Datasheet

LT8612
15
8612fa
For more information www.linear.com/LT8612
APPLICATIONS INFORMATION
tantalum or electrolytic capacitor at the output. Low noise
ceramic capacitors are also available.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8612. As
previously mentioned, a ceramic input capacitor com-
bined with trace or cable inductance forms a high qual-
ity (underdamped) tank circuit. If the LT8612 circuit is
plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8612’s
rating. This situation is easily avoided (see Linear
Technology Application Note 88).
Enable Pin
The LT8612 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.0V, with 40mV of hysteresis. The EN pin
can be tied to V
IN
if the shutdown feature is not used, or
tied to a logic level if shutdown control is required.
Adding a resistor divider from V
IN
to EN programs the
LT8612 to regulate the output only when V
IN
is above a
desired voltage (see the Block Diagram). Typically, this
threshold, V
IN(EN)
, is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
latch low under low source voltage conditions. The V
IN(EN)
threshold prevents the regulator from operating at source
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
they satisfy the following equation:
V
IN(EN)
=
R3
R4
+1




•1.0V
(10)
where the LT8612 will remain off until V
IN
is above V
IN(EN)
.
Due to the comparator’s hysteresis, switching will not
stop until the input falls slightly below V
IN(EN)
.
When operating in Burst Mode operation for light load
currents, the current through the V
IN(EN)
resistor network
can easily be greater than the supply current consumed
by the LT8612. Therefore, the V
IN(EN)
resistors should be
large to minimize their effect on efficiency at low loads.
INTV
CC
Regulator
An internal low dropout (LDO) regulator produces the
3.4V supply from V
IN
that powers the drivers and the
internal bias circuitry. The INTV
CC
can supply enough cur-
rent for the LT8612’s circuitry and must be bypassed to
ground with a minimum of 1μF ceramic capacitor. Good
bypassing is necessary to supply the high transient cur
-
rents required by the power MOSFET gate drivers. To
improve efficiency the internal LDO can also draw cur-
rent from the BIAS pin when the BIAS pin is at 3.1V or
higher. Typically the BIAS pin can be tied to the output of
the LT8612, or can be tied to an external supply of 3.3V or
above. If BIAS is connected to a supply other than V
OUT
,
be sure to bypass with a local ceramic capacitor. If the
BIAS pin is below 3.0V, the internal LDO will consume
current from V
IN
. Applications with high input voltage and
high switching frequency where the internal LDO pulls
current from V
IN
will increase die temperature because
of the higher power dissipation across the LDO. Do not
connect an external load to the INTV
CC
pin.
Output Voltage Tracking and Soft-Start
T he LT8612 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2.1μA
pulls up the TR/SS pin to INTV
CC
. Putting an external
capacitor on TR/SS enables soft starting the output to
prevent current surge on the input supply. During the soft-
start ramp the output voltage will proportionally track the
TR/SS pin voltage. For output tracking applications, TR/
SS can be externally driven by another voltage source.
From 0V to 0.97V, the TR/SS voltage will override the
internal 0.97V reference input to the error amplifier, thus
regulating the FB pin voltage to that of TR/SS pin. When
TR/SS is above 0.97V, tracking is disabled and the feed-
back voltage will regulate to the internal reference voltage.
The TR/SS pin may be left floating if the function is not
needed.
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
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