Datasheet

LT8641
20
Rev B
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APPLICATIONS INFORMATION
The LT8641 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation. The LT8641
may be synchronized over a 200kHz to 3MHz range. The
R
T
resistor should be chosen to set the LT8641 switching
frequency equal to or below the lowest synchronization
input. For example, if the synchronization signal will be
500kHz and higher, the R
T
should be selected for 500kHz.
The slope compensation is set by the R
T
value, while the
minimum slope compensation required to avoid subhar-
monic oscillations is established by the inductor size,
input voltage, and output voltage. Since the synchroniza-
tion frequency will not change the slopes of the inductor
current waveform, if the inductor is large enough to avoid
subharmonic oscillations at the frequency set by R
T
, then
the slope compensation will be sufficient for all synchro-
nization frequencies.
For some applications it is desirable for the LT8641 to
operate in pulse-skipping mode, offering two major differ-
ences from Burst Mode operation. First is the clock stays
awake at all times and all switching cycles are aligned
to the clock. Second is that full switching frequency is
reached at lower output load than in Burst Mode operation.
These two differences come at the expense of increased
quiescent current. To enable pulse-skipping mode, the
SYNC pin is floated. Leakage current on this pin should
be <1µA. See Block Diagram for internal pull-up and pull-
down resistance.
The LT8641 features spread spectrum operation to further
reduce EMI emissions. To enable spread spectrum opera-
tion, the SYNC/MODE pin should be tied high either to
INTV
CC
(~3.4V) or an external supply of 3V to 4V. In this
mode, triangular frequency modulation is used to vary the
switching frequency between the value programmed by RT
to approximately 20% higher than that value. The modula-
tion frequency is approximately 3kHz. For example, when
the LT8641 is programmed to 2MHz, the frequency will
vary from 2MHz to 2.4MHz at a 3kHz rate. When spread
spectrum operation is selected, Burst Mode operation is
disabled, and the part will run in pulse-skipping mode.
The LT8641 does not operate in forced continuous mode
regardless of SYNC signal.
Shorted and Reversed Input Protection
The LT8641 will tolerate a shorted output. Several features
are used for protection during output short-circuit and
brownout conditions. The first is the switching frequency
will be folded back while the output is lower than the set
point to maintain inductor current control. Second, the
bottom switch current is monitored such that if inductor
current is beyond safe levels switching of the top switch
will be delayed until such time as the inductor current
falls to safe levels.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the pro-
grammed level. If the SYNC pin is connected to a clock
source, floated, or tied high, the LT8641 will stay at the
programmed frequency without foldback and only slow
switching if the inductor current exceeds safe levels.
There is another situation to consider in systems where the
output will be held high when the input to the LT8641 is
absent. This may occur in battery charging applications or
in battery-backup systems where a battery or some other
supply is diode ORed with the LT8641’s output. If the V
IN
pin is allowed to float and the EN pin is held high (either by
a logic signal or because it is tied to V
IN
), then the LT8641’s
internal circuitry will pull its quiescent current through its
SW pin. This is acceptable if the system can tolerate several
μA in this state. If the EN pin is grounded the SW pin current
will drop to near 1µA. However, if the V
IN
pin is grounded
while the output is held high, regardless of EN, parasitic
body diodes inside the LT8641 can pull current from the
output through the SW pin and the V
IN
pin, which may
damage the IC. Figure 4 shows a connection of the V
IN
and
EN/UV pins that will allow the LT8641 to run only when
the input voltage is present and that protects against a
shorted or reversed input.
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