LTC3114-1
17
31141fb
For more information www.linear.com/LTC3114-1
operaTion
The RUN comparator is relatively noise insensitive, but
there may be cases dues to PCB layout, very large value
resistors for R1 and R2 or proximity to noisy components
where noise pickup is unavoidable and may cause the
turn on or turn off of the IC to be intermittent. In these
cases, a filter capacitor can be added across R2 to ensure
proper operation.
THERMAL CONSIDERATIONS
The power switches of the LTC3114-1 are designed to
operate continuously with currents up to the internal
current limit thresholds. However, when operating at high
current levels, there may be significant heat generated
within the IC. In addition, the LDO regulator can generate
a significant amount of heat when V
IN
is very high. This
adds to the total power dissipation of the IC. As described
elsewhere in this data sheet, bootstrapping of the LDO
for 5V output applications can essentially eliminate the
LDO power dissipation term and significantly improve
efficiency. As a result, careful consideration must be given
to the thermal environment of the IC in order to provide
a means to remove heat from the IC and ensure that the
LTC3114-1 is able to provide its full rated output current.
Specifically, the exposed die attach pad of both the DHC
and FE packages must be soldered to a copper layer on
the PCB to maximize the conduction of heat out of the IC
package. This can be accomplished by utilizing multiple
vias from the die attach pad connection underneath the IC
package to other PCB layer(s) containing a large copper
plane. A typical board layout incorporating these concepts
is shown in Figure 4.
If the IC die temperature exceeds approximately 165°C,
overtemperature shutdown will be invoked and all switching
will be inhibited. The part will remain disabled until the die
temperature cools by approximately 10°C. The soft-start
circuit is re-initialized in overtemperature shutdown to
provide a smooth recovery when the IC die temperature
cools enough to resume operation.
Start-Up Into a Pre-Biased V
OUT
Some applications require the LTC3114-1 to start up into
an output voltage (V
OUT
), that is pre-biased by an external
source to some level. It is desirable at LTC3114-1 start-up
to minimize current taken from the pre-bias voltage source
and V
OUT
storage capacitor to prevent V
OUT
glitches and
currents fed backwards into the V
IN
power source of the
LTC3114-1.
If the LTC3114-1 V
IN
voltage is higher than the pre-biased
V
OUT
, indicating buck mode operation, then there will
be minimal reverse current at start-up. However, if the
LTC3114-1 V
IN
voltage is lower than the pre-biased V
OUT
,
indicating boost mode operation, then it is possible for a
brief, but substantial reverse current to be taken by the
LTC3114-1 from V
OUT
. The duration of this reverse cur-
rent is approximately 100µs
to 200µs. The magnitude is
inversely proportional to the V
IN
voltage and dependent
upon external component values.
Prevention of pre-biased V
OUT
reverse current in boost
mode can be achieved in two ways. The preferred method
is to ensure that the pre-biased V
OUT
voltage level is set
higher than the nominal V
OUT
regulation level. For example,
if V
OUT
is pre-biased to 13V, then setting the V
OUT
regu-
lation voltage of the LTC3114
-1 to less than 13V, taking
into account error margins, will result in negligible or
zero reverse current at start-up. If this is not possible,
then a Schottky diode can be connected in series between
V
OUT
of the LTC3114-1 and the converter output to block
reverse current .
High Transient Input Voltage Applications.
Two-layer printed circuit board (PCB) applications that
are subject to ≤ 100μs low to high dV/dT transitions on
V
IN
, where the maximum V
IN
can exceed 20V, require
a 1A or higher current Schottky diode connected from
SW1 to PGND for robust performance. The Schottky
diode is optional, but not required, with four-layer PCB
designs similar to the example in Figure 4. Refer to the
Typical Applications schematics for examples using this
Schottky diode.
