Datasheet

LT8609/LT8609A/LT8609B
18
Rev. H
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APPLICATIONS INFORMATION
amplitude should have valleys that are below 0.9V and
peaks above 2.7V (up to 5V).
The LT8609/LT8609A will not enter Burst Mode opera-
tion at low output loads while synchronized to an exter-
nal clock, but instead will pulse skip to maintain regu-
lation. The LT8609/LT8609A may be synchronized over
a 200kHz to 2.2MHz range. The R
T
resistor should be
chosen to set the LT8609/LT8609A 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 subharmonic oscil-
lations is established by the inductor size, input voltage,
and output voltage. Since the synchronization frequency
will not change the slopes of the inductor current wave
-
form, if the inductor is large enough to avoid subharmonic
oscillations at the frequency set by RT, then the slope
compensation will be sufficient for all synchronization
frequencies.
For some applications, it is desirable for the LT8609/
LT8609A to operate in pulse-skipping mode, which is the
only mode available to the LT8609B. Pulse-skipping mode
offers two major differences from Burst Mode operation.
First is the clock stays awake at all times and all switch-
ing cycles are aligned to the clock. Second is that full
switching frequency is reached at lower output load than
in Burst Mode operation as shown in Figure1b in an ear-
lier section. These two differences come at the expense
of increased quiescent current. To enable pulse-skipping
mode the SYNC pin is floated.
For some applications, reduced EMI operation may be
desirable, which can be achieved through spread spec-
trum modulation. This mode operates similar to pulse
skipping mode operation, with the key difference that the
switching frequency is modulated up and down by a 3kHz
triangle wave. The modulation has the frequency set by RT
as the low frequency, and modulates up to approximately
20% higher than the frequency set by RT. To enable spread
spectrum mode, tie SYNC to INTV
CC
or drive to a voltage
between 3.2V and 5V.
The LT8609/LT8609A/LT8609B does not operate in forced
continuous mode regardless of SYNC signal.
Shorted and Reversed Input Protection
The LT8609/LT8609A/LT8609B 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 cur-
rent control. Second, the bottom switch current is moni-
tored 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. This allows
for tailoring the LT8609/LT8609A/LT8609B to individual
applications and limiting thermal dissipation during short
circuit conditions.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low or high, or floated the
switching frequency will slow while the output voltage is
lower than the programmed level. If the SYNC pin is con
-
nected to a clock source, the LT8609/LT8609A/LT8609B
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 LT8609/
LT8609A/LT8609B 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
LT8609/LT8609A/LT8609B’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 LT8609/
LT8609A/LT8609B’s internal circuitry will pull its quies-
cent 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 0.7µA.
However, if the V
IN
pin is grounded while the output is
held high, regardless of EN, parasitic body diodes inside
the LT8609/LT8609A/LT8609B can pull current from the
output through the SW pin and the V
IN
pin. Figure3 shows
a connection of the V
IN
and EN/UV pins that will allow the
LT8609/LT8609A/LT8609B to run only when the input
voltage is present and that protects against a shorted or
reversed input.
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