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LTC1702A

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Burst Mode Operation

Discontinuous mode removes the resistive loss drop term

in QB, but the LTC1702A is still switching QT and QB on

and off once a cycle. Each time an external MOSFET is

turned on, the internal driver must charge its gate to VCC.

Each time it is turned off, that charge is lost to ground. At

the high switching frequencies that the LTC1702A oper-

ates at, the charge lost to the gates can add up to tens of

milliamps from VCC. As the load current continues to drop,

this quickly become the dominant power loss term, reduc-

ing efficiency once again.

Once again, the LTC1702A switches to a new mode to

minimize efficiency loss: Burst Mode operation. As the

circuit goes deeper and deeper into discontinuous mode,

the QB on-time reduces. When the load drops to the point

that the output begins to rise, the LTC1702A senses this

rise and shuts both QT and QB off completely, skipping

several switching cycles until the output falls back into

range. It then resumes switching in discontinuous mode

and the burst sequence repeats. The total deviation from

the regulated output is within the 1% regulation tolerance

of the LTC1702A.

In Burst Mode operation, both resistive loss and switching

loss are minimized while keeping the output in regulation.

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 LTC1702A—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 fre-

quency 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 LTC1702A 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

LTC1702A run in continuous mode regardless of load.

APPLICATIONS INFORMATION

This does not affect output regulation but does reduce

efficiency 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.

Paralleling Outputs

Synchronous regulators (like the LTC1702A) 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 LTC1702A discontinuous mode allows it to be paral-

leled with another regulator without fighting. A typical

system might use the LTC1702A as a primary regulator

and a small LDO as a backup regulator to keep SRAM alive

when the main power is off. When the LTC1702A 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 LTC1702A 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 LTC1702A from pulling down the

output. The Typical Applications section shows an ex-

ample of such a circuit.

OVERVOLTAGE FAULT

The LTC1702A 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

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 LTC1702A stops all switching, turns both

QB (bottom synchronous) MOSFETs on continuously and

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