LTC3118_15 Datasheet, PDF (15/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Operation
Current Mode Control
The LTC3118 utilizes average current mode control for the
pulse width modulator, as shown in Figure 3. Current mode
control, both average and the better known peak method,
enjoy some benefits compared to other control methods,
including: simplified loop compensation, rapid response
to load transients and inherent line voltage rejection.
Referring to Figure 3, an internal high gain transcon-
ductance error amplifier, labeled V_AMP, monitors V_OUT
through a voltage divider connected to the FB pin and
provides an output, VC, used by the current mode control
loop to command the appropriate inductor current level.
To ensure stability, external frequency compensation
components (R_Z, C_P1 and C_P2) must be installed be-
tween VC and GND. 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 3 as I_AMP. The inverting
input of the average current amplifier is connected to the
inductor current sense resistor R_CS through a gain-setting
resistor R_A1 and to its output (I_COMP) through an internal
frequency compensation network comprised of R_A2 and
C_A. The average current amplifier’s output provides the
cycle-by-cycle duty cycle command into the buck-boost
PWM circuitry.
The non-inverting reference level input to the average
current 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 A1 (or A2) and B. The output of
the sensing circuitry produces a voltage across resistor R_CS
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, I_COMP, will increase momentarily to
command a larger duty cycle. This duty cycle increase
will result in a higher inductor current level, ultimately
raising the average voltage across R_CS. Once the average
INDUCTOR
CURRENT
SW1
I_L
SENSE
SW2
V_OUT
R1 _FB
V_AMP
–
R2 ^1V +
R_A1 IAVG R_A2
R_CS
– I_AMP
+
VC
C_A
I_COMP
1.2MHz RAMPS/
OSCILLATOR
R_Z
C_P2
C_P1
PWM
TO
SWITCHES
DRIVE LOGIC
3118 F03
Figure 3. Average Current Mode Control Loop
For more information www.linear.com/LTC3118
3118f
15