LTC3545 LINER [Linear Technology], LTC3545 Datasheet - Page 11

• Manufacturer Part Number: LTC3545
• Description: Triple 800mA Synchronous Step-Down Regulator-2.25MHz
• Manufacturer: LINER [Linear Technology]

OPERATION
SOFT-START
Soft-start reduces surge currents on V
overshoot during start-up. Soft-start on the LTC3545/
LTC3545-1 is implemented by internally ramping the refer-
ence signal fed to the error amplifi er 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
IN
and output
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
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).
(ALL 1V/DIV)
2V/DIV
V
V
V
RUNX
OUT1
OUT2
OUT3
Figure 1. Start-Up from Shutdown, No Load
T
V
I
LOAD
A
IN
= 25°C
= 3.6V
LTC3545/LTC3545-1
= 0mA, ALL CHANNELS
200μs/DIV
DS(ON)
of the P-channel switch
3545 F01
11
35451fb