Datasheet FL7701 (ON Semiconductor) - 6

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FL7701 Dimming Function
In DCM Mode, inductance is: The FL7701 uses the ADIM pin for analog or 0 V to 10 V n @ V dimming by using a resistive divider. The peak voltage of L f @ (1 * Dmin) m + [H] f (eq. 2) internal reference, which is DAC_OUT signal in Figure 5, s @ Dirip is changed by the VADIM level, as shown in Figure 7, and has If the peak current is fixed at 350 mApk, the formula for different peak level according to the operating mode. the peak current is: Dirip V I [A] DAC_OUT LED(ave.peak) + Dicon ) 2 (eq. 3) AC mode 0.5 V In FL7701, the LED RMS current determines the inductance parameter. To drive for CCM Mode, define LED DC mode 0.365 V RMS current first, as: ILED(ave.peak) ILED(rms) + [A] 2 Ǹ (eq. 4) Substituting Equation (2) for Equation (4), the inductance 0.5 V 3.5 V VADIM of inductor is obtained.
Figure 7. VADIM vs. VDAC_OUT(peak)
VIN
Inductor Design
The FL7701 prefixed internal duty ratio range is below p 2 p 3p 4pt Bridge 50%, or around 400 ns, from timing point of view. The range Diode Vbridge Output is dependent on the input voltage and LED numbers in its Voltage Input Voltage V Chargi dd ng string. Peak Voltage Minimum duty is calculated as: t ZDC n @ V t D f DAC min + h @ V (eq. 1) in(max) t VLED where: h is efficiency of system; t Vdrain Input Voltage VIN(max) is maximum input voltage; Peak Vf is forward drop voltage of LED; and Input t ILED Voltage n is LED number in series connection. Peak t Current Limit IFRD Average LED Current t i rip IMOSFET D t I t input on t off Dmin 1−Dmin (a) DCM Mode t Current peak at LED current T / 2 = 1 / (Input Frequency * 2) maximum point (ILED(peak)) Current peak at LED average Average T = 1 / Input Frequency current maximum point LED Current (ILED(ave)) (ILED(ave.peak))
Figure 9. Typical Performance Characteristics
DCM 0.5 i D DCM D i 0.5 iD CCM Current min at LED current maximum point (ILED(min)) Dmin 1−D t min on t off (b) CCM Mode
Figure 8. DCM and CCM Operation www.onsemi.com 6