LT8611
18
8611fa
For more information www.linear.com/LT8611
APPLICATIONS INFORMATION
Figure 5. Reverse V
IN
Protection
The LT8611 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 LT8611
may be synchronized over a 200kHz to 2.2MHz range. The
R
T
resistor should be chosen to set the LT8611 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
LT8611
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
tied high either to a logic output or to the INTVCC pin.
The LT8611 does not operate in forced continuous mode
regardless of SYNC signal. Never leave the SYNC pin
floating.
Shorted and Reversed Input Protection
The LT8611 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
s
our
ce or tied high, the LT8611 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 LT8611
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 LT8611’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 LT8611’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, regard-
less of EN, parasitic body diodes inside the LT8611 can
pull current from the output through the SW pin and
the V
IN
pin. Figure 5 shows a connection of the V
IN
and
EN/UV pins that will allow the LT8611
to run only when
the input voltage is present and that protects against a
shorted or reversed input.
V
IN
V
IN
D1
LT8611
EN/UV
8611 F05
GND
