LTC3129_15 Datasheet, PDF (13/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129
OPERATION
functions of the IC, significantly reducing the quiescent
current of the LTC3129 to just 1.3µA typical. This greatly
improves overall power conversion efficiency when the
output load is light. Since the converter is not operating
in sleep, the output voltage will slowly decay at a rate
determined by the output load resistance and the output
capacitor value. When the output voltage has decayed by
a small amount, typically 1%, the LTC3129 will wake and
resume normal PWM switching operation until the volt-
age on V_OUT is restored to the previous level. If the load
is very light, the LTC3129 may only need to switch for a
few cycles to restore V_OUT and may sleep for extended
periods of time, significantly improving efficiency. If the
load is suddenly increased above the burst transition
threshold, the part will automatically resume continuous
PWM operation until the load is once again reduced.
A feedforward capacitor on the feedback divider can be
used to reduce Burst Mode V_OUT ripple. This is discussed
in more detail in the Applications Information section of
this data sheet.
Note that Burst Mode operation is inhibited until soft-start
is done, the MPPC pin is greater than 1.175V and V_OUT
has reached regulation.
Soft-Start
The LTC3129 soft-start circuit minimizes input current
transients and output voltage overshoot on initial power up.
The required timing components for soft-start are internal
to the LTC3129 and produce a nominal soft-start dura-
tion of approximately 3ms. The internal soft-start circuit
slowly ramps the error amplifier output, V_C. In doing so,
the current command of the IC is also slowly increased,
starting from zero. It is unaffected by output loading or
output capacitor value. Soft-start is reset by the UVLO on
both V_IN and V_CC, the RUN pin and thermal shutdown.
V_CC Regulator
An internal low dropout regulator (LDO) generates a nomi-
nal 4.1V V_CC rail from V_IN. The V_CC rail powers the internal
control circuitry and the gate drivers of the LTC3129. The
V_CC regulator is disabled in shutdown to reduce quiescent
current and is enabled by raising the RUN pin above its
logic threshold. The V_CC regulator includes current-limit
protection to safeguard against accidental short-circuiting
of the V_CC rail.
Undervoltage Lockout (UVLO)
There are two undervoltage lockout (UVLO) circuits within
the LTC3129 that inhibit switching; one that monitors V_IN
and another that monitors V_CC. Either UVLO will disable
operation of the internal power switches and keep other
IC functions in a reset state if either V_IN or V_CC are below
their respective UVLO thresholds.
The V_IN UVLO comparator has a falling voltage threshold
of 1.8V (typical). If V_IN falls below this level, IC operation
is disabled until V_IN rises above 1.9V (typical), as long as
the V_CC voltage is above its UVLO threshold.
The V_CC UVLO has a falling voltage threshold of 2.19V
(typical). If the V_CC voltage falls below this threshold, IC
operation is disabled until V_CC rises above 2.25V (typical)
as long as V_IN is above its nominal UVLO threshold level.
Depending on the particular application, either of these
UVLO thresholds could be the limiting factor affecting the
minimum input voltage required for operation. Because the
V_CC regulator uses V_IN for its power input, the minimum
input voltage required for operation is determined by the
V_CC minimum voltage, as input voltage (V_IN) will always
be higher than V_CC in the normal (non-bootstrapped)
configuration. Therefore, the minimum V_IN for the part
to startup is 2.25V (typical).
In applications where V_CC is bootstrapped (powered
through a Schottky diode by either V_OUT or an auxiliary
power rail), the minimum input voltage for operation will
be limited only by the V_IN UVLO threshold (1.8V typical).
Please note that if the bootstrap voltage is derived from
the LTC3129 V_OUT and not an independent power rail, then
the minimum input voltage required for initial startup is
still 2.25V (typical).
Note that if either V_IN or V_CC are below their UVLO thresh-
olds, or if RUN is below its accurate threshold of 1.22V
(typical), then the LTC3129 will remain in a soft shutdown
state, where the V_IN quiescent current will be only 1.9µA
typical.
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