LTC3544B

8

3544bfa

OPERATION

MAIN CONTROL LOOP

The LTC3544B uses a constant frequency, current mode

step-down architecture. Both the main (P-channel

MOSFET) and synchronous (N-channel 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,

of error amplifier EA. When the load current increases, it

causes a slight decrease in the feedback voltage FB rela-

tive to the 0.8V reference, which in turn, causes the EA

amplifier’s output voltage to increase 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, as indicated by

of the next clock cycle.

PULSE SKIPPING MODE OPERATION

At light loads, the inductor current may reach zero or

reverse on each pulse. The bottom MOSFET is turned off

voltage will ring. This is discontinuous mode operation,

and is normal behavior for the switching regulator. At very

light loads, the LTC3544B will automatically skip pulses

to maintain output regulation.

SOFT-START

overshoot during start-up. Soft-start on the LTC3544B is

implemented by internally ramping the reference signal

fed to the error amplifier over approximately a 1ms period.

Figure 1 shows the behavior of the four regulator channels

during soft-start.

Short-Circuit Protection

Short circuit protection is achieved by monitoring the in-

ductor current. When the current exceeds a predetermined

level, the main switch is turned off, and the synchronous

switch is turned on long enough to allow the current in the

inductor to decay below the fault threshold. This prevents

a catastrophic inductor current, run-away condition, but

will still provide current to the output. Output voltage

regulation in this condition is not achieved.

DROPOUT OPERATION

As the input supply voltage decreases to a value approach-

ing the output voltage, the duty cycle increases toward the

maximum on-time. Further reduction of the supply voltage

forces the main switch to remain on for more than one cycle

until it reaches 100% duty cycle. The output voltage will

then be determined by the input voltage minus the voltage

drop across the P-channel MOSFET and the inductor. An

important detail to remember is that at low input supply

(see Typical Performance Characteristics). Therefore,

the user should calculate the power dissipation when

the LTC3544B is used at 100% duty cycle with low input

voltage (See Thermal Considerations in the Applications

Information section).

Figure 1. Regulator Soft-Start

RUNx

200

μs/DIV

ALL CHANNELS UNLOADED

3544B G10