LM3502 datasheet(14/18 Pages) NSC

14 / 18 page

Application Information (Continued)

CONTINUOUS AND DISCONTINUOUS MODES OF

OPERATION

Since the LM3502 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 LM3502 is in. The two operational modes of the

LM3502 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 current never goes to and stays

at zero for any significant amount of time during the switch-

ing cycle. Discontinuous conduction mode refers to the

mode of operation where during the switching cycle, the

inductor current goes to and stays at zero for a significant

amount of time during the switching cycle. Figure 6 illus-

trates the threshold between CCM and DCM operation. In

Figure 6, 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 6, the R factor is equal to

the average inductor current, I

L(avg), divided by half the

inductor ripple current,

∆i

L. Using Figure 6 the following

equation can be used to compute R factor:

20131737

20131738

20131739

20131740

IN:

Input Voltage.

OUT:

Output Voltage.

Eff:

Efficiency of the LM3502.

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:

20131741

20131742

Duty Cycle for CCM Operation.

OUT: Output Voltage.

IN:

Input Voltage.

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

20131743

20131736

FIGURE 6. Inductor Current Waveform

LM3502 Folha de dados(PDF) 14 Page - National Semiconductor (TI)

LM3502 Folha de dados(HTML) 14 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Folha de dados Download

Ligação URL