Datasheet

LTC3129-1
13
31291fa
For more information www.linear.com/LTC3129-1
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
L
2
• 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 LTC3129-1 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 LTC3129-1 is capable of providing a mini
-
mum of
200
mA 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 LTC3129-1 can provide up
to 150mA to the load. Refer to the Typical Performance
Characteristics section for more detail on output current
capability.
operaTion
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 I
SENSE
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 LTC3129-1 features near discontinuous inductor
current operation at light output loads by virtue of the
I
ZERO
comparator 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 out
-
put 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 LTC3129-1 is con
-
figured for
automatic Burst Mode operation. As a result,
the
buck-boost DC/DC converter will operate with normal
continuous PWM switching above a predetermined mini
-
mum 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 of this data sheet 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