LM2707 datasheet(12/19 Pages) NSC

12 / 19 page

Product Description (Continued)

Figure 5 graphs the relationship between inductor current

and current limit. Figure 6 plots ideal inductor current wave-

forms to illustrate inductor current behavior. Figure 7 gives

peak inductor current versus input voltage and shows the

two regions where the oscillator and current limit, respec-

tively, determine peak inductor current.

Note 12: VIN is a good approximation of the voltage across the inductor

during the charge phase. Actual voltage across the inductor will be slightly

lower due to the VDS voltage of the power transistor, but this factor is minimal

due to the low RDS-ON of the power transistor.

INCREASING CURRENT LIMIT AND PEAK INDUCTOR

CURRENT

It is possible to increase the current limit by placing an

external resistor between the V

IN and LX pins, as shown in

Figure 8. With the addition of the external resistor, only a

fraction of the total inductor current passes through internal

sense resistor. Thus, it takes more inductor current for the

voltage across the internal sense resistor to become large

enough to trip the current limit comparator.

The addition of an external current limit resistor modifies the

associated peak inductor equation to:

EXT: External Current Limit Adjust Resistance

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FIGURE 5. Peak Inductor Current and Current Limit vs.

Input Voltage

20099232

FIGURE 6. Ideal Inductor Current Waveforms

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FIGURE 7. Peak Inductor Current vs. Input Voltage.

L = 22µH, C

X = 68pF.

20099233

FIGURE 8. Increase Current Limit and Peak Inductor

Current by Adding R

EXT

LM2707 Datasheet(PDF) 12 Page - National Semiconductor (TI)