Datasheet

LT3681
16
3681f
Caution should be used when synchronizing more than
50% above the initial switching frequency (as set by the
RT resistor) because at higher clock frequencies the
amplitude of the internal slope compensation used to
prevent subharmonic switching is reduced. This type of
subharmonic switching only occurs at input voltages less
than twice output voltage. Higher inductor values will tend
to reduce this problem.
Reversed Input Protection
In some systems, the output may be held high when the
input to the LT3681 is absent. This may occur in battery
charging applications or in battery backup systems where
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied
to the Output. It Also Protects the Circuit from a Reversed Input. The LT3681 Runs Only
When the Input is Present
V
IN
BOOST
DA
BD
FB
RUN/SS
V
C
SW
DC
D4
MBRS140
V
IN
LT3681
3681 F08
V
OUT
BACKUP
GND
APPLICATIONS INFORMATION
a battery or some other supply is diode ORed with the
LT3681’s output. If the V
IN
pin is allowed to fl oat and the
RUN/SS
pin is held high (either by a logic signal or because
it is tied to V
IN
), then the LT3681’s internal circuitry will
pull its quiescent current through its SW pin. This is fi ne
if your system can tolerate a few mA in this state. If you
ground the RUN/SS pin, the SW pin current will drop to
essentially zero. However, if the V
IN
pin is grounded while
the output is held high, then parasitic diodes inside the
LT3681 can pull large currents from the output through
the SW pin and the V
IN
pin. Figure 8 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.