15
Rev. C
APPLICATIONS INFORMATION
500kHz. The slope compensation is set by the R
T
value,
while the minimum slope compensation required to avoid
subharmonic oscillations is established by the inductor
size, input voltage, and output voltage. Since the syn-
chronization 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
synchronization frequencies.
For some applications it is desirable for the LT8607 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 opera-
tion as shown in Figure 2 in an earlier 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 R
T
as the low frequency, and modulates up to approximately
20% higher than the frequency set by R
T
. To enable spread
spectrum mode, tie SYNC to INTV
CC
or drive to a voltage
between 3.2V and 5V.
The LT8607 does not operate in forced continuous mode
regardless of SYNC signal. The LT8607 DFN is always
programmed for Burst Mode operation and cannot enter
pulse-skipping mode. The LT8607B DFN is programmed
for pulse-skipping mode and cannot enter Burst Mode
operation.
Shorted and Reversed Input Protection
The LT8607 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. This allows for tailoring the LT8607
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 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, tied high or floated, the LT8607 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 LT8607
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 LT8607’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 LT8607’s internal circuitry will pull its quiescent
current through its SW pin. This is acceptable if the sys-
tem 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 LT8607 can pull current from the output through the
SW pin and the V
IN
pin. Figure 4 shows a connection of
the V
IN
and EN/UV pins that will allow the LT8607 to run
only when the input voltage is present and that protects
against a shorted or reversed input.
V
IN
V
IN
LT8607
GND
D1
8607 F04
EN/UV
Figure 4. Reverse V
IN
Protection
