LM3557 datasheet(8/11 Pages) NSC

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Application Information (Continued)

CONTINUOUS AND DISCONTINUOUS MODES OF

OPERATION

Since the LM3557 is a constant frequency pulse-width-

modulated step-up regulator, care must be taken to make

sure the maximum duty cycle specification is not violated.

The duty cycle equation depends on which mode of opera-

tion the LM3557 is in. The two operational modes of the

LM3557 are continuous conduction mode (CCM) and dis-

continuous conduction mode (DCM). Continuous conduction

mode refers to the mode of operation where during the

switching cycle, the inductor’s current never goes to and

stays at zero for any significant amount of time during the

switching cycle. Discontinuous conduction mode refers to

the mode of operation where during the switching cycle, the

inductor’s current goes to and stays at zero for a significant

amount of time during the switching cycle. Figure 4 illus-

trates the threshold between CCM and DCM operation. In

Figure 4, the inductor current is right on the CCM/DCM

operational threshold. Using this as a reference, a factor can

be introduced to calculate when a particular application is in

CCM or DCM operation. R is a CCM/DCM factor we can use

to compute which mode of operation a particular application

is in. If R is

≥ 1, then the application is operating in CCM.

Conversely, if R is < 1, the application is operating in DCM.

The R factor inequalities are a result of the components that

make up the R factor. From Figure 4, the R factor is equal to

the average inductor current, I

L(avg), divided by half the

inductor ripple current,

∆i

L. Using Figure 4, the following

equation can be used to compute R factor:

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IN:

Input Voltage.

OUT:

Output Voltage.

Eff:

Efficiency of the LM3557.

Fs:

Switching Frequency.

OUT:

White LED Current/Load Current.

Inductance Magnitude/Inductor Value.

Duty Cycle for CCM operation.

∆i

Inductor Ripple Current.

L(avg): Average Inductor Current.

For CCM operation, the duty cycle can be computed with:

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ON:

Internal Power FET On-Time.

Switching Period of Operation.

Duty Cycle for CCM Operation.

OUT: Output Voltage.

IN:

Input Voltage.

For DCM operation, the duty cycle can be computed with:

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FIGURE 4. Inductor Current Waveform

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