Datasheet

LTC3129-1
16
31291fa
For more information www.linear.com/LTC3129-1
operaTion
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.
Maximum Power-Point Control (MPPC)
The MPPC input of the LTC3129-1 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 noninverting 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 the input filter capacitor, C
IN
, is
at least 22µF. See Typical Applications for an example of
external compensation that can be added in applications
where C
IN
must be less than the recommended minimum
value.
The divider resistor values can be in the megohm 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.
The MPPC pin controls the converter in a linear fashion
when using sources that can provide a minimum of 5mA
to 10mA of continuous input current. For operation from
weaker input sources, refer to the Application Information
section to see how the programmable RUN pin can be used
to control the converter in a hysteretic manner to provide
an effective MPPC function for sources that can provide
as little as 5µA or less.
If the MPPC function is not required, the MPPC
pin should
be tied to V
CC
.
V
OUT
Programming Pins
The LTC3129-1 has a precision internal voltage divider on
V
OUT
, eliminating the need for high-value external feedback
resistors. This not only eliminates two external compo-
nents, it
minimizes
no-load quiescent current by using very
Figure 3. MPPC Amplifier with External Resistor Divider
LTC3129-1
1.175V
V
C
CURRENT
COMMAND
VOLTAGE
ERROR AMP
31291 F03
MPPC
R5
R6
R
S
V
SOURCE
*C
IN
V
IN
V
IN
+
–
+
–
+
–
* C
IN
SHOULD BE AT
LEAST 22µF FOR
MPPC APPLICATIONS