LTC3554 datasheet(21/36 Pages) LINER

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LTC3554

3554f

PGOOD Operation

The PGOOD pin is an open-drain output which indicates

that all enabled step-down switching regulators have

reached their ﬁnal regulation voltage. It goes high-imped-

ance 230ms after all enabled switching regulators reach

92% of their regulation value. The delay allows ample time

for an external processor to reset itself. PGOOD may be

used as a power-on reset to a microprocessor powered

by the step-down switching regulators. Since PGOOD is

an open-drain output, a pull-up resistor to an appropriate

power source is needed. A suggested approach is to con-

nect the pull-up resistor to one of the step-down switching

regulator output voltages so that power is not dissipated

while the regulators are disabled.

In hard reset, the PGOOD pin is placed in high impedance

state to minimize current draw from the battery in this

ultralow power state. This will cause the PGOOD pin to

signal the wrong state (high level) if it is pulled up to a

supply that is not shut down in hard reset (e.g. BAT). If

PGOOD is pulled up to one of the step-down switching

regulator outputs then the PGOOD pin will indicate the

correct state (low level) in hard reset because the switch-

ing regulator output will be low.

Normal Operating Mode (STBY Pin Low)

In normal mode (STBY pin low), the regulators perform

as traditional constant-frequency current mode switch-

ing regulators. Switching frequency is determined by an

internal oscillator whose frequency is selectable via the

FSEL pin. An internal latch is set at the start of every

oscillator cycle, turning on the main P-channel MOSFET

switch. During each cycle, a current comparator compares

the inductor current to the output of an error ampliﬁer.

The output of the current comparator resets the internal

latch, which causes the main P-channel MOSFET switch

to turn off and the N-channel MOSFET synchronous

rectiﬁer to turn on. The N-channel MOSFET synchronous

rectiﬁer turns off at the end of the clock cycle, or when

the current through the N-channel MOSFET synchronous

rectiﬁer drops to zero, whichever happens ﬁrst. Via this

mechanism, the error ampliﬁer adjusts the peak inductor

current to deliver the required output power. All neces-

sary compensation is internal to the step-down switching

regulator requiring only a single ceramic output capacitor

for stability.

At light load and no-load conditions, the buck automatically

switches to a power-saving hysteretic control algorithm

that operates the switches intermittently to minimize

switching losses. Known as Burst Mode operation, the

buck cycles the power switches enough times to charge

the output capacitor to a voltage slightly higher than

the regulation point. The buck then goes into a reduced

quiescent current sleep mode. In this state, power loss is

minimized while the load current is supplied by the output

capacitor. Whenever the output voltage drops below a

predetermined value, the buck wakes from sleep and cycles

the switches again until the output capacitor voltage is once

again slightly above the regulation point. Sleep time thus

depends on load current, since the load current determines

the discharge rate of the output capacitor.

Standby Mode (STBY Pin High)

There are situations where even the low quiescent current

of Burst Mode operation is not low enough. For instance,

in a static memory keep alive situation, load current may

fall well below 1μA. In this case, the 25μA typical BVIN

quiescent current per active regulator in Burst Mode

operation becomes the main factor determining battery

run time.

Standby mode cuts BVIN quiescent current down to just

1.5μA per active regulator, greatly extending battery run

time in this essentially no-load region of operation. The

application circuit commands the LTC3554 into and out

of standby mode via the STBY pin logic input. Bringing

the STBY pin high places both regulators into standby

mode, while bringing it low returns them to Burst Mode

operation. In standby mode, load capability drops to 5mA

per regulator.

OPERATION

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