LT8610
APPLICATIONS INFORMATION
Output Power Good
twodifferencescomeattheexpenseofincreasedquiescent
current. To enable pulse-skipping mode, the SYNC pin is
tied high either to a logic output or to the INTVCC pin.
When the LT8610’s output voltage is within the ±±9
window of the regulation point, which is a V voltage in
FB
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 1.39 of hysteresis.
The LT8610 does not operate in forced continuous mode
regardless of SYNC signal. Never leave the SYNC pin
floating.
Shorted and Reversed Input Protection
TheLT8610willtolerateashortedoutput.Severalfeatures
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.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTV has fallen too
CC
low, V is too low, or thermal shutdown.
IN
Synchronization
ToselectlowrippleBurstModeoperation,tietheSYNCpin
below 0.4V (this can be ground or a logic low output). To
synchronizetheLT8610oscillatortoanexternalfrequency
connect a square wave (with 209 to 809 duty cycle) to
theSYNCpin.Thesquarewaveamplitudeshouldhaveval-
leys that are below 0.4V and peaks above 2.4V (up to 6V).
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
sourceortiedhigh,theLT8610willstayattheprogrammed
frequency without foldback and only slow switching if the
inductor current exceeds safe levels.
The LT8610 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 LT8610
may be synchronized over a 200kHz to 2.2MHz range. The
R resistor should be chosen to set the LT8610 switching
There is another situation to consider in systems where
the output will be held high when the input to the LT8610
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 LT8610’s output. If
T
frequency equal to or below the lowest synchronization
input. For example, if the synchronization signal will be
500kHz and higher, the R should be selected for 500kHz.
T
The slope compensation is set by the R value, while the
T
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
the V pin is allowed to float and the EN pin is held high
IN
(either by a logic signal or because it is tied to V ), then
IN
theLT8610’sinternalcircuitrywillpullitsquiescentcurrent
through its SW pin. This is acceptable if the system can
tolerate several ꢀA in this state. If the EN pin is grounded
the SW pin current will drop to near 1μA. However, if the
subharmonic oscillations at the frequency set by R , then
T
the slope compensation will be sufficient for all synchro-
V pin is grounded while the output is held high, regard-
IN
nization frequencies.
less of EN, parasitic body diodes inside the LT8610 can
pull current from the output through the SW pin and
For some applications it is desirable for the LT8610 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
theclock.Secondisthatfullswitchingfrequencyisreached
at lower output load than in Burst Mode operation. These
the V pin. Figure 3 shows a connection of the V and
IN
IN
EN/UV pins that will allow the LT8610 to run only when
the input voltage is present and that protects against a
shorted or reversed input.
8610p
16
QQ：2881677436 复制
