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LTC1879 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1879 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LTC1879 1879f OPERATIO Main Control Loop The LTC1879 uses a constant frequency, current mode step-down architecture. Both the top MOSFET and syn- chronous bottom MOSFET switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP turns the top MOSFET off is controlled by the voltage on the ITH pin, which is the output of error amplifier EA. When the load current increases, it causes a slight decrease in the feedback voltage, VFB, relative to the 0.8V internal reference, which, in turn, causes the ITH voltage to in- crease until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse direction or the next clock cycle begins. Comparator OVDET guards against transient overshoots > 7.5% by turning the main switch off and keeping it off until the fault is removed. Burst Mode Operation The LTC1879 is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. To enable Burst Mode operation, simply tie the SYNC/MODE pin to SVIN or connect it to a logic high (VSYNC/MODE > 1.5V). To disable Burst Mode operation and enable PWM pulse skipping mode, connect the SYNC/ MODE pin to SGND. In this mode, the efficiency is lower at light loads but becomes comparable to Burst Mode opera- tion when the output load exceeds 100mA. The advantage of pulse skipping mode is lower output ripple. When the converter is in Burst Mode operation, the peak current of the inductor is set to approximately 400mA, even though the voltage at the ITH pin indicates a lower value. The voltage at the ITH pin drops when the inductor’s average current is greater than the load requirement. As the ITH voltage drops below approximately 0.45V, the BURST comparator trips, turning off both power MOSFETs. The ITH pin is then disconnected from the output of the EA amplifier and held 0.65V above ground. In sleep mode, both power MOSFETs are held off and the internal circuitry is partially turned off, reducing the quies- cent current to 15 µA. The load current is now being supplied from the output capacitor. When the output voltage drops, the ITH pin reconnects to the output of the EA amplifier and the top MOSFET is again turned on and this process repeats. Soft-Start/Run Function The RUN/SS pin provides a soft-start function and a means to shut down the LTC1879. Soft-start reduces the input current surge by gradually increasing the regulator’s maximum output current. This pin can also be used for power supply sequencing. Pulling the RUN/SS pin below 0.7V shuts down the LTC1879, which then draws < 1 µA current from the sup- ply. This pin can be driven directly from logic circuits as shown in Figure 1. It is recommended that this pin is driven to VIN during normal operation. Note that there is no current flowing out of this pin. Soft-start action is accom- plished by connecting an external RC network to the RUN/ SS pin as shown in Figure 1. The LTC1879 actively pulls the RUN/SS pin to ground under low input supply voltage conditions. (Refer to Block Diagram) Figure 1. RUN/SS Pin Interfacing 3.3V OR 5V VIN RUN/SS D1* 0.32V RSS CSS *ZETEX BAT54 1879 F01 |
Similar Part No. - LTC1879_15 |
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Similar Description - LTC1879_15 |
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