LTC3425 datasheet(13/24 Pages) LINER

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LTC3425

3425f

SWITCH A

VOLTAGE

SWITCH B

VOLTAGE

SWITCH C

VOLTAGE

SWITCH D

VOLTAGE

INDUCTOR A

CURRENT

INDUCTOR B

CURRENT

INDUCTOR C

CURRENT

INDUCTOR D

CURRENT

RECTIFIER A

CURRENT

RECTIFIER B

CURRENT

RECTIFIER C

CURRENT

RECTIFIER D

CURRENT

OUTPUT RIPPLE

CURRENT

INPUT CURRENT

3432 F03

OPERATIO

Figure 3. Simplified Voltage and Current Waveforms

for 4-Phase Operation at 75% Duty Cycle

internally controlled. The device can start up under some

load (see the graph Start-Up Current vs Input Voltage).

Soft-start and inrush current limiting is provided during

start-up as well as normal mode. The same soft-start

capacitor is used for each operating mode.

During start-up, all four phases switch in unison. When

either VIN or VOUT exceeds 2.3V, the IC enters normal

operating mode. Once the output voltage exceeds the

input by 0.3V, the IC powers itself from VOUT instead of

VIN. At this point the internal circuitry has no dependency

on the VIN input voltage, eliminating the requirement for a

large input capacitor. The input voltage can drop as low as

0.5V without affecting circuit operation. The limiting factor

for the application becomes the ability of the power source

to supply sufficient energy to the output at the low volt-

ages, and the maximum duty cycle that is clamped at 90%.

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