LTC3545-1_15 Datasheet, PDF (11/20 Pages) Linear Technology – Triple 800mA Synchronous Step-Down Regulator–2.25MHz

LTC3545/LTC3545-1
OPERATION
SOFT-START
Soft-start reduces surge currents on V_IN and output
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
input supply voltages, the R_DS(ON) of the P-channel switch
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).
V_OUT1
V_OUT2
V_OUT3
(ALL 1V/DIV)
RUNX
2V/DIV
T_A = 25°C
200μs/DIV
V_IN = 3.6V
I_LOAD = 0mA, ALL CHANNELS
3545 F01
Figure 1. Start-Up from Shutdown, No Load
35451fb
11