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MP4689DN Datasheet(PDF) 8 Page - Monolithic Power Systems |
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MP4689DN Datasheet(HTML) 8 Page - Monolithic Power Systems |
8 / 10 page MP4689 – 100V INPUT, 1A HIGH POWER LED DRIVER MP4689 Rev. 1.0 www.MonolithicPower.com 8 4/29/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2011 MPS. All Rights Reserved. APPLICATION INFORMATION COMPONENT SELECTION Setting the LED Current The LED current I LED is set using a sensing resistor R FB, which is in series with the LEDs and connected to GND. The voltage on the sensing resistor R FB is connected to FB pin. FB LED FB V I R = For example, for a 700mA LED current, R FB is 287mΩ. Inductor Selection and Frequency Setting The inductor LO is required to supply a constant current to the LED. The inductor value is related to the switching frequency fs setting: OUT IN OUT s 8 IN IN LED V(V V ) f V(8 10 V L 15% I ) − ⋅− = ⋅⋅ ⋅ + ⋅ ⋅ Where, VIN is the input voltage, VOUT is the output voltage to drive the LEDs. A larger value inductor will result in smaller switching frequency. The peak inductor current is about: 8 IN LP LED LED 410 V II ( 1.075) LI − ⋅⋅ =⋅ + ⋅ Choose an inductor that will not saturate under the maximum inductor peak current and the switching frequency under 2MHz. Output Rectifier Diode The output rectifier diode supplies the current to the inductor when the high-side switch is off. To reduce losses due to the diode forward voltage and recovery times, use a Schottky diode. The RMS current through the diode is about: OUT DLED IN V II 1 V =⋅ − Choose a diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating is greater than the maximum diode current. Input Capacitor CIN The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance especially under high switching frequency applications. The RMS current through the input capacitor is about: OUT DLED IN V II V =⋅ ’ With low ESR capacitors, the input voltage ripple can be estimated by: OUT OUT LED IN sIN IN IN VV I V1 fC V V ⎛⎞ ∆= × × − ⎜⎟ × ⎝⎠ Choose the input capacitor with enough RMS current rating and enough capacitance for small input voltage ripple. When electrolytic or tantalum capacitors are applied, a small, high quality ceramic capacitor, i.e. 0.1µF, should be placed as close to the IC as possible. Output Capacitor COUT The output capacitor (COUT) is not necessary for MP4689. The LED current ripple (peak-to-peak value) is about 15% of the LED DC current. Output capacitor can be used to further reduce the LED current ripple. Low ESR capacitors are preferred to keep the output voltage ripple low so that the AC ripple current through the LEDs is small. The output voltage ripple can be estimated by: OUT OUT OUT ESR SO IN S OUT VV 1 V1 R fL V 8 f C ⎛⎞ ⎛⎞ ∆= × − × + ⎜⎟ ⎜⎟ ×× × ⎝⎠ ⎝⎠ Where RESR is the equivalent series resistance (ESR) value of the output capacitor. |
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