-Skipping Mode) Shutdown, Soft-Start and Tracking Start-Up (MODE Pin) (RUN and TRACK/SS Pins)
The LTC3809-1 can be programmed for either high effi ciency The LTC3809-1 is shut down by pulling the RUN pin low. Burst Mode operation, forced continuous conduction mode In shutdown, all controller functions are disabled and the or pulse-skipping mode at low load currents. To select chip draws only 9μA. The TG output is held high (off) and Burst Mode operation, tie the MODE pin to VIN. To select the BG output low (off) in shutdown. Releasing the RUN forced continuous operation, tie the MODE pin to a DC pin allows an internal 0.7μA current source to pull up the voltage below 0.4V (e.g., GND). Tying the MODE pin to a RUN pin to VIN. The controller is enabled when the RUN DC voltage above 0.4V and below 1.2V (e.g., VFB) enables pin reaches 1.1V. pulse-skipping mode. The 0.4V threshold between forced continuous operation and pulse-skipping mode can be The start-up of VOUT is based on the three different con- used in secondary winding regulation as described in the nections on the TRACK/SS pin. The start-up of VOUT is Auxiliary Winding Control Using the MODE Pin discussion controlled by the LTC3809-1’s internal soft-start when in the Applications Information section. TRACK/SS is connected to VIN. During soft-start, the error amplifi er EAMP compares the feedback signal VFB to the When the LTC3809-1 is in Burst Mode operation, the peak internal soft-start ramp (instead of the 0.6V reference), current in the inductor is set to approximately one-fourth which rises linearly from 0V to 0.6V in about 1ms. This al- of the maximum sense voltage even though the voltage on lows the output voltage to rise smoothly from 0V to its fi nal the ITH pin indicates a lower value. If the average induc- value while maintaining control of the inductor current. tor current is higher than the load current, the EAMP will decrease the voltage on the I The 1ms soft-start time can be changed by connecting TH pin. When the ITH voltage drops below 0.85V, the internal SLEEP signal goes high the optional external soft-start capacitor CSS between the and the external MOSFET is turned off. TRACK/SS and GND pins. When the controller is enabled 38091fc 9
