lm2707 National Semiconductor Corporation, lm2707 Datasheet - Page 12

• Manufacturer Part Number: lm2707
• Description: Inductive-boost Series Led Driver With Programmable Oscillator Frequency
• Manufacturer: National Semiconductor Corporation

www.national.com
Product Description
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.
FIGURE 5. Peak Inductor Current and Current Limit vs.
FIGURE 6. Ideal Inductor Current Waveforms
Input Voltage
(Continued)
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12
Note 12: V
during the charge phase. Actual voltage across the inductor will be slightly
lower due to the V
due to the low R
INCREASING CURRENT LIMIT AND PEAK INDUCTOR
CURRENT
It is possible to increase the current limit by placing an
external resistor between the V
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:
FIGURE 8. Increase Current Limit and Peak Inductor
FIGURE 7. Peak Inductor Current vs. Input Voltage.
R
EXT
: External Current Limit Adjust Resistance
IN
is a good approximation of the voltage across the inductor
DS-ON
DS
voltage of the power transistor, but this factor is minimal
Current by Adding R
of the power transistor.
L = 22µH, C
X
IN
= 68pF.
and L
EXT
X
pins, as shown in
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