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FAN5333A датащи(PDF) 8 Page - Fairchild Semiconductor |
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FAN5333A датащи(HTML) 8 Page - Fairchild Semiconductor |
8 / 11 page 8 www.fairchildsemi.com FAN5333A/FAN5333B Rev. 1.0.1 Inductor Selection The inductor parameters directly related to device performances are saturation current and dc resistance. The FAN5333A/ FAN5333B operates with a typical inductor value of 10µH. The lower the dc resistance, the higher the efficiency. Usually a trade-off between inductor size, cost and overall efficiency is needed to make the optimum choice. The inductor saturation current should be rated around 1A, in an application having the LED current near the maximum cur- rent as indicated in “Typical Performance Characteristics”. The peak inductor current is limited to 1.5A by the current sense loop. This limit is reached only during the start-up and with heavy load condition; when this event occurs the converter can shift over in discontinuous conduction mode due to the auto- matic turn-off of the switching transistor, resulting in higher rip- ple and reduced efficiency. Some recommended inductors are suggested in the table below: Table 1: Recommended Inductors Capacitors Selection For best performance, low ESR input and output capacitors are required. Ceramic capacitors of CIN = 10µF and COUT = 1µF placed as close to the IC pins, are required for the maximum load(65mA). For the lighter load ( ≤ 20mA) the capacitances may be reduced to CIN = 4.7µF and COUT = 0.47µF or even to 0.1µF, if higher ripple is acceptable. The output capacitor voltage rating should be according to the VOUT setting. Some capacitors are suggested in the table below. Table 2: Recommended Capacitors Diode Selection The external diode used for rectification is usually a Schottky diode. Its average forward current and reverse voltage maxi- mum ratings should exceed the load current and the voltage at the output of the converter respectively. A barrier Schottky diode such as BAT54 is preferred, due to its lower reverse current over the temperature range. Care should be taken to avoid any short circuit of VOUT to GND, even with the IC disabled, since the diode can be instantly dam- aged by the excessive current. Brightness Control 1. Dimming Using PWM Logic Signal A PWM signal applied to SHDN Table 5 on page 8 can control the LED’s brightness in direct dependence with the duty cycle. The maximum frequency should not exceed 1kHz to ensure a linear dependence of the LED’s average current. The amplitude of the PWM signal should be suitable to turn the FAN5333 ON and OFF. Alternatively, a PWM logic signal can be used to switch a FET ON/OFF to change the resistance that sets the LED’s current Table 6 on page 8. Adjusting the duty cycle from 0% to 100% results in varying the LED’s current between IMIN and IMAX. Where Figure 5. Dimming Using a PWM Signal Figure 6. Dimming Using a PWM Logic Signal 2. Dimming Using DC Voltage An external adjustable DC voltage Table 7 on page 8 between 0V to 2V can control the LED’s current from 15mA to 0mA, respectively. Figure 7. Dimming Using DC Voltage Inductor Value Vendor Part Number Com- ment 10µH TDK SLF6025&-100M1R0 10µH MURATA LQH66SN100M01C Highest Efficiency 10µH COOPER SD414-100 Small Size Capacitor Value Vendor Part Number 0.47µF Panasonic ECJ-3YB1E474K 1µF Murata GRM21BR61E105K 10µF Murata GRM21BR61A106K I MIN V FB R MIN ------------- = and I MAX V FB R MIN R SET -------------------------------- = SHDN FAN5333 FB FAN5333 RMIN RSET FB VDC 90K Ω 5 Ω 1.6K Ω FAN5333A FB VDC 90K Ω 15 Ω 4.7K Ω FAN5333B |
Аналогичный номер детали - FAN5333A |
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Аналогичное описание - FAN5333A |
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