Datasheet

LTC3112
12
3112fc
For more information www.linear.com/LTC3112
operaTion
THERMAL CONSIDERATIONS
For the LTC3112 to provide maximum output power, it is
imperative that a good thermal path be provided to dis
-
sipate the heat generated within the package. This can be
accomplished by taking advantage of the large thermal
pad on the underside of the IC. It is recommended that
multiple vias in the printed circuit board be used to conduct
the heat away from the IC and into a copper plane with as
much area as possible.
The efficiency and maximum output current capability of
the LTC3112 will be reduced if the converter is required to
continuously deliver large amounts of power or operate at
high ambient temperatures. The amount of output current
derating is dependent upon factors such as board ground
plane or heat sink area, ambient operating temperature,
and the input/output voltages of the application. A poor
thermal design can cause excessive heating, resulting in
impaired performance or reliability.
The temperature rise curves given in the Typical Perfor
-
mance Characteristics
section can be used as a general
guide to predict junction temperature rise from ambient.
These curves were generated by mounting the LTC3112
to the 4-layer FR4 Demo Board printed circuit board layout
shown
in Figure 3. The curves were taken with the board
at
room temperature, elevated ambient temperatures will
result in greater thermal rise rates due to increased initial
R
DS(ON)
of the N-Channel MOSFETs. The die temperature
of the LTC3112 should be kept below the maximum junc-
tion rating of 150°C.
In
the event that the junction temperature gets too high
(approximately 150ºC), the current limit will be linearly
decreased from its typical value. If the junction temperature
continues to rise and exceeds approximately 170°C the
LTC3112 will be disabled. All power devices are turned off
and all switch nodes put to a high impedance state. The
soft-start circuit for the converter is reset during thermal
shutdown to provide a smooth recovery once the overtem
-
perature condition is eliminated. When the die temperature
drops to approximately 160°C the LTC3112 will re-start.
UNDERVOLTAGE LOCKOUTS
The LTC3112 buck-boost converter is disabled and all
power devices are turned off until the V
CC
supply reaches
2.35V (typical). The soft-start circuit is reset during under-
voltage lockout
to provide a smooth restart once the input
voltage rises above the undervoltage lockout threshold. A
second UVLO circuit disables all
power devices if V
IN
is
below 2.3V rising, 2.0V falling (typical). This can provide
a lower V
IN
operating range in applications where V
CC
is
powered from an alternate source or V
OUT
after start-up.
INDUCTOR DAMPING
When the LTC3112 is disabled (RUN = 0V) or sleeping
during Burst Mode operation (PWM/SYNC = 0V), active
circuits “damp” the inductor voltage through a 250Ω (typi
-
cal) impedance
from SW1 and SW2 to GND to minimize
ringing and reduce EMI.
PWM MODE OPERATION
When the PWM/SYNC pin is held high, the LTC3112 buck-
boost converter operates in a fixed frequency pulse width
modulation (PWM) mode using voltage mode control. Full
output current capability is only available in PWM mode. A
proprietary switching algorithm allows the converter to tran
-
sition between
buck,
buck-boost, and boost modes without
discontinuity in inductor current. The switch topology for
the buck-boost converter is shown in Figure 1.
V
IN
V
OUT
A
L
B
D
C
3112 F01
Figure 1. Buck-Boost Switch Topology
When the input voltage is significantly greater than the
output voltage, the buck-boost converter operates in buck
mode. Switch D turns on at maximum duty cycle and switch
C turns on just long enough to refresh the voltage on the
BST2 capacitor used to drive switch D. Switches A and B