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ISL97682 데이터시트(PDF) 15 Page - Renesas Technology Corp |
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ISL97682 데이터시트(HTML) 15 Page - Renesas Technology Corp |
15 / 18 page ISL97682, ISL97683, ISL97684 FN7689 Rev.2.00 Page 15 of 18 Sep 19, 2017 Components Selections According to the inductor Voltage-Second Balance principle, the change of inductor current during the switching regulator On-time is equal to the change of inductor current during the switching regulator Off-time. Since the voltage across an inductor is as shown in Equation 10: and IL @ On = IL @ Off, therefore: where D is the switching duty cycle defined by the turn-on time over the switching periods. VD is a Schottky diode forward voltage that can be neglected for approximation. Rearranging the terms without accounting for VD gives the boost ratio and duty cycle as Equations 12 and 13: Input Capacitor Switching regulators require input capacitors to deliver peak charging current and to reduce the impedance of the input supply. This reduces interaction between the regulator and input supply, thereby improving system stability. The high switching frequency of the loop causes almost all ripple current to flow in the input capacitor, which must be rated accordingly. A capacitor with low internal series resistance should be chosen to minimize heating effects and improve system efficiency, such as X5R or X7R ceramic capacitors, which offer small size and a lower value of temperature and voltage coefficient compared to other ceramic capacitors. It is recommended that an input capacitor of at least 10µF be used. Ensure the voltage rating of the input capacitor is suitable to handle the full supply range. Inductor The selection of the inductor should be based on its maximum and saturation current (ISAT) characteristics, power dissipation (DCR), EMI susceptibility (shielded vs unshielded), and size. Inductor type and value influence many key parameters, including ripple current, current limit, efficiency, transient performance and stability. The inductor’s maximum current capability must be adequate enough to handle the peak current at the worst case condition. Additionally if an inductor core is chosen with too low a current rating, saturation in the core will cause the effective inductor value to fall, leading to an increase in peak to average current level, poor efficiency and overheating in the core. The series resistance, DCR, within the inductor causes conduction loss and heat dissipation. A shielded inductor is usually more suitable for EMI susceptible applications, such as LED backlighting. The peak current can be derived from the voltage across the inductor during the Off-period, expressed in Equation 14: The choice of 85% is just an average term for the efficiency approximation. The first term is the average current, which is inversely proportional to the input voltage. The second term is the inductor current change, which is inversely proportional to L and FSW as a result, for a given switching. Applications Low Voltage Operations The ISL97682, ISL97683, ISL97684 VIN pin can be separately biased from the LEDs power input to allow low voltage operation. For systems that have only single supply, VOUT can be tied to the driver VIN pin to allow initial start-up; see Figure 26. The circuit works as follows; when the input voltage is available and the device is not enabled, the VOUT follows VIN with a Schottky diode voltage drop. The VOUT bootstrapped to VIN pin allows an initial start-up once the part is enabled. Once the driver starts up with VOUT regulating to the target, the VIN pin voltage also increases. As long as the VOUT does not exceed 26.5V and the extra power loss on VIN is acceptable, this configuration can be used for input voltage as low as 3.0V. For systems where a single input supply of 4V to 5.5V is available, the VIN pin can be shorted to VDC, allowing a slight gain in efficiency due to bypassing the internal LDO. For systems that have dual supplies, the VIN pin can be biased from 5V to 12V. The input voltage can be as low as 2.7V without the limitations previously mentioned; see Figure 27. VL L IL t = (EQ. 10) V I 0 L DtS VO VD VI – – = L1 D tS – – (EQ. 11) VO VI 11 D – = (EQ. 12) DVO VI VO – = (EQ. 13) ILpeak VO IO 85% VI 12 VI VO VI L VO fSW – + = (EQ. 14) FIGURE 26. SINGLE SUPPLY 3V OPERATION VIN = 3V ~ 24V VIN VDC EN PWMI FSW RSET COMP LX OVP PGND CH1 CH2 CH3 CH4 AGND ISL97684 VOUT < 26.5V 20mA |
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