LTC3129 datasheet(21/30 Pages) LINER

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LTC3129

3129fc

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applicaTions inForMaTion

When using high value divider resistors (in the MΩ

range) to minimize current draw on VIN, a small noise

filter capacitor may be necessary across the lower divider

resistor to prevent noise from erroneously tripping the

RUNcomparator.Thecapacitorvalueshouldbeminimized

so as not to introduce a time delay long enough for the

input voltage to drop significantly below the desired VIN

threshold before the converter is turned off. Note that

larger VIN decoupling capacitor values will minimize this

effect by providing more holdup time on VIN.

Programming the MPPC Voltage

As discussed in the previous section, the LTC3129 in-

cludes an MPPC function to optimize performance when

operating from voltage sources with relatively high source

resistance. Using an external voltage divider from VIN, the

MPPCfunctiontakescontroloftheaverageinductorcurrent

when necessary to maintain a minimum input voltage, as

programmed by the user. Referring to Figure 3:

VIN(MPPC) = 1.175V • (1 + R5/R6)

This is useful for such applications as photovoltaic pow-

ered converters, since the maximum power transfer point

occurs when the photovoltaic panel is operated at about

75%ofitsopen-circuitvoltage.Forexample,whenoperat-

ing from a photovoltaic panel with an open-circuit voltage

of 5V, the maximum power transfer point will be when

the panel is loaded such that its output voltage is about

3.75V. Choosing values of 2MΩ for R5 and 909kΩ for R6

will program the MPPC function to regulate the maximum

input current so as to maintain VIN at a minimum of 3.74V

(typical). Note that if the panel can provide more power

than the LTC3129 can draw, the input voltage will rise

above the programmed MPPC point. This is fine as long

as the input voltage doesn't exceed 15V.

For weak input sources with very high resistance (hun-

dreds of Ohms or more), the LTC3129 may still draw

more current than the source can provide, causing VIN to

drop below the UVLO threshold. For these applications, it

is recommended that the programmable RUN feature be

used, as described in the previous section.

MPPC Compensation and Gain

When using MPPC, there are a number of variables that

affect the gain and phase of the input voltage control

loop. Primarily these are the input capacitance, the MPPC

divider ratio and the VIN source resistance (or current). To

simplify the design of the application circuit, the MPPC

control loop in the LTC3129 is designed with a relatively

low gain, such that external MPPC loop compensation is

generally not required when using a VIN capacitor value

of at least 22µF. The gain from the MPPC pin to the in-

ternal VC control voltage is about 12, so a drop of 50mV

on the MPPC pin (below the 1.175V MPPC threshold),

corresponds to a 600mV drop on the internal VC voltage,

which reduces the average inductor current all the way

to zero. Therefore, the programmed input MPPC voltage

will be maintained within about 4% over the load range.

Note that if large value VIN capacitors are used (which may

have a relatively high ESR) a small ceramic capacitor of

at least 4.7µF should be placed in parallel across the VIN

input, near the VIN pin of the IC.

Bootstrapping the VCC Regulator

The high and low side gate drivers are powered through

the VCCrail,whichisgeneratedfromtheinputvoltage,VIN,

through an internal linear regulator. In some applications,

especially at high input voltages, the power dissipation

in the linear regulator can become a major contributor to

thermal heating of the IC and overall efficiency. The Typical

Performance Characteristics section provides data on the

VCC current and resulting power loss versus VIN and VOUT.

Asignificantperformanceadvantagecanbeattainedinhigh

VIN applications where converter output voltage (VOUT) is

programmed to 5V, if VOUT is used to power the VCC rail.

PoweringVCCinthismannerisreferredtoasbootstrapping.

This can be done by connecting a Schottky diode (such

as a BAT54) from VOUT to VCC as shown in Figure 6.

With the bootstrap diode installed, the gate driver currents

are supplied by the buck-boost converter at high efficiency

rather than through the internal linear regulator. The in-

ternal linear regulator contains reverse blocking circuitry

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