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ISL78100 Datasheet(PDF) 13 Page - Intersil Corporation |
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ISL78100 Datasheet(HTML) 13 Page - Intersil Corporation |
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13 / 19 page  13 FN6626.0 December 17, 2007 Over-Temperature Shutdown An internal sense circuit disables PWM switching if the die temperature exceeds +135°C. Switching is re-enabled when the temperature falls below +100°C. Internal 5V LDO An internal LDO between VIN and VDC regulates VDC to 5V, to power control and gate drive circuits when VIN exceeds 5.1V. In normal operation decouple VDC with at least 3.3µF. In applications where the input supply is less than 5.5V, VDC should be tied directly to VIN. LED Temperature Control LED lifetime reduces dramatically with elevated temperature. An over-temperature control circuit utilizing the thermistor voltage at TEMP reduces the LED bias current when VTEMP exceeds the threshold voltage on TMAX. To minimize noise injection, use a potential divider between VDC and GND to set the voltage on TMAX, as shown in Figure 34. The value of TMAX for a specific threshold temperature is determined by the choice of thermistor temperature coefficient. Disable the function by connecting the TMAX pin to VDC and TEMP pin to GND. Component Selection 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, 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. Considerably more input current ripple is generated in buck mode than boost mode. In buck mode input current is alternately switched between IOUT and zero. The RMS current flow in the input capacitor is given by Equation 3: Where: D = Duty Cycle 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 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. In boost mode input current flows continuously into the inductor, with an AC ripple component proportional to the rate of inductor charging only and smaller value input capacitors may be used. 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. In automotive applications, the input capacitor can be protected from exposure to high voltages present during fault conditions (load dump) by connecting it downstream of the fault protection switch, as shown in Figures 39 and 40. Inductor Careful selection of inductor value will optimize circuit operation. Inductor type and value influence many key parameters, including ripple current, current limit, efficiency, transient performance and stability. Internal slope compensation has been optimized for inductor values between 4.7µH and 10µH. 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 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. FIGURE 33. EXTERNAL OVP CIRCUIT COUT 20µF R1 100 VIN FB SWS1 SWS2 SWD1 SWD2 VBAT FAULT ISL78100 VBAT ZOVP VOUT L1 10µH 0.5 RSENSE FIGURE 34. OVER-TEMPERATURE CIRCUIT ISL78100 GND VIN TEMP THERMISTOR CLOSE VDC LDO CREG 0.47µF RM1 20k RT 10k + - TEMPERATURE COMPENSATION RM2 80k TMAX FB LEVEL CURRENT TO LEDs I CAPRMS I OUT DD 2 – () • = (EQ. 3) ISL78100 |
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