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

LTC3129
OPERATION
PGOOD Comparator
The LTC3129 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
if the part is in shutdown (RUN below its logic threshold)
while V_CC is being held up (or back-driven). PGOOD is
not affected by V_IN UVLO or the accurate RUN threshold.
In cases where V_CC is not being back-driven in shutdown,
PGOOD will not be held low indefinitely. The internal PGOOD
pull-down will be disabled as the V_CC voltage decays below
approximately 1V.
Maximum Power-Point Control (MPPC)
The MPPC input of the LTC3129 can be used with an
optional external voltage divider to dynamically adjust
the commanded inductor current in order to maintain
a minimum input voltage when using high resistance
sources, such as photovoltaic panels, so as to maximize
input power transfer and prevent V_IN from dropping too
low under load. Referring to Figure 3, the MPPC pin is
internally connected to the non-inverting input of a g_m
amplifier, whose inverting input is connected to the 1.175V
reference. If the voltage at MPPC, using the external volt-
age divider, falls below the reference voltage, the output of
the amplifier pulls the internal V_C node low. This reduces
the commanded average inductor current so as to reduce
the input current and regulate V_IN to the programmed
minimum voltage, as given by:
V_IN(MPPC) = 1.175V • (1 + R5/R6)
Note that external compensation should not be required
for MPPC loop stability if input filter capacitor, C_IN, is at
least 22µF.
The divider resistor values can be in the MΩ range to
minimize the input current in very low power applications.
However, stray capacitance and noise pickup on the MPPC
pin must also be minimized.
R_S
*C_IN
+
V_SOURCE
–
R5
V_IN
MPPC
+
R6
–
1.175V
* C_IN SHOULD BE AT
LEAST 22µF FOR
FB
MPPC APPLICATIONS
+
–
VOLTAGE
ERROR AMP
LTC3129
V_C
CURRENT
COMMAND
3129 F03
Figure 3. MPPC Amplifier with External Resistor Divider
For more information www.linear.com/LTC3129
3129fb
15