Datasheet

LT8640/LT8640-1
13
Rev.C
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OPERATION
The LT8640/LT8640-1 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
internal 0.97V 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
voltage 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 more than 10A 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 LT8640/LT8640-1 is shut
down and draws 1µA from the input. When the EN/UV pin
is above 1V, the switching regulator will become active.
To optimize efficiency at light loads, the LT8640/LT8640-1
operates in Burst Mode operation in 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/MODE pin is tied low to use Burst Mode
operation and can be floated to use pulse-skipping mode
(LT8640) or forced continuous mode (FCM) (LT8640-1).
If a clock is applied to the SYNC/MODE pin, the part will
synchronize to an external clock frequency and operate in
either pulse-skipping mode (LT8640) or FCM (LT8640-1).
While in pulse-skipping mode (LT8640 only), the oscilla
-
tor 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 current
will be several hundred µA.
The LT8640-1 can operate in forced continuous mode
(FCM) for fast transient response and full frequency opera
-
tion over a wide load range. When in FCM the oscillator
operates continuously and positive SW transitions are
aligned to the clock. Negative inductor current is allowed.
The LT8640-1 can sink current from the output and return
this charge to the input in this mode, improving load step
transient response.
To improve EMI/EMC, the LT8640/LT8640-1 can operate in
spread spectrum mode. This feature varies the clock with
a triangular frequency modulation of +20%. For example,
if the LT8640/LT8640-1’s frequency is programmed to
switch at 2MHz, spread spectrum mode will modulate the
oscillator between 2MHz and 2.4MHz. The SYNC/MODE
pin should be tied high to INTV
CC
(~3.4V) or an external
supply of 3V to 4V to enable spread spectrum modula
-
tion with either pulse-skipping mode (LT8640) or forced
continuous mode (LT8640-1).
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will
draw current from V
IN
. The BIAS pin should be connected
to V
OUT
if the LT8640/LT8640-1 output is programmed
at 3.3V to 25V.
Comparators monitoring the FB pin voltage will pull the
PG pin low if the output voltage varies more than ±8%
(typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8640/LT8640-1’s operating
frequency when the voltage at the FB pin is low. This
frequency foldback helps to control the inductor current
when the output voltage is lower than the programmed
value which occurs during start-up or overcurrent condi
-
tions. When a clock is applied to the SYNC/MODE pin, the
SYNC/MODE pin is floated, or held DC high, the frequency
foldback is disabled and the switching frequency will slow
down only during overcurrent conditions.
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