LTC3129-1_15 Datasheet, PDF (15/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129-1
Operation
V_OUT Undervoltage
There is also an undervoltage comparator that monitors
the output voltage. Until V_OUT reaches 1.15V (typical), the
average current limit is reduced by a factor of two. This
reduces power dissipation in the device in the event of a
shorted output. In addition, N-channel switch D, which
feeds V_OUT, will be disabled until V_OUT exceeds 1.15V.
RUN Pin Comparator
In addition to serving as a logic level input to enable cer-
tain functions of the IC, the RUN pin includes an accurate
internal comparator that allows it to be used to set custom
rising and falling ON/OFF thresholds with the addition of
an optional external resistor divider. When RUN is driven
above its logic threshold (0.9V typical), the V_CC regulator
is enabled, which provides power to the internal control
circuitry of the IC. If the voltage on RUN is increased
further so that it exceeds the RUN comparator’s accurate
analog threshold (1.22V typical), all functions of the buck-
boost converter will be enabled and a start-up sequence
will ensue (assuming the V_IN and V_CC UVLO thresholds
are satisfied).
If RUN is brought below the accurate comparator threshold,
the buck-boost converter will inhibit switching, but the V_CC
regulator and control circuitry will remain powered unless
RUN is brought below its logic threshold. Therefore, in
order to completely shut down the IC and reduce the V_IN
current to 10nA (typical), it is necessary to ensure that
RUN is brought below its worst case low logic threshold of
0.5V. RUN is a high voltage input and can be tied directly
to V_IN to continuously enable the IC when the input supply
is present. Also note that RUN can be driven above V_IN
or V_OUT as long as it stays within the operating range of
the IC (up to 15V).
With the addition of an optional resistor divider as shown
in Figure 2, the RUN pin can be used to establish a user-
programmable turn-on and turn-off threshold. This feature
can be utilized to minimize battery drain below a certain
input voltage, or to operate the converter in a hiccup mode
from very low current sources.
LTC3129-1
V_IN
1.22V
R3
RUN
ACCURATE THRESHOLD
–
ENABLE SWITCHING
+
R4
+
0.9V –
ENABLE LDO AND
CONTROL CIRCUITS
LOGIC THRESHOLD
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Figure 2. Accurate RUN Pin Comparator
Note that once RUN is above 0.9V typical, the quiescent
input current on V_IN (or V_CC if back-driven) will increase to
about 1.9µA typical until the V_IN and V_CC UVLO thresholds
are satisfied.
The converter is enabled when the voltage on RUN exceeds
1.22V (nominal). Therefore, the turn on voltage threshold
on V_IN is given by:
V_IN(TURN-ON) = 1.22V • (1 + R3/R4)
The RUN comparator includes a built-in hysteresis of
approximately 80mV, so that the turn off threshold will
be 1.14V.
There may be cases due to PCB layout, very large value
resistors for R3 and R4, or proximity to noisy components
where noise pickup may cause the turn-on or turn-off of the
IC to be intermittent. In these cases, a small filter capaci-
tor can be added across R4 to ensure proper operation.
PGOOD Comparator
The LTC3129-1 provides an open-drain PGOOD output that
pulls low if V_OUT falls more than 7.5% (typical) below its
programmed value. When V_OUT rises to within 5% (typical)
of its programmed value, the internal PGOOD pull-down
will turn off and PGOOD will go high if an external pull-
up resistor has been provided. An internal filter prevents
nuisance trips of PGOOD due to short transients on V_OUT.
Note that PGOOD can be pulled up to any voltage, as long
as the absolute maximum rating of 18V is not exceeded,
and as long as the maximum sink current rating is not
exceeded when PGOOD is low. Note that PGOOD will
also be driven low if V_CC is below its UVLO threshold or
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