LT8612
16
8612f
For more information www.linear.com/LT8612
applicaTions inForMaTion
Output Power Good
When the LT8612’s output voltage is within the ±9%
window 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 output). 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 val
-
leys 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 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 LT8612 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 INT
VCC 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 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 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 the
V
IN
pin is grounded while the output is held high, regard-
less of EN, parasitic body diodes inside the LT8612 can
pull
current from the output through the SW pin and
the V
IN
pin. Figure 3 shows a connection of the V
IN
and
EN/UV pins that will allow the LT8612 to run only when
the
input voltage is present and that protects against a
shorted or reversed input.
