LTC3114-1_15 Datasheet, PDF (14/34 Pages) Linear Technology – 40V, 1A Synchronous Buck-Boost DC/DC Converter with Programmable Output Current

LTC3114-1
OPERATION
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 cur-
rent for a given output current compared to buck mode,
the efficiency in boost mode will be lower due to higher
I_INDUCTOR2 • R_DS(ON) losses in the power switches. This
will further 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, unlike peak
current mode control.
With peak current mode control, the maximum output
current capability is reduced by the magnitude of inductor
ripple current because the peak inductor current level is the
control variable, but the average inductor current is what
determines the output current. The LTC3114 -1 measures
and controls average inductor current, and therefore, the
inductor ripple current magnitude has little effect on the
maximum current capability in contrast to an equivalent
peak current mode converter. Under most conditions in
buck mode, the LTC3114 -1 is capable of providing 1A to
the load. Under certain conditions, more output current is
possible, refer to the Typical Performance Characteristics
section for more details. In boost mode, as described
previously, the output current capability is related to the
boost ratio or duty cycle (D). For a 3.6V V_IN to 5V output
application, the LTC3114-1 can provide up to 500mA to
the load. Refer to the Typical Performance Characteristics
section for more detail on output current capability.
At VC levels below 135mV, the LTC3114-1 will not com-
mand any current because the internal current sense signal
has a built-in 135mV offset. Therefore, the active range of
VC is between approximately 135mV (zero current) and
1V (full current). In some applications, an external circuit
may be used to control the VC voltage level. Any such
circuit needs to have the capability to sink or source the
approximate 12µA provided by the internal error amplifier
and to pull below 135mV to disable the current command,
if necessary.
OVERLOAD CURRENT LIMIT AND ZERO CURRENT
COMPARATOR
The internal current sense waveform is also used by the
peak overload current (I_PEAK) and zero current (I_ZERO)
comparators. The I_PEAK current comparator monitors I_S-
ENSE ^{and halts converter operation if the inductor current}
level exceeds its maximum internal threshold, which is
approximately 50% above the normal maximum current
level commanded by the current control loop. An inductor
current level of this magnitude will only occur during a fault,
such as an output short circuit or a fast V_IN (line)transient.
If the I_PEAK comparator is engaged, the PWM is halted for
the remainder of the switching cycle with SW1 and SW2
held low. If V_OUT is less than approximately 1.8V when
the peak limit occurs, then a soft-start cycle is initiated.
In the event that the current overload is the result of an
output short-circuit condition, the LTC3114‑1 will remain
in a low frequency restart mode, keeping the on-chip
power dissipation to very low levels. If the short circuit is
removed, the LTC3114-1 will restart in the normal fashion.
The LTC3114-1 exhibits discontinuous inductor current
operation at light output loads by virtue of the I_ZERO
comparator circuit under most operating conditions. This
improves efficiency at light output loads if PWM mode
operation compared to continuous conduction mode. If
the internal current sense waveform transitions below the
internally set zero current threshold, the LTC3114-1 will
disconnect the inductor from V_OUT, by shutting off switch
D, to prevent discharge of the output capacitor. The I_ZERO
circuitry is reset by the oscillator clock at the end of the
switching cycle. The I_ZERO comparator threshold is set
slightly above zero current to compensate for comparator
propagation delay. In some cases, the inductor current
may reverse slightly if there is a very high voltage output
or small inductor resulting in a small amount of residual
energy left in the inductor following a zero current event.
In this case the LTC3114-1 SW1 waveform will display a
characteristic half sine wave between the time at which
Izero is detected and when the next switching cycle com-
mences. This is because SWC is the only active (on) switch
following an I_ZERO event and this behavior is not harmful
to the LTC3114-1.
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