Datasheet

LT8612
16
8612fa
For more information www.linear.com/LT8612
APPLICATIONS INFORMATION
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, V
IN
voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8612’s output voltage is within the ±9% win -
dow of the regulation point, which is a V
FB
voltage in the
range of 0.883V to 1.057V (typical), the output voltage
is considered good and the open-drain PG pin goes high
impedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and
lower thresholds include 1.3% of hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTV
CC
has fallen too
low, V
IN
is too low, or thermal shutdown.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.4V (this can be ground or a logic low out
-
put). To synchronize the LT8612 oscillator to an external
frequency connect a square wave (with 20% to 80% duty
cycle) to the SYNC pin. The square wave amplitude should
have valleys that are below 0.4V and peaks above 2.0V
(up to 6V).
The LT8612 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 LT8612
may be synchronized over a 200kHz to 2.2MHz range. The
R
T
resistor should be chosen to set the LT8612 switch-
ing frequency equal to or below the lowest synchroni-
zation 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 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 LT8612 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 LT8612 does not operate in forced continuous mode
regardless of SYNC signal. Never leave the SYNC pin
floating.
Shorted and Reversed Input Protection
The LT8612 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 source or tied high, the
LT8612 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 LT8612 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 LT8612’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
LT8612’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
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