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EL7630 Datasheet(PDF) 5 Page - Intersil Corporation |
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EL7630 Datasheet(HTML) 5 Page - Intersil Corporation |
5 / 7 page 5 FN7371.1 February 22, 2006 Detailed Description EL7630 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. It can drive up to 6 LEDs in series or 15 LEDs in parallel/series configuration, with efficiencies of up 86%. EL7630 operates from an input voltage of 2.7V to 5.5V and can boost up to 27V. Steady-State Operation EL7630 operates with constant frequency PWM. The switching frequency is around 1.2MHz. Depending on the input voltage, inductance, number of LEDs and the LED current, the converter operates in either continuous conduction mode or discontinuous conduction mode. Both are normal. The forward current of the LED is set using the RSET resistor. In steady state mode, this current is given by the equation: Shut-Down The ENAB pin, when taken low places EL7630 into power down mode. When in power down, the supply current reduced to less than 1µA. Dimming Control The ENAB pin also doubles as a brightness control. There are two different types of dimming control methods. The first dimming control is controlled through the duty-cycle of the ENAB input PWM waveform, which can operate at frequencies of 400Hz to 1kHz. The LEDs operate at either zero or full current. This is called PWM dimming control method. The relationship between the average LED current and the duty-cycle (D) of the ENAB pin’s waveform is as follows: The magnitude of the PWM signal should be higher than the minimum ENAB voltage high. The bench PWM dimming test results are shown in Figure 8. In the test, two PWM frequencies 400Hz and 1kHz are chosen to compare the linear dimming range. It is clear that for lower PWM frequency, the linear dimming range is wider than one for higher PWM frequency. In the PWM dimming test, the output capacitor is 0.22µF. The second dimming control is to apply a variable DC voltage to adjust the LED current. This is called analog dimming control. The dimming control using a DC voltage is shown in Figure 9. As the DC dimming signal voltage increases, the voltages drop on R1 and R2 increases and the voltage drop on RSET decreases. Thus, the LED current decreases. The DC dimming signal voltage can be a variable DC voltage or a DC voltage generated from a PWM control signal. For some application areas, the PWM control signal is a high frequency signal. To make dimming controllable with these high frequency PWM signals, the high frequency components of the PWM control signal should be filtered to get the equivalent DC voltage. The equivalent DC voltage is then used as the variable DC voltage for dimming LED current. where F is the brightness with respect to the undimmed value. ILED VFB RSET --------------- = (EQ. 1) average ILED VFB RSET --------------- D ⋅ = (EQ. 2) 0 5 10 15 20 25 0 102030 4050 60 708090 10 DUTY-CYCLE (%) FIGURE 8. PWM DIMMING LINEAR RANGE (FOR 400Hz AND 1kHz PWM FREQUENCIES CONDITION, COUT = 0.22µF) 1kHz 400Hz ILED VFB RSET --------------- R1 R2 + R2 --------------------- VDim R1 ⋅ RSET R2 ⋅ --------------------------- – ⋅ = (EQ. 3) VDim R2 R1 ------- VFB 1 R1 R2 ------- F – + ⋅⋅ = (EQ. 4) EL7630 |
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