EL7801 input supply, improving system stability. The high switching frequency of the loop causes almost all ripple current to flow VDC in the input capacitor, which must be rated accordingly. MURATA Considerably more input current ripple is generated in buck NCP18XH103F03RB mode than boost mode. In buck mode input current is R1 23k alternately switched between IOUT and zero. The rms R2 current flow in the input capacitor is given by: 300 I = • ( ) CAPRMS IOUT D D2 – TEMP Where: D = Duty Cycle R3 4k The input current is maximum for D = 0.5 and when IOUT approaches current limit (2.4A) giving a value of around 1.2A. A capacitor with low internal series resistance should be chosen to minimize heating effects and improve system FIGURE 11. THERMISTOR VOLTAGE COEFFICIENTADJUSTMENT efficiency, such as X5R or X7R ceramic capacitors, which offer small size and a lower value of temperature and voltage LED Temperature Control coefficient compared to other ceramic caps. LED lifetime reduces dramatically with elevated In boost mode input current flows continuously into the temperature. An over temperature control circuit utilizing the inductor, with an ac ripple component proportional to the rate thermistor voltage at TEMP reduces the LED bias current of inductor charging only and smaller value input capacitors when VTEMP exceeds the threshold voltage on TMAX. To may be used. It is recommended that an input capacitor of at minimize noise injection use a potential divider between least 10µF be used. Ensure the voltage rating of the input VDC and GND to set the voltage on TMAX, as shown in capacitor is suitable to handle the full supply range. Figure 12. The value of TMAX for a specific shut down temperature is determined by the choice of thermistor In automotive applications the input capacitor can be temperature coefficient. Disable the function by connecting protected from exposure to high voltages present during the TMAX pin to GND. fault conditions (load dump) by connecting it downstream of the fault protection switch, as shown in Figures 19 and 20. Inductor Thermistor Careful selection of inductor value will optimise circuit 0.47uF CREG Close to LED's operation. Inductor type and value influence many key parameters, including ripple current, current limit, efficiency, VIN VDC RM1 20k transient performance and stability. Internal slope E S LDO TMAX N E .5 0 compensation has been optimised for inductor values S RM2 R 80k between 4.7uH and 10uH. Ensure the inductor current rating is capable of handling the current limit value in the + configuration used (2.4A for buck, 3.5A for boost). If an TEMP - inductor core is chosen with too low a current rating, FB Level Adjust Current RT 10K Temp saturation in the core will cause the effective inductor value Compensation GND to fall, leading to an increase in peak to average current EL7801 level, poor efficiency and overheating in the core. Rectifier DiodeFIGURE 12. OVER-TEMPERATURE CIRCUIT A high speed rectifier diode is necessary to prevent excessive voltage overshoot, especially in the boost Component Selection configuration. Low forward voltage and reverse leakage current will minimize losses, making Schottky diodes the Input Capacitor preferred choice. Similarly to the Inductor, a diode with a Switching regulators require input capacitors to deliver peak suitable current rating to handle current limit in the charging current and to reduce the impedance of the input configuration must be used. supply. This reduces interaction between the regulator and 9 FN7354.0 December 22, 2005
