6
LTC1474/LTC1475
OPERATIO
U
(Refer to Functional Diagram)
The LTC1474/LTC1475 are step-down converters with
internal power switches that use Burst Mode operation to
keep the output capacitor charged to the proper output
voltage while minimizing the quiescent current. Burst
Mode operation functions by using short “burst” cycles to
ramp the inductor current through the internal power
switch and external Schottky diode, followed by a sleep
cycle where the power switch is off and the load current is
supplied by the output capacitor. During sleep mode, the
LTC1474/LTC1475 draw only 9µA typical supply current.
At light loads, the burst cycles are a small percentage of
the total cycle time; thus the average supply current is very
low, greatly enhancing efficiency.
Burst Mode Operation
At the beginning of the burst cycle, the switch is turned on
and the inductor current ramps up. At this time, the internal
current comparator is also turned on to monitor the switch
current by measuring the voltage across the internal and
optional external current sense resistors. When this volt-
age reaches 100mV, the current comparator trips and
pulses the 1-shot timer to start a 4.75µs off-time during
which the switch is turned off and the inductor current
ramps down. At the end of the off-time, if the output
voltage is less than the voltage comparator threshold, the
switch is turned back on and another cycle commences. To
minimize supply current, the current comparator is turned
on only during the switch-on period when it is needed to
monitor switch current. Likewise, the 1-shot timer will only
be on during the 4.75µs off-time period.
The average inductor current during a burst cycle will
normally be greater than the load current, and thus the
output voltage will slowly increase until the internal volt-
age comparator trips. At this time, the LTC1474/LTC1475
go into sleep mode, during which the power switch is off
and only the minimum required circuitry is left on: the
voltage comparator, reference and low battery compara-
tor. During sleep mode, with the output capacitor supply-
ing the load current, the V
FB
voltage will slowly decrease
until it reaches the lower threshold of the voltage com-
parator (about 5mV below the upper threshold). The
voltage comparator then trips again, signaling the LTC1474/
LTC1475 to turn on the circuitry necessary to begin a new
burst cycle.
Peak Inductor Current Programming
The current comparator provides a means for program-
ming the maximum inductor/switch current for each switch
cycle. The 1X sense MOSFET, a portion of the main power
MOSFET, is used to divert a sample (about 5%) of the
switch current through the internal 5Ω sense resistor. The
current comparator monitors the voltage drop across the
series combination of the internal and external sense
resistors and trips when the voltage exceeds 100mV. If the
external sense resistor is not used (Pins 6 and 7 shorted),
the current threshold defaults to the 400mA maximum due
to the internal sense resistor.
Off-Time
The off-time duration is 4.75µs when the feedback voltage
is close to the reference; however, as the feedback voltage
drops, the off-time lengthens and reaches a maximum
value of about 65µs when this voltage is zero. This ensures
that the inductor current has enough time to decay when
the reverse voltage across the inductor is low such as
during short circuit.
Shutdown Mode
Both LTC1474 and LTC1475 provide a shutdown mode
that turns off the power switch and all circuitry except for
the low battery comparator and 1.23V reference, further
reducing DC supply current to 6µA typical. The LTC1474’s
run/shutdown mode is controlled by a voltage level at the
RUN pin (ground = shutdown, open/high = run). The
LTC1475’s run/shutdown mode, on the other hand, is
controlled by an internal S/R flip-flop to provide pushbutton
on/off control. The flip-flop is set (run mode) by a momen-
tary ground at the ON pin and reset (shutdown mode) by
a momentary ground at the LBI/OFF pin.
Low Battery Comparator
The low battery comparator compares the voltage on the
LBI pin to the internal reference and has an open drain
N-channel MOSFET at its output. If LBI is above the
reference, the output FET is off and the LBO output is high
impedance. If LBI is below the reference, the output FET is
on and sinks current. The comparator is still active in
shutdown.
