LT8614
19
8614fc
For more information www.linear.com/LT8614
APPLICATIONS INFORMATION
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 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 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 frequency
will slow while the output voltage is lower than the pro
-
grammed level. If the SYNC pin is connected to a clock
source 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 applications
or in battery-backup systems where a battery or some
other supply is diode ORed with the LT8614’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 LT8614’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 LT8614 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 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
further. The maximum load current should be derated
as the ambient temperature approaches the maximum
junction rating. Power dissipation within the LT8614 can
be estimated 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.
