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RT8402GJ6 Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT8402GJ6 Datasheet(HTML) 9 Page - Richtek Technology Corporation |
9 / 13 page RT8402 Copyright © 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. DS8402-00 January 2017 www.richtek.com 9 Application Information The RT8402 is a boundary mode, high efficiency constant current controller with internal high side driver, which can be used in buck configuration, to provide a constant output current to the (LED) load. It contains special circuitry for achieving high power factor and low input current THD, while minimizing external component count. The small SOT23-6 package keeps application footprint small, and makes the RT8402 a cost effective solution for off-line LED drivers. The RT8402 can achieve high accuracy LED output current via the average current feedback loop control. The internal sense voltage (250mV typ.) is used to set the average output current. The average current is set by the external resistor, RS. The sense voltage is also used for over current protection (OCP) function. The typical OCP threshold is about seven times of the sense voltage threshold. Under Voltage Lockout (UVLO) The RT8402 includes a UVLO function with 11.2V hysteresis. For system start up, the VIN must rise over 18V (typ.) to turn on the internal MOSFET. The internal MOSFET will turn off if VIN falls below 6.8V (typ.) Setting Average Output Current The output current that flows through the LED string is set by an external resistor, RS, which is connected between the SGND and SENSE pins. The relationship between output current, IOUT, and RS is shown below : OUT S 250 I = mA R Start-up Resistor The start-up resistor should be chosen to set the start up current exceeds certain minimum value. Otherwise, the RT8402 may latch off and the system will never start. The start-up current equals 2 90V / R1 (for 110VAC regions), and equals 2 180V / R1 (for 220VAC regions). The typical required minimum start-up current is 100 A. The typical total start up resistance (R1) is around 1M Ohm for universal inputs. Input Diode Bridge Rectifier Selection The current rating of the input bridge rectifier is dependent on the VOUT /VIN conversion ratio and out LED current. The voltage rating of the input bridge rectifier, VBR, on the other hand, is only dependent on the input voltage. Thus, the VBR rating is calculated as below : BR AC(MAX) V = 1.2 2 V where VAC(MAX) is the maximum input voltage (RMS) and the parameter 1.2 is used for safety margin. For this example : BR AC(MAX) V = 1.2 2 V = 1.2 2 264 = 448V If the input source is universal, VBR will reach 448V. In this case, a 600V, 0.5A bridge rectifier can be chosen. Input Capacitor Selection For High Power Factor application, the input Capacitor CIN should use a small value capacitance to achieve line voltage sine-wave. The voltage rating of the input filter capacitor, VCIN, should be large enough to handle the input voltage. VCIN ≥ (1.2× 2 × VAC(MAX) ) = (1.2× 2 × 264) = 448V Thus, a 0.1 F / 500V film capacitor can be chosen in this case. Inductor Selection For high power factor application, the RT8402 operates the converter in BCM (Boundary-Condition Mode). The inductance range is defined by peak current of inductor、 maximum and minimum value of switching on time and off time, for ensuring the inductor operates in BCM. The peak current of inductor is showed as below : PEAK PEAK 2Pin I = V F a OUT PEAK V where a = V and 4 3 2 F a -0.411a +0.296a -0.312a +0.638a-0.0000846, a|0~0.7 The inductance range is showed as below : |
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