Datasheet

LT8614
19
Rev. E
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APPLICATIONS INFORMATION
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 LT8614 to
operate in pulse-skipping mode, offering two major dif-
ferences 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 fre-
quency 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 LT8614 does not operate in forced continuous mode
regardless of SYNC signal. Never leave the SYNC pin
floating.
Shorted and Reversed Input Protection
The LT8614 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 fre-
quency will slow while the output voltage is lower than
the programmed level. If the SYNC pin is connected to
a clock sour
ce or tied high, the LT8614 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 LT8614
is absent. This may occur in battery charging applica-
tions or in battery-backup systems where a battery or
some other supply is diode ORed with the LT8614’s out-
put. 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 LT8614’s internal circuitry will pull its
quiescent current through its SW pin. This is accept-
able 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 LT8614 can pull current from the output
through the SW pin and the V
IN
pin, which may dam -
age the IC. Figure4 shows a connection of the V
IN
and
EN/UV pins that will allow the LT8614
to run only when
the input voltage is present and that protects against a
shorted or reversed input.
Figure4. Reverse V
IN
Protection
V
IN
V
IN
D1
LT8614
EN/UV
8614 F04
GND
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT8614. The ground pins on the bottom of the package
should be soldered to a ground plane. This ground should
be tied to large copper layers below with thermal vias;
these layers will spread heat dissipated by the LT8614.
Placing additional vias can reduce thermal resistance fur-
ther. The maximum load current should be derated as the
ambient temperature approaches the maximum junction
rating. Power dissipation within the LT8614 can be esti
-
mated by calculating the total power loss from an efficiency
measurement and subtracting the inductor loss. The die
temperature is calculated by multiplying the LT8614 power
dissipation by the thermal resistance from junction to
ambient. The LT8614 will stop switching and indicate a
fault condition if safe junction temperature is exceeded.
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