LTC3114-1_15 Datasheet, PDF (12/34 Pages) Linear Technology – 40V, 1A Synchronous Buck-Boost DC/DC Converter with Programmable Output Current

LTC3114-1
OPERATION
Referring to Figure 2, an internal high gain transconduc-
tance error amplifier monitors V_OUT through a voltage
divider connected to the FB pin and provides an output, VC,
that is used by the current mode control loop to command
the appropriate inductor current level. To ensure stability,
external frequency compensation components (C_P1, C_P2
and R_Z) must be installed between VC and ground. The
procedure for determining these components is provided
in the Applications Information section of this data sheet.
VC is internally connected to the noninverting input of a
high gain, integrating, operational amplifier, referred to
in Figure 2, as the average current amp. The inverting
input of the average current amplifier is connected to
the inductor current sense circuit through a gain setting
resistor (R_CS1) and to its output (VIA) through an internal
frequency compensation network comprised of R_CS2,
R_A
250mΩ
C_CS1 and C_CS2. The average current amplifier’s output
provides the cycle-by-cycle duty cycle command into the
buck-boost PWM circuitry.
The noninverting reference level input to the average cur-
rent amplifier is VC and the feedback or inverting input
is driven from the inductor current sensing circuitry. The
inductor current sensing circuitry alternately measures
the current through switches A and B. The output of the
sensing circuitry produces a voltage across resistor R_X
that resembles the inductor current waveform transformed
to a voltage. If there is an increase in the power converter
load on V_OUT, the instantaneous level of V_OUT will drop
slightly, which will increase the voltage level on VC by
the inverting action of the voltage error amplifier. When
the increase on VC first occurs, the output of the current
averaging amplifier, VIA, will also increase momentarily
V_DS
SENSE
V_IN
SWA
L1
V_DS
SENSE
SWB
INDUCTOR
I_SENSE
g_m = 111µA/V
SWD PV_OUT
SWC
C_OUT
R_COESR
V_OUT
R_LOAD
R_X
12
ABCD
CONTROL
LOGIC
PWM
C_CS2
R_CS1
R_CS2
C_CS1
VIA
AVERAGE
CURRENT
AMP
SOFT-START
1V
ACTIVE RANGE OF
VC = 135mV TO 1V
FB
g_m
VOLTAGE
1V
ERROR
AMP
g_m = 120µS
R_O = 3.6M
VC
GND
R_TOP
R_BOT
R_Z
C_P2
C_P1
31141 F02
Figure 2. Average Current Mode Control Loop
For more information www.linear.com/LTC3114-1
31141fa