LTC1702IGN datasheet(15/36 Pages) LINER

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LTC1702

Once again, the LTC1702 switches to a new mode to

minimize efficiency loss: Burst Mode operation. As the

circuit goes deeper and deeper into discontinuous mode,

the total time QT and QB are on reduces. However, the ratio

of the time that QT is on to the time that QB is on must

remain constant for the output to stay in regulation. An

internal timer circuit forces QT to stay on for at least 10%

of a normal switching cycle. When the load drops to the

point that the output requires less than 10% on-time at QT,

the output voltage will begin to rise. The LTC1702 senses

this rise and shuts both QT and QB off completely, skip-

ping several switching cycles until the output falls back

into range. It then resumes switching in discontinuous

mode with QT at 10% duty cycle and the burst sequence

repeats. The total deviation from the regulated output is

within the 1% regulation tolerance of the LTC1702.

In Burst Mode operation, both resistive loss and switching

loss are minimized while keeping the output in regulation.

The ripple current will be set by the 10% QT on-time and

the input supply voltage and is the lowest of all three

operating modes. As the load current falls to zero in Burst

Mode operation, the most significant loss term becomes

the 3mA quiescent current drawn by each side of the

LTC1702—usually much less than the minimum load

current in a typical low voltage logic system. Burst Mode

operation maximizes efficiency at low load currents, but

can cause low frequency ripple in the output voltage as the

cycle-skipping circuitry switches on and off.

FCB Pin

In some circumstances, it is desirable to control or disable

discontinuous and Burst Mode operations. The FCB (Force

Continuous Bar) pin allows the user to do this. When the

FCB pin is high, the LTC1702 is allowed to enter discon-

tinuous and Burst Mode operations at either side as

required. If FCB is taken low, discontinuous and Burst

Mode operations are disabled and both sides of the

LTC1702 run in continuous mode regardless of load. This

does not affect output regulation but does reduce effi-

ciency at low output currents. The FCB pin threshold is

specified at 0.8V

±50mV, and includes 20mV of hyster-

esis, allowing it to be used as a precision small-signal

comparator.

APPLICATIONS INFORMATION

Paralleling Outputs

Synchronous regulators (like the LTC1702) are known for

their bullheadedness when their outputs are paralleled

with other regulators. In particular, a synchronous regu-

lator paralleled with another regulator whose output is

slightly higher (perhaps just by millivolts) will happily sink

amps of current attempting to pull its own output back

down to what it thinks is the right value.

The LTC1702 discontinuous mode allows it to be paral-

leled with another regulator without fighting. A typical

system might use the LTC1702 as a primary regulator and

a small LDO as a backup regulator to keep SRAM alive

when the main power is off. When the LTC1702 is shut

down (by pulling RUN/SS to ground), both QT and QB turn

off and the output goes into a high impedance state,

allowing the smaller regulator to support the output volt-

age. However, if the LTC1702 is powered back up in

continuous mode, it will begin a soft-start cycle with a low

duty cycle, pulling the output down and corrupting the

data stored in SRAM. The solution is to tie FCB high,

allowing the device to start in discontinuous mode. Any

reverse current flow in QB will trip the discontinuous mode

circuitry, preventing the LTC1702 from pulling down the

output. The Typical Applications section shows an

example of such a circuit.

OVERVOLTAGE FAULT

The LTC1702 includes a single overvoltage fault flag for

both channels: FAULT. FAULT is an open-drain output with

an internal 10

µA pull-up. If either FB pin rises more than

15% above the nominal 800mV value for more than 25

µs,

the overvoltage comparator will trip, setting an internal

latch. This latch releases the pull-down at FAULT, allowing

the 10

µA pull-up to take it high. When FAULT goes high,

the LTC1702 stops all switching, turns both QB (bottom

synchronous) MOSFETs on continuously and remains in

this state until both RUN/SS pins are pulled low simulta-

neously, the power supply is recycled, or the FAULT pin is

pulled low externally. This behavior is intended to protect

a potentially expensive load from overvoltage damage at

all costs. Under some conditions, this behavior can cause

the output voltage to undershoot below ground. If latched

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