LTC3545/LTC3545-1

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Figure 1. Start-Up from Shutdown, No Load

SOFT-START

overshoot during start-up. Soft-start on the LTC3545/

LTC3545-1 is implemented by internally ramping the refer-

ence signal fed to the error amplifier over approximately a

1ms period. Figure 1 shows the behavior of the regulator

channels during start-up.

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 volt-

age 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

increases (see Typical Performance Characteristics).

Therefore, the user should calculate the power dissipation

when the LTC3545/LTC3545-1 is used at 100% duty cycle

with low input voltage (See Thermal Considerations in the

Applications Information section).

OPERATION

200μs/DIV

RUNX

2V/DIV

(ALL 1V/DIV)

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