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ISL97634IRT18Z-TK Datasheet(PDF) 5 Page - Intersil Corporation |
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ISL97634IRT18Z-TK Datasheet(HTML) 5 Page - Intersil Corporation |
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5 / 10 page  5 FN6264.3 March 7, 2008 Detailed Description The ISL97634 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. There are three OVP models for driving 3, 4 and 7 LEDs (3.5V/20mA type) and their OVP thresholds are set at 14V, 18V and 26V respectively. The ISL97634 operates from an input voltage of 2.4V to 5.5V and ambient temperature from -40°C to +85°C. The switching frequency is around 1.45MHz and allows the driver circuit to employ small LC components. The peak forward current of the LED is set using the RSET resistor. In the steady state mode, the LED peak current is given by Equation1: PWM Dimming The ISL97634’s PWM/EN pin can be tied permanently to high for a fixed current operation. On the other hand, the ISL97634 can be applied with an external PWM signal to pulse width modulated output current. It is well understood that the LED brightness is a linear function of the LED current. In addition, the average LED current corresponds to the duty cycle “D” of the PWM signal as shown in Equation 2: As a result, PWM signal provides a means to dim the LED brightness. PWM dimming offers the best LEDs matching over DC dimming. It is because the LED peak current operating point is far away from the knee of the diode I-V curve where part to part variations are high. The PWM dimming test results are shown in Figure 6 with two PWM frequencies, 1kHz and 20kHz. The vertical scale parameter FB is proportional to the current and therefore the brightness. For the ISL97634, PWM dimming provides linear dimming adjustment with low frequency signal, such as 1kHz and FIGURE 7. PWM DIMMING AT 1kHz, D = 1% ZOOM IN FIGURE 8. PWM DIMMING AT 1kHz, D = 99% FIGURE 9. PWM DIMMING AT 20kHz, D = 50% ILED PWM/EN LX VOUT VIN = 4V R1 = 4Ω L1 = 22µH ILED PWM/EN LX VOUT VIN = 4V R1 = 4Ω L1 = 22µH ILED PWM/EN LX VOUT VIN = 4V L1 = 22µH R1 = 4Ω I LED V FB R SET --------------- = (EQ. 1) I LED-AVG V FB R SET --------------- D ⋅ = (EQ. 2) ISL97634 |
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