LTC3129_15 Datasheet, PDF (12/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129
OPERATION
inductor current is approximately 275mA (typical). In
Buck mode, the output current is approximately equal to
the inductor current I_L.
I_OUT(BUCK) ≈ I_L • 0.89
The 90ns SW1/SW2 forced low time on each switching
cycle briefly disconnects the inductor from V_OUT and V_IN
resulting in about 11% 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) • Efficiency,
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 I_L² • 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 cur-
rent 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 LTC3129 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 LTC3129 is capable of providing a mini-
mum of 200mA to the load. In boost mode, as described
previously, the output current capability is related to the
boost ratio or duty cycle (D). For example, for a 3.6V V_IN
to 5V output application, the LTC3129 can provide up
to 150mA to the load. Refer to the Typical Performance
characteristics section for more detail on output current
capability.
Overload Current Limit and I_ZERO Comparator
The internal current sense waveform is also used by the
peak overload current (I_PEAK) and zero current (I_ZERO) com-
parators. The I_PEAK current comparator monitors Isense
and turns off switch A if the inductor current level exceeds
its maximum internal threshold, which is approximately
500mA. An inductor current level of this magnitude will
occur during a fault, such as an output short-circuit, or
during large load or input voltage transients.
The LTC3129 features near discontinuous inductor current
operation at light output loads by virtue of the I_ZERO com-
parator circuit. By limiting the reverse current magnitude
in PWM mode, a balance between low noise operation and
improved efficiency at light loads is achieved. The I_ZERO
comparator threshold is set near the zero current level in
PWM mode, and as a result, the reverse current magnitude
will be a function of inductance value and output voltage
due to the comparator's propagation delay. In general,
higher output voltages and lower inductor values will
result in increased reverse current magnitude.
In automatic Burst Mode operation (PWM pin low), the
I_ZERO comparator threshold is increased so that reverse
inductor current does not normally occur. This maximizes
efficiency at very light loads.
Burst Mode OPERATION
When the PWM pin is held low, the LTC3129 is configured
for automatic Burst Mode operation. As a result, the buck-
boost DC/DC converter will operate with normal continu-
ous PWM switching above a predetermined minimum
output load and will automatically transition to power
saving Burst Mode operation below this output load level.
Note that if the PWM pin is low, reverse inductor current is
not allowed at any load. Refer to the Typical Performance
Characteristics section to determine the Burst Mode
transition threshold for various combinations of V_IN and
V_OUT. If PWM is low, at light output loads, the LTC3129
will go into a standby or sleep state when the output volt-
age achieves its nominal regulation level. The sleep state
halts PWM switching and powers down all non-essential
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