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Selecting the appropriate value of capacitor for the high frequency oscillator (CHF) will set the inductor switching
frequency of the D371 IC. High inductor frequency allows for more efficient use of inductor coils with lower values.
However, care must be taken that the charge pumping does not reach a continuous mode at very high frequency when
the voltage is not efficiently transferred to the lamp load. Figure 3 graphically represents the effect of the CHF value
on the oscillator frequency at V+ = 3.0V.
II. High Frequency Capacitor (CHF) Selection
Figure 3: Typical Inductor Frequency vs. CHF Capacitor
The inductor switching frequency may also be controlled with an external clock signal. The inductor will charge
during the low portion of the clock signal and discharge into the EL lamp during the high portion of the clock signal.
The positive duty cycle used for the external high frequency clock signal is usually between 15%-75% with a typical
value of 15%-20% for maximum brightness. The clock signal input voltage should not exceed V+.
10
15
20
25
50
75 100 125 150 175 200 225
CHF (pF)
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