LTC3554EPD datasheet(22/36 Pages) LINER

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LTC3554

3554f

In standby mode, each regulator operates hysteretically.

When the FB pin voltage falls below the internal 0.8V

reference, a current source from BVIN to SW turns on,

delivering current through the inductor to the switching

regulator output capacitor and load. When the FB pin

voltage rises above the reference plus a small hysteresis

voltage, that current is shut off. In this way, output regula-

tion is maintained.

Since the power transfer from BVIN to SW is through a

high impedance current source rather than through a low

impedance MOSFET switch, power loss scales with load

current as in a linear low dropout (LDO) regulator, rather

than as in a switching regulator. For near-zero load condi-

tions where regulator quiescent current is the dominant

power loss, standby mode is ideal. But at any appreciable

load current, Burst Mode operation yields the best overall

conversion efﬁciency.

Shutdown

Each step-down switching regulator is shut down and

enabled via the pushbutton interface. In shutdown, each

switching regulator draws only a few nanoamps of leak-

age current from the BVIN pin. Each disabled regulator

also pulls down on its output with a 10k resistor from its

switch pin to ground.

Dropout Operation

It is possible for a step-down switching regulator’s input

voltage to fall near or below its programmed output volt-

age (e.g., a battery voltage of 3.4V with a programmed

output voltage of 3.3V). When this happens, the PMOS

switch duty cycle increases to 100%, keeping the switch on

OPERATION

continuously. Known as dropout operation, the respective

output voltage equals the regulator’s input voltage minus

the voltage drops across the internal P-channel MOSFET

and the inductor.

Soft-Start Operation

In normal operating mode, soft-start works by gradually

increasing the peak inductor current for each step-down

switching regulator over a 500μs period. This allows each

output to rise slowly, helping minimize the inrush current

needed to charge up the output capacitor. A soft-start cycle

occurs whenever a given switching regulator is enabled.

Soft-start occurs only in normal operation, but not in

standby mode. Standby mode operation is already in-

herently current-limited, since the regulator works by

intermittently turning on a current source from BVIN to

SW. Changing the state of the STBY pin while the regu-

lators are operating doesn’t trigger a new soft-start cycle,

to avoid glitching the outputs.

Frequency/Slew Rate Select

The FSEL pin allows an application to dynamically trade

off between highest efﬁciency and reduced electromag-

netic interference (EMI) emission.

When FSEL is high, the switching regulator frequency is

set to 2.25MHz to stay out of the AM radio band. Also,

new patented circuitry is enabled which limits the slew rate

of the switch nodes (SW1 and SW2). This new circuitry

is designed to transition the switch node over a period of

a few nanoseconds, signiﬁcantly reducing radi-ated EMI

and conducted supply noise.

LTC3554EPD Datasheet(PDF) 22 Page - Linear Technology

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