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LTC3569EFE-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3569EFE-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTC3569 11 3569f OPERATION tor (NCOMP) signals to turn-off the N-channel switch, so that is does not discharge the output capacitor. When a rising clock edge occurs, the P-channel switch turns on repeating the cycle. The peak inductor current is controlled by the error amplifier (EA) and is influenced by the slope compensation. The error amplifier compares the FB pin voltage to the programmed internal reference (REF). When the load current increases, the FB voltage decreases. When the FB voltage falls below the reference voltage, the error amplifier output rises to increase the peak inductor current until the average inductor current matches the new load current. With the inductor current equal to the load current, the duty cycle will stabilize to a value equal to VOUT/VIN. Low Current Operation At light loads, the FB voltage may rise above the refer- ence voltage. If this occurs the error amplifier signals the control loop to go to sleep, and the P-channel turns off immediately. The inductor current then discharges through the N-channel switch until the inductor current approaches zero; whereupon the SW goes Hi-Z, and the output capacitor supplies power to the load. When the load discharges the output capacitor the feedback voltage falls and the error amp wakes up the buck, restarting the main control loop as if a clock cycle has just begun. This sleep cycle helps minimize the switching losses which are dominated by the gate charge losses of the power devices. Two operating modes are available to control the operation of the LTC3569 at low currents, Burst Mode operation and pulse skip mode. Select Burst Mode operation to optimize efficiency at low output currents. In Burst Mode operation the inductor cur- rent reaches a fixed current before the P-channel switch compares inductor current against the value determined by ITH. This burst clamp causes the output voltage to rise above the regulation voltage and forces a longer sleep cycle. Thisgreatlyreducesswitchinglossesandaveragequiescent current at light loads, at the cost of higher ripple voltage. Pulse skip mode is intended for lower output voltage ripple at light load currents. Here, the peak P-channel current is compared with the value determined by the error amplifier output. Then, the P-channel is turned off and the N-channel switch is turned on until either the next cycle begins or the N-channel comparator (NCOMP) turns off the N-channel switch. If the NCOMP trips, the SW node goes Hi-Z and the buck operates discontinuously. In pulse skip mode the LTC3569 continues to switch at a constant frequency down to very low currents; where it eventually begins skipping pulses. Because the LTC3569 remains active at lighter load currents in pulse skip mode, the efficiency performance is traded off against output voltage ripple and electromagnetic interference (EMI). Dropout Operation When the input supply voltage decreases towards the out- put voltage the duty cycle automatically increases to 100%; which is the dropout condition. In dropout, the P-channel switch is turned on continuously with the output voltage being equal to the input voltage minus the voltage drop across the internal P-channel switch and the inductor. Low Supply Operation The LTC3569 incorporates an undervoltage lockout circuit which shuts down the part when the input voltage drops below 2.5V to prevent unstable operation. The UVLO function does not reset the reference voltage DAC. (See Programming the Reference.) Slave Power Stage When the FB pin of one of the two 600mA regulators is tied to SVIN that regulator’s control circuits are disabled and the regulator’s switch pin is configured to follow a master regulator; either the first 600mA regulator (regulator 2) or the 1.2A regulator (regulator 1). In this way, two regulator power stages are ganged together (e.g. switch pins shorted together to a single inductor) to support higher current levels. This permits three permutations of power levels: three independent regulators at 1.2A, 600mA and 600mA; |
Similar Part No. - LTC3569EFE-PBF |
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Similar Description - LTC3569EFE-PBF |
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