Datasheet

LTC3129
13
3129fa
For more information www.linear.com/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.