Electronic Components Datasheet Search |
|
ISL97687 Datasheet(PDF) 17 Page - Renesas Technology Corp |
|
ISL97687 Datasheet(HTML) 17 Page - Renesas Technology Corp |
17 / 24 page ISL97687 FN7714 Rev.3.00 Page 17 of 24 Sep 13, 2017 Inductor The selection of the inductor should be based on its maximum current (ISAT) characteristics, power dissipation, EMI susceptibility (shielded vs unshielded), and size. Inductor type and value influence many key parameters, including the inductor ripple current, current limit, efficiency, transient performance and stability. The inductor’s maximum current capability must be large enough to handle the peak current at the worst case condition. If an inductor core is chosen with a lower 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, as expressed in Equation 10: 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 frequency, minimum input voltage must be used to calculate the input/inductor current as shown in Equation 10. For a given inductor size, the larger the inductance value, the higher the series resistance because of the extra number of turns required, thus, higher conductive losses. The ISL97687 current limit should be less than the inductor saturation current. Output Capacitors The output capacitor acts to smooth the output voltage and supplies load current directly during the conduction phase of the power switch. Output ripple voltage consists of the discharge of the output capacitor during the FET turn-on period and the voltage drop due to load current flowing through the ESR of the output capacitor. The ripple voltage is shown in Equation 11: where IO represents the output current, CO is the output capacitance, D is the duty ratio as described in Equation 9. ESR is the equivalent series resistance of the output capacitance and fsw is the switching frequency of the converter. Equation 11 shows the importance of using a low ESR output capacitor for minimizing output ripple. As shown in Equation 11, the output ripple voltage, VCo, can be reduced by increasing the output capacitance, CO or the switching frequency, fSW, or using output capacitors with small ESR. In general, ceramic capacitors are the best choice for output capacitors in small to medium sized LCD backlight applications due to their cost, form factor, and low ESR. The choice of X7R over Y5V ceramic capacitors is highly recommended because the X7R type capacitor is less sensitive to capacitance change overvoltage. Y5V’s absolute capacitance can be reduced to 10%~20% of its rated capacitance at the maximum voltage. Because of this, Y5V type ceramic capacitors should be avoided. A larger output capacitor will also ease the driver response during PWM dimming off period due to the longer sample and hold effect of the output drooping. The driver does not need to boost as much on the next on period, which minimizes transient current. The output capacitor also plays an important role for system compensation. Channel Capacitor It is recommended to use at least 1nF capacitors from CH pins to VOUT. Larger capacitors will reduce LED current ripple at boost frequency, but will degrade transient performance at high PWM frequencies. The best value is dependant on PCB layout. Up to 4.7nF is sufficient for most configurations. Schottky Diode A high speed rectifier diode is necessary to prevent excessive voltage overshoot, especially in the boost configuration. Low forward voltage and reverse leakage current will minimize losses, making Schottky diodes the preferred choice. Although the Schottky diode turns on only during the boost switch off period, it carries the same peak current as the inductor, therefore, a suitable current rated Schottky diode must be used. High Current Applications Each channel of the ISL97687 can support up to 160mA. For applications that need higher current, multiple channels can be grouped to achieve the desirable current. For example, in Figure 27, the cathodes of the last LEDs can be connected to CH1/CH2 and CH3/CH4, this configuration can be treated as a single string with up to 350mA current driving capability. ILpeak VO IO 85% VI 12 VI VO VI L VO fSW – + = (EQ. 10) VCO I O CO DfSw IO ESR + = (EQ. 11) FIGURE 27. GROUPING MULTIPLE CHANNELS FOR HIGH CURRENT APPLICATIONS CH1 CH2 CH3 CH4 BOOST OUTPUT |
Similar Part No. - ISL97687 |
|
Similar Description - ISL97687 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |