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NCP5010 Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP5010 Datasheet(HTML) 10 Page - ON Semiconductor |
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10 / 18 page  NCP5010 http://onsemi.com 10 LED Current Selection The feedback resistor (RFB) determines the average maximum current through the LED string. The control loop regulated the current such that the average voltage at the FB input is 500 mV (nom). For example, should one need a 20 mA output current in the primary branch, RFB should be selected according to the following equation: RFB + FBV IOUT + 500 mV 20 mA + 25 W In white LED applications it is desirable to operate the LEDs at a specific operating current as the color will shift as the bias current is changed. As a result of this effect, it is recommended to dim the LED string by a pulse width modulation techniques. A low frequency PWM signal can be applied to the CTRL input and by varying the duty cycle the brightness of the LED can be changed. To avoid any optical flicker, the frequency must be higher than 100 Hz and preferably less than 1 kHz. Due to the soft−start function set at 600 ms (nom) with higher frequency the device remains active but the brightness can decrease. Nevertheless in this case, a dimming control using a filtered PWM signal (See Figure 33) can be used. Also for DC voltage control the same technique is suitable and the filter is takes away. Inductor Selection To choose the inductor there are three different electrical parameters that need to be considered, the absolute value of the inductor, the saturation current and the DCR. In normal operation, this device is intended to operate in Continuous Conduction Mode (CCM) so the following equation below can be used to calculate the peak current: IPEAK + IOUT h(1 * D) ) VIND 2LF In the equation above, VIN is the battery voltage, IOUT is the load current, L the inductor value, F the switching frequency, and the duty cycle D is given by: D + 1 * VIN VOUT h is the global converter efficiency which can vary with load current (see Figure 3 thru Figure 8). A good approximation is to use h = 0.8. Figure 24 − Figure 26 are a graphical representation of the above equations, as a function of the desired IOUT, VIN, and number of LEDs in series (VF = 3.5 V nominal). The curves are limited to an IPEAK_MAX of 300 mA. It is important to analyze this at worst case Vf conditions to ensure that the inductor current rated is high enough such that it not saturate. The recommended inductor value should range between 10 mH and 22 mH. As can be seen from the curves, as the inductor size is reduced, the peak current for a given set of conditions increases along with higher current ripple so it is not possible to deliver maximum output power at lower inductor values. 50 100 150 200 250 300 10 20 30 40 50 60 70 80 L = 10 mH V IN = 3.1 V IN = 4.2 V Figure 24. Peak Inductor Currents vs. IOUT (mA) @ 3 LEDs, 10.5 V L = 15 mH L = 22 mH V IOUT (mA) 50 100 150 200 250 300 10 20 30 40 50 60 70 80 L = 15 mH Figure 25. Peak Inductor Currents vs. IOUT (mA) @ 4 LEDs, 14 V V IN = 3.1 V IN = 4.2 V V L = 22 mH L = 10 mH IOUT (mA) 50 100 150 200 250 300 10 20 30 40 50 60 70 80 Figure 26. Peak Inductor Currents vs. IOUT (mA) @ 5 LEDs, 17.5 V L = 15 mH V IN = 3.1 V IN = 4.2 V V L = 22 mH L = 10 mH IOUT (mA) |
Similar Part No. - NCP5010_06 |
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Similar Description - NCP5010_06 |
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