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CAT4104V-GT3 Datasheet(PDF) 9 Page - ON Semiconductor |
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CAT4104V-GT3 Datasheet(HTML) 9 Page - ON Semiconductor |
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9 / 14 page  CAT4104 © 2009 SCILLC. All rights reserved. 9 Doc. No. MD-5041 Rev. B Characteristics subject to change without notice APPLICATION INFORMATION SINGLE 12 V SUPPLY The circuit shown in Figure 3 shows how to power the LEDs from a single 12 V supply using the CAT4104. Three external components are needed to create a lower voltage necessary for the VIN pin (below 5.5 V). The resistor R2 and zener diode Z provide a regulated voltage while the quiescent current runs through the N-Channel transistor M. The recommended parts are ON Semiconductor MM3Z6V2 zener diode (in SOD-323 package), and 2N7002L N-Channel transistor (in SOT23). VIN R2 5k Ω Z 6.2V M CAT4104 12V R1 C1 0.1µF EN/PWM RSET LED1 LED2 LED3 LED4 GND C2 1µF Figure 3. Single Supply Driving 12 LEDs DAYLIGHT DETECTION The circuit in Figure 4 shows how to use CAT4104 in an automatic light sensor application. The light sensor allows the CAT4104 to be enabled during the day and disabled during the night. Two external components are required to configure the part for ambient light detection and conserve power. Resistor R1 sets the bias for the light sensor. The recommended part is Microsemi LX1972 light sensor. For best performance, the LED light should not interfere with the light sensor. VIN VDD VSS Light Sensor R1 100 k Ω CAT4104 5 V R1 C1 0.1 µF EN/PWM RSET LED1 LED2 LED3 LED4 GND ON OFF C2 1 µF Figure 4. Daylight Detection NIGHTLIGHT DETECTION The circuit shown in Figure 5 illustrates how to use the CAT4104 in an automatic night light application. The light sensor allows the CAT4104 to be disabled during the day and enabled during the night. Five external components are needed to properly configure the part for night detection. Resistor R3 limits the quiescent current through the N-Channel transistor M. Resistors R1 and R2 act as a voltage divider to create the required voltage to turn on transistor M, which disables the CAT4104. The recommended parts are ON Semiconductor 2N7002L N-Channel transistor (in SOT23) and the Microsemi LX1972 light sensor. For best performance, the LED light should not interfere with the light sensor. VIN VDD VSS Light Sensor R1 100 k Ω CAT4104 5V R4 M C1 0.1 µF EN/PWM RSET LED1 LED2 LED3 LED4 GND ON OFF C2 1µF R2 1M Ω R3 100 k Ω Figure 5. Night Detection LED CURRENT DERATING The circuit shown in Figure 6 provides LED tempe- rature derating to avoid over-driving the LED under high ambient temperatures, by reducing the LED current to protect the LED from over-heating. The positive thermo coefficient (PTC) thermistor RPTC is used for temperature sensing and should be located near the LED. As the temperature of RPTC increases, the gate voltage of the MOSFET M1 decreases. This causes the transistor M1 on-resistance to increase which results in a reduction of the LED current. The circuit is powered from a single VCC voltage of 5 V. The recommended parts are Vishay 70ºC thermistor PTCSS12T071DTE and ON Semiconductor 2N7002L N-Channel transistor (in SOT23). The PCB and heatsink for the LED should be designed such that the LED current is constant within the normal temperature range. But as soon as the ambient temperature exceeds a max threshold, the LED current drops to protect the LEDs from overheating. |
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