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LT3840

3840fa

For more information www.linear.com/LT3840

Switching Frequency

The choice of switching frequency is a trade-off between

converter efficiency and component size. Low frequency

operation improves efficiency by reducing MOSFET

switching losses and gate charge losses. However, lower

frequency operation requires more inductance for a given

amount of ripple current, resulting in larger inductor size.

Increasing the ripple current requires additional output

capacitance to maintain the same output ripple voltage.

For converters with extremely high or low step-down VIN

to VOUT ratios, another consideration is the minimum

on and off times of the LT3840. A final consideration for

operating frequency is in noise-sensitive systems where

it is often desirable to keep the switching noise out of a

sensitive frequency band.

The LT3840 uses a constant frequency architecture pro-

grammablewithasingleresistor(RT)overa50kHzto1MHz

range. The value of RT for a given operating frequency can

be chosen from Table 1 or from the following equation:

RT(kΩ) = 2.32 • 104 • fSW(–1.08)

Table 1. Recommended 1% Standard Values

RT (kΩ)

fSW (kHz)

348

158

100

76.8

200

49.9

300

36.5

400

28.0

500

23.2

600

19.1

700

16.5

800

14.3

900

13.7

1000

applicaTions inForMaTion

Inductor Selection

The critical parameters for the selection of an inductor

are minimum inductance value, saturation current and

RMS current. For a given ΔIL, the inductance value is

calculated as follows:

L ≥ VOUT •

VIN(MAX)– VOUT

fSW • VIN(MAX)•ΔIL

The typical range of values for ΔIL is (0.2 • IOUT(MAX)) to

(0.5 • IOUT(MAX)), where IOUT(MAX) is the maximum load

current of the supply. Using ΔIL = 0.3 • IOUT(MAX) yields a

good design compromise between inductor performance

versusinductorsizeandcost.AvalueofΔIL=0.3•IOUT(MAX)

producesa±15%ofIOUT(MAX)ripplecurrentaroundtheDC

output current of the supply. Lower values of ΔIL require

larger and more costly magnetics. Higher values of ΔIL

will increase the peak currents, requiring more filtering

on the input and output of the supply. If ΔIL is too high,

the slope compensation circuit is ineffective and current

mode instability may occur at duty cycles greater than

50%. To satisfy slope compensation requirements the

minimum inductance is calculated as follows:

LMIN> VOUT •

2DCMAX –1

DCMAX

•

RSENSE •30

fSW

Magneticsvendorsspecifythesaturationcurrent,theRMS

current or both. When selecting an inductor based on

inductor saturation current, use the peak current through

the inductor, IOUT(MAX) + ΔIL/2. The inductor saturation

currentspecificationisthecurrentatwhichtheinductance,

measuredatzerocurrent,decreasesbyaspecifiedamount,

typically 30%. When selecting an inductor based on RMS

currentrating,usetheaveragecurrentthroughtheinductor,

IOUT(MAX).TheRMScurrentspecificationistheRMScurrent

at which the part has a specific temperature rise, typically

40°C, above 25°C ambient. After calculating the minimum

inductance value, the saturation current and the RMS

current for your design, select an off-the-shelf inductor.

Contact the Applications Group at Linear Technology for

furthersupport.Formoredetailedinformationonselecting

an inductor, please see the Inductor Selection section of

Linear Technology Application Note 44.

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