LTC3114-1_15 Datasheet, PDF (13/34 Pages) Linear Technology – 40V, 1A Synchronous Buck-Boost DC/DC Converter with Programmable Output Current

LTC3114-1
OPERATION
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_X. Once the average
value of the voltage on R_X is equivalent to the VC level,
the voltage on VIA will revert very closely to its previous
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_X) will be equivalent to the voltage on its noninverting,
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 the output voltage,
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 com-
pensation for the inner current loop unlike peak current
mode control. The compensation network must have high
DC gain to minimize errors between the commanded av-
erage current level and actual, high bandwidth to quickly
change the commanded current level following transient
load steps and a controlled mid-band gain to provide a
form of slope compensation unique to average current
mode control. Fortunately, the compensation components
required to ensure these sometimes conflicting require-
ments have been carefully selected and are integrated
within the LTC3114-1. With the inner loop compensation
fixed internally, compensation of the outer voltage loop
as is detailed in the applications section, is similar to well
known techniques used with peak current mode control.
Inductor Current Sense and Maximum Output Current
As part of the current control loop required for current
mode control, the LTC3114-1 includes a pair of current
sensing circuits that directly measure the buck-boost
converter inductor current as shown in Figure 2. These
circuits measure the voltage dropped across switches A
and B separately and produce output currents proportional
to the switches’ voltage drop. By sensing current in this
manner, there is no additional power loss incurred, which
improves converter efficiency. The amplifier output ter-
minals are summed together into a common resistor, R_X
connected to ground. Since switches A and B are never
conducting at the same time, the resultant waveform on R_X
resembles the inductor current. This replica of the induc-
tor current is used as one input to the current averaging
amplifier as described in the previous section.
The voltage error amplifier output, VC, is internally clamped
to a nominal level of 1V. Since the average inductor current
is proportional to VC, the 1V clamp level sets the maxi-
mum 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_X, the maximum
average inductor current is approximately 1.7A (typical).
In buck mode, the output current is approximately equal
to the inductor current, I_L.
I_OUT(BUCK) ≈ I_L • 0.9
The SW1/SW2 forced low time on each switching cycle
briefly disconnects the inductor from V_OUT and V_IN result-
ing 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.
31141fa
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