LTC3129-1_15 Datasheet, PDF (16/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129-1
Operation
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 LTC3129-1 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
V_IN
R_S
*C_IN
+
V_SOURCE
–
R5
V_IN
MPPC
+
R6
–
1.175V
* C_IN SHOULD BE AT
LEAST 22µF FOR
MPPC APPLICATIONS
+
–
VOLTAGE
ERROR AMP
LTC3129-1
V_C
CURRENT
COMMAND
31291 F03
Figure 3. MPPC Amplifier with External Resistor Divider
16
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