Datasheet

LT8609/LT8609A/LT8609B
12
Rev. H
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OPERATION
The LT8609/LT8609A/LT8609B is a monolithic constant
frequency current mode step-down DC/DC converter. An
oscillator with frequency set using a resistor on the RT pin
turns on the internal top power switch at the beginning of
each clock cycle. Current in the inductor then increases
until the top switch current comparator trips and turns off
the top power switch. The peak inductor current at which
the top switch turns off is controlled by the voltage on the
internal VC node. The error amplifier servos the VC node
by comparing the voltage on the V
FB
pin with an inter-
nal 0.782V reference. When the load current increases it
causes a reduction in the feedback voltage relative to the
reference leading the error amplifier to raise the VC volt
-
age until the average inductor current matches the new
load current. When the top power switch turns off the
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in excess current flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
If the EN/UV pin is low, the LT8609/LT8609A/LT8609B is
shut down and draws 1µA from the input. When the EN/UV
pin is above 1V, the switching regulator becomesactive.
To optimize efficiency at light loads, the LT8609/LT8609A
enters Burst Mode operation during light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input supply
current to 1.7μA. In a typical application, 2.5μA will be
consumed from the input supply when regulating with no
load. The SYNC pin is tied low to use Burst Mode opera-
tion and can be floated to use pulse-skipping mode. If a
clock is applied to the SYNC pin the part will synchronize
to an external clock frequency and operate in pulse-skip
-
ping mode. While in pulse-skipping mode the oscillator
operates continuously and positive SW transitions are
aligned to the clock. During light loads, switch pulses
are skipped to regulate the output and the quiescent cur-
rent will be several mA. The SYNC pin may be tied high
for spread spectrum modulation mode, and the LT8609/
LT8609A will operate similar to pulse-skipping mode but
vary the clock frequency to reduce EMI.
Comparators monitoring the FB pin voltage will pull the PG
pin low if the output voltage varies more than ±8.5% (typi -
cal) from the set point, or if a fault condition is present.
The oscillator reduces the LT8609/LT8609A’s operating
frequency when the voltage at the FB pin is low. This fre
-
quency foldback helps to control the inductor current when
the output voltage is lower than the programmed value
which occurs during start-up. When a clock is applied to
the SYNC pin the frequency foldback is disabled.
The LT8609B has no SYNC pin and only operates in pulse-
skipping mode.
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