ltc1702a Linear Technology Corporation, ltc1702a Datasheet - Page 15

• Manufacturer Part Number: ltc1702a
• Description: Dual 550khz Synchronous 2-phase Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

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 V
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 V
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
CC
. As the load current continues to drop,
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INFORMATION
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.
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 10µA pull-up. If either FB pin rises more
than 15% above the nominal 800mV value for more than
25µs, the overvoltage comparator will trip, setting an
internal latch. This latch releases the pull-down at FAULT,
allowing the 10µA pull-up to take it high. When FAULT
goes high, the LTC1702A stops all switching, turns both
QB (bottom synchronous) MOSFETs on continuously and
LTC1702A
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