LTC3112_15 Datasheet, PDF (14/32 Pages) Linear Technology – 15V, 2.5A Synchronous Buck-Boost DC/DC Converter

LTC3112
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.
Applications Information
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.
2.5V < V_OUT < 14V
V_OUT
R3
R1
OVP FB
LTC3112
C1 _R4
R2
GND
3112 F02
Figure 2. Setting the Output Voltage
The resistor divider values determine the buck-boost output
voltage according to the following formula:
V_OUT = 0.8V •1+ _RR21^{}
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
3112fc
14
For more information www.linear.com/LTC3112