Datasheet

LTC3112
14
3112fc
For more information www.linear.com/LTC3112
The basic LTC3112 application circuit is shown on the front
page of this data sheet. The external component selection
is dependent upon the required performance of the IC in
each particular application given trade-offs such as PCB
area, output voltages, output currents, ripple voltages
and efficiency. This section of the data sheet provides
some basic guidelines and considerations to aid in the
selection of external components and the design of the
application circuit.
V
OUT
AND OVP PROGRAMMING
The buck-boost output voltage is set with an external resis-
tor divider
connected to the FB pin as shown in Figure 2.
The
resistor divider values determine the buck-boost output
voltage according to the following formula:
applicaTions inForMaTion
V
OUT
2.5V < V
OUT
< 14V
R3
R4
R1
R2
C1
LTC3112
GND
OVP FB
3112 F02
Figure 2. Setting the Output Voltage
V
OUT
= 0.8V • 1+
R1
R2






If accurate overvoltage protection is required, a second
resistor divider (R3 and R4) may be connected to the OVP
pin to program the overvoltage protection threshold where
the LTC3112 will stop switching.
V
OVP
= 0.83V • 1+
R3
R4






A small capacitor, C1, in parallel with R4 may be needed
to provide filtering to prevent nuisance trips during a load
step. A soft-start cycle will be initiated if an overvoltage
event occurs.
INDUCTOR SELECTION
To achieve high efficiency, a low ESR inductor should be
utilized for the buck-boost converter. In addition, the buck-
boost inductor must have a saturation current rating that
is greater than the worst case average inductor current
plus half the ripple current. The peak-to-peak inductor
current ripple for buck or boost mode operation can be
calculated from the following formulas:
∆I
L,P−P, BUCK
=
V
OUT
f•L
V
IN
− V
OUT
V
IN






A
∆I
L,P−P, BOOST
=
V
IN
f•L
V
OUT
− V
IN
V
OUT






A
operaTion
Burst Mode operation, the error amplifier is configured for
low power operation and used to hold the compensation
pin COMP, to reduce transients that may occur during
transitions from and to burst and PWM mode.
OUTPUT CURRENT MONITOR
The LTC3112 includes a circuit that sources an approximate
24µA/A current replica of the V
OUT
(or SWD) current. This
current is typically passed through a resistor from I
OUT
to GND and filtered to produce a DC voltage proportional
to average load current. This voltage can be monitored
by an A/D converter to track load conditions. The I
OUT
pin voltage can also control LTC3112’s feedback loop to
regulate I
OUT
current instead of V
OUT
voltage. The accuracy
of the I
OUT
replica depends on factors such as duty cycle,
V
IN
and V
OUT
voltages, operating frequency etc. The I
OUT
pin’s DC voltage must be less than V
CC
- 1V to provide an
accurate representation of output current.