LTC3118_15 Datasheet, PDF (16/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Operation
value of the voltage on R_CS is equivalent to the VC level,
the voltage on I_COMP will revert very closely to its previ-
ous level into the PWM, and force the correct duty cycle
to maintain voltage regulation at this new higher inductor
current level. The average current amplifier is configured as
an integrator, so in steady state, the average value of the
voltage applied to its inverting input (voltage across R_CS)
will be equivalent to the voltage on its noninverting input
VC. As a result, the average value of the inductor current
is controlled in order to maintain voltage regulation. The
entire current amplifier and PWM can be simplified as a
voltage controlled current source, with the driving volt-
age coming from VC. VC is commonly referred to as the
current command for this reason, and the voltage on VC
is directly proportional to average inductor current, which
can prove useful for many applications.
The voltage error amplifier monitors V_OUT through a voltage
divider and makes adjustments to the current command
as necessary to maintain regulation. The voltage error
amplifier therefore controls the outer voltage regulation
loop. The average current amplifier makes adjustments
to the inductor current as directed by the voltage error
amplifier output via VC and is commonly referred to as
the inner current loop amplifier. The average current mode
control technique is similar to peak current mode control
except that the average current amplifier, by virtue of its
configuration as an integrator, controls average current
instead of the peak current. This difference eliminates the
peak-to-average current error inherent to peak current
mode control, while maintaining most of the advantages
inherent to peak current mode control.
Average current mode control requires appropriate
compensation for the inner current loop, unlike peak
current mode control. The compensation network must
have high DC gain to minimize V_OUT regulation errors
and high bandwidth to quickly change the commanded
current level following transient load steps. The inner
loop compensation components are fixed internally on
the LTC3118. External compensation of the voltage loop
is detailed in the Applications Information section and is
similar to techniques used for peak current mode control.
Inductor Current Sense and Maximum Output Current
As part of the current control loop, the LTC3118 has cur-
rent sense circuitry that measures the inductor current
of the buck-boost converter, as shown in Figure 3. This
circuitry measures the current through switches A1 (or
A2) and B separately and produces proportional output
currents that are summed at the current sense resistor R_CS.
Sensed A and B switch currents form a voltage replica of
the inductor current at R_CS, which is used by the average
current amplifier, as described in the previous section.
The voltage amplifier output, VC, is internally clamped to
a nominal value of 1V. Since the average inductor current
is proportional to VC, the 1V clamp sets the maximum
average inductor current that can be programmed by the
inner current loop. Taking into account the current sense
amplifier’s gain, and the value of R_CS, the maximum average
inductor current is 3.6A typical. In buck mode, the output
current is approximately equal to the inductor current I_L.
I_OUT(BUCK) ≈ I_L • 0.85
The 100ns SW1/SW2 forced low time on each switching
cycle briefly disconnects the inductor from V_OUT and V_IN,
resulting in slightly less output current in either buck or
boost mode for a given inductor current. In boost mode,
the output current is related to average inductor current
and duty cycle by:
I_OUT(BOOST) ≈ I_L • (1 – D)
where D is the converter duty cycle.
Since the output current in boost mode is reduced by the
duty cycle (D), the output current rating in buck mode is
always greater than in boost mode. Also, because boost
mode operation requires a higher inductor current for a
given output current compared to buck mode, the efficiency
in boost mode will be lower due to higher conduction
(I_L² • R_DS(ON)) losses in the power switches. This will fur-
ther reduce the output current capability in boost mode. In
either operating mode, however, the inductor peak-to-peak
ripple current does not play a major role in determining
the output current capability.
3118f
16
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