LTC3124 datasheet(21/36 Pages) AD

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LT3120

Rev. 0

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One half of this inductor ripple current must be added to

the highest expected average inductor current in order to

select the proper saturation current rating for the inductor.

Programming the Output Voltage

The output voltage is set via the external resistive divider

comprisedofresistorsRTOPandRBOTasshowninFigure 4.

Theresistordividervaluesdeterminetheoutputregulation

voltage according to Equation 12.

VOUT = 0.795V• 1+

RTOP

RBOT

⎛

⎝⎜

⎞

⎠⎟

(12)

Programming the MPPC Voltage

The LT3120 includes an MPPC function to optimize

performance when operating from current limited input

sources. Using an external voltage divider from VIN, the

MPPC function takes control of the average inductor cur-

rent when necessary to maintain a minimum input voltage

VMPPC, as programmed by the user. (See Figure 5).

VMPPC = 0.795V• 1+

⎛

⎝⎜

⎞

⎠⎟

(13)

Thisisusefulforsuchapplicationsasphotovoltaicpowered

converters,sincethemaximumpowertransferpointoccurs

when the photovoltaic panel is operated at approximately

75%ofitsopen-circuitvoltage.Forexample,whenoperat-

ing from a photovoltaic panel with an open-circuit voltage

of10V,themaximumpowertransferpointwillbewhenthe

panel is loaded such that its output voltage is about 7.5V.

WhenusingtheMPPCfunction,theinputcapacitorshould

be sized between 100µF and 470µF. Resistor R6 should be

chosen between 50k and 250k. Lower values will result in

smaller undershoot of the MPPC tracking point during line

and load transient conditions, but will draw more current

from the input supply. For this example, a value of 100kΩ

will be used. The value of R5 can then be determined us-

ing Equation 14 to set the desired input MPPC voltage.

VMPPC

0.795V

−1

⎛

⎝⎜

⎞

⎠⎟ •

7.5V

0.795V

−1

⎛

⎝⎜

⎞

⎠⎟ •

100k

= 843kΩ ≅ 845kΩ

(14)

Using these resistor values, the MPPC function is pro-

grammed to control the maximum input current so as to

maintain VIN at a minimum of 7.56V. Note that if the pho-

tovoltaic panel can provide more power than the LT3120

can draw or the load requires, the input voltage will rise

abovetheprogrammedMPPCpoint.Higherinputvoltages

do not present a problem so long as the input voltage

does not exceed the maximum operating input voltage.

For photovoltaic panel applications, it may be also desir-

able to use the programmable EN/UVLO feature to disable

the part when VIN drops too low due to lack of sufficient

light. Using the EN/UVLO pin provides a well-controlled

behavior when the input power source is dropping out by

halting switching to prevent discharge of the output. This

custom input UVLO voltage should be programmed to be

below the MPPC tracking voltage with sufficient margin to

ensurethepartdoesnotdisableundertransientconditions.

The MPPC loop requires compensation to maintain

stability of the input voltage regulation loop. This can be

accomplished by means of a pole-zero pair on the MPPC

pin created with a series RC network in parallel with the

lower MPPC resistor R6 as shown in Figure 5.

The pole and zero locations should be selected to create a

low frequency pole at or below approximately 360Hz and

a zero at a frequency that is scaled based on the size of

the input capacitor. Equation 15 determines the values for

the compensation capacitor CC2, and zero resistor RC2.

CC2 =

π•R6 •360Hz

RC2 =

CIN

π•CC2

(15)

Using the divider values from the previous example and

a 220µF input capacitor, the value of the compensation

components can be calculated as shown in Equation 16.

CC2 =

π•100kΩ•360Hz

= 4.42nF ≅ 4.3nF

RC2 =

220µF

π•4.3nF

= 8.14kΩ ≅ 8.25kΩ

(16)

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