LTC3115-2_15 Datasheet, PDF (17/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

LTC3115-2
Operation
threshold. Therefore, in order to place the part in shut-
down and reduce the input current to its minimum level
(3µA typical) it is necessary to ensure that the RUN pin
is brought below the worst-case logic threshold (0.3V).
The RUN pin is a high voltage input and can be connected
directly to V_IN to continuously enable the part when the
input supply is present. If the RUN pin is forced above
approximately 5V it will sink a small current as given by
the following equation:
I_RUN
≅
V_RUN – 5V
5MΩ
With the addition of an external resistor divider as shown
in Figure 3, the RUN pin can be used to establish a custom
input undervoltage lockout threshold. The buck-boost con-
verter is enabled when the RUN pin reaches 1.21V which
allows the rising UVLO threshold to be set via the resis-
tor divider ratio. Once the RUN pin reaches the threshold
voltage, the comparator switches and the buck-boost
converter is enabled. In addition, an internal 0.5µA (typi-
cal) current source is enabled which sources current out
of the RUN pin raising the RUN pin voltage away from
the threshold. In order to disable the part, V_IN must be
reduced sufficiently to overcome the hysteresis generated
by this current as well as the 100mV hysteresis of the RUN
comparator. As a result, the amount of hysteresis can be
independently programmed without affecting the rising
UVLO threshold by scaling the values of both resistors.
THERMAL CONSIDERATIONS
The power switches in the LTC3115-2 are designed to op-
erate 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, in many applications the V_CC regulator is
operated with large input-to-output voltage differentials
resulting in significant levels of power dissipation in its
pass element which can add significantly to the total power
dissipated within the IC. As a result, careful consideration
must be given to the thermal environment of the IC in order
to optimize efficiency and ensure that the LTC3115-2 is
able to provide its full-rated output current. Specifically,
the exposed die attach pad of both the DHD and FE pack-
ages should be soldered to the PC board and the PC board
should be designed 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 to other
PCB layers containing a large area of exposed copper.
If the die temperature exceeds approximately 165°C, the
IC will enter overtemperature shutdown and all switching
will be inhibited. The part will remain disabled until the
die cools by approximately 10°C. The soft-start circuit is
re-initialized in overtemperature shutdown to provide a
smooth recovery when the fault condition is removed.
V_IN
LTC3115-2
0.5µA
V_IN
R1
1.21V
RUN
R2
+
0.8V –
–
+ _ENA
INPUT LOGIC
THRESHOLD
ENABLE
SWITCHING
ENABLE
V_CC REGULATOR AND
CONTROL CIRCUITS
31152 F03
Figure 3. Accurate RUN Pin Comparator
For more information www.linear.com/LTC3115-2
31152fa
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