Datasheet LTC3544B (Analog Devices) - 9

LTC3544B operaTion
MAIN CONTROL LOOP
The LTC3544B uses a constant frequency, current mode VOUT100 step-down architecture. Both the main (P-channel VOUT200A MOSFET) and synchronous (N-channel MOSFET) switches VOUT200B are internal. During normal operation, the internal top power VOUT300 MOSFET is turned on each cycle when the oscillator sets RUNx the RS latch, and turned off when the current comparator, ICOMP , resets the RS latch. The peak inductor current at V 3544B F01 IN = 3.6V 200µs/DIV which ICOMP resets the RS latch, is controlled by the output TA = 25°C of error amplifier EA. When the load current increases, it ALL CHANNELS UNLOADED causes a slight decrease in the feedback voltage FB rela-
Figure 1. Regulator Soft-Start
tive to the 0.8V reference, which in turn, causes the EA
Short-Circuit Protection
amplifier’s output voltage to increase until the average inductor current matches the new load current. While the Short circuit protection is achieved by monitoring the in- top MOSFET is off, the bottom MOSFET is turned on until ductor current. When the current exceeds a predetermined either the inductor current starts to reverse, as indicated by level, the main switch is turned off, and the synchronous the current reversal comparator, I switch is turned on long enough to allow the current in the RCMP , or the beginning of the next clock cycle. inductor to decay below the fault threshold. This prevents a catastrophic inductor current, run-away condition, but
PULSE SKIPPING MODE OPERATION
will still provide current to the output. Output voltage regulation in this condition is not achieved. At light loads, the inductor current may reach zero or reverse on each pulse. The bottom MOSFET is turned off
DROPOUT OPERATION
by the current reversal comparator, IRCMP , and the switch voltage will ring. This is discontinuous mode operation, As the input supply voltage decreases to a value approach- and is normal behavior for the switching regulator. At very ing the output voltage, the duty cycle increases toward the light loads, the LTC3544B will automatically skip pulses maximum on-time. Further reduction of the supply voltage to maintain output regulation. forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will
SOFT-START
then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. An Soft-start reduces surge currents on VIN and output important detail to remember is that at low input supply overshoot during start-up. Soft-start on the LTC3544B is voltages, the RDS(ON) of the P-channel switch increases implemented by internally ramping the reference signal (see Typical Performance Characteristics). Therefore, fed to the error amplifier over approximately a 1ms period. the user should calculate the power dissipation when Figure 1 shows the behavior of the four regulator channels the LTC3544B is used at 100% duty cycle with low input during soft-start. voltage (See Thermal Considerations in the Applications Information section). 3544bfb Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagrams Operation Applications Information Package Description Revision History Related Parts