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LTC3119

3119fb

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applicaTions inForMaTion

internal switch resistance (30mΩ), or output current capa-

bility and efficiency will be compromised. Larger inductor

values reduce inductor current ripple but may not increase

output current capability as is the case with peak current

mode control as described in the Maximum Output Current

section. Larger value inductors also tend to have a higher

DC series resistance for a given case size, which will have a

negative impact on efficiency. Larger values of inductance

will also lower the right half plane zero (RHPZ) frequency

whenoperatinginboostmode,whichcancompromiseloop

stability. Nearly all LTC3119 application circuits deliver the

best performance with an inductor value between 1.5μH

and 15μH. Buck mode only applications can use the larger

inductor values as they are unaffected by the RHPZ, while

mostlyboostapplicationsgenerallyrequireinductanceonthe

lower end of this range depending on how large the step-up

ratio is. Regardless of inductor value, the saturation current

ratingshouldbeselectedsuchthatitisgreaterthantheworst

case average inductor current plus half of the ripple current.

The peak-to-peak inductor current ripple for each opera-

tional mode can be calculated from the following formula,

where fSW is the programmed switching frequency, L is

the inductance and tLOW is the switch pin minimum low

time, typically 90ns.

∆IL P-P

( ) BUCK

(

) @

VOUT

VIN − VOUT

VIN









fSW

− tLOW









∆IL P-P

( ) BOOST

(

) @

VIN

VOUT − VIN

VOUT









fSW

− tLOW









It should be noted that the worst-case peak-to-peak in-

ductor ripple current occurs when the duty cycle in buck

mode is minimum (highest VIN) and in boost mode when

the duty cycle is 50% (VOUT @ 2 • VIN).

As an example, if VIN (minimum) = 2.5V and VIN (maxi-

mum) = 15V, VOUT = 5V, fSW = 1MHz and L = 4.7μH, the

peak-to-peakinductorripplesatthevoltageextremes(15V

VIN for buck and 2.5V VIN for boost) are:

∆IL P-P

( ) BUCK

(

) @

4.7µH

−5











 • 910ns=645mA

∆IL P-P

( ) BOOST

(

) @

2.5

4.7µH

−2.5











 • 910ns=242mA

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:

VOUT =0.795V• 1+

RTOP

RBOT













Programming the MPPC Voltage

The LTC3119 includes an MPPC function to optimize

performance when operating from current limited input

sources.UsinganexternalvoltagedividerfromVIN(referto

Figure 5), the MPPC function takes control of the average

inductor current when necessary to maintain a minimum

input voltage VMPPC, as programmed by the user.

VMPPC =0.798V• 1+













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 100k will be used.

VMPPC

0.798V

−1











 •R6=

7.5

0.798

−1











 • 100kΩ

= 838kΩ @ 845kΩ

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