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SM8205 Fiches technique(PDF) 9 Page - SamHop Microelectronics Corp. |
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SM8205 Fiches technique(HTML) 9 Page - SamHop Microelectronics Corp. |
9 / 13 page SM8205 Over-Voltage Protection (OVP) Over voltage protection is achieved by connecting the output voltage to the OVP pin through a resistor divider. The voltage at the OVP pin is constantly compared to the internal 1.2V. When the output voltage at LED load terminal is exceeded 1.2V, the IC is turned off, immediately, at the same time the GATE and FAULT pins goes low. Power on again to resume this situation. The output voltage can be set by selecting the values of R1 and R2 (see figure 1) according to the following equation: R1 R2 VOUT OVP SM8205 R2 R1+R2 1.2 • VOUT = Figure 1. Over-voltage protection resistor connection 1.2V PWM Dimming (PWMD) PWM dimming can be achieved by driving the PWMD pin with a TTL compatible source. The PWM signal is connected internally to the three different node – the transconductance amplifier, the FAULT output, and the GATE output. When the PWMD signal is high, the GATE and FAULT pins are enabled, and the output of the EA amplifier is connected to the external compensation network. Thus, the internal amplifier controls the LED current. When the PWMD signal goes low, the output of the EA amplifier is disconnected from the compensation network. So, the integrating capacitor maintains the voltage across it. The GATE is disabled, the converter stops switching and the FAULT pin goes low, turning off the disconnect switch. By turning off the disconnect switch action, the output capacitor is prevented from being discharged, and thus the PWM dimming response of the boost converter improvs dramatically. When the voltage at PWMD is greater than 2.0V, the PWM dimming MOSFET turns on and when the voltage on PWMD is below 0.8V, the PWM dimming MOSFET turns off. LED Current Reference (IREF) The LED current is propotional to the voltage at IREF. Applying an external DC voltage at IREF or using a potentionmeter from IREF to GND allow analog dimming of the LED current. Gate driver (GATE) External MOSFETs are driven by the SM8205's internal low impedance gate driver. These driver are biased from the VDD and have a source current of 150mA and a sink current of 300mA, to switch a ground-referenced N- channel MOSFET in high-power applications. The average current demanded from the supply to drive the external MOSFET depends on the total gate charge (Qg) and the operating frequency of the converter, FSW. Use the following equation to calculate the driver supply current IGATE required for the switching MOSFET: Dimming MOSFET Driver (FAULT) IGATE = Qg x FSW The SM8205 requires an external N-channel MOSFET for PWM dimming. Connect the gate of the MOSFET to the output of the dimming driver, FAULT, for normal operation. The dimming driver is capable of sourcing or sinking up to 50mA of current. LED Current-Sense Input (FDBK) The current through the LED string is set via the value chosen for the current sense resistor, RSN. This value can be calculated using equation of below: VIREF RSN ILED = FDBK RSN FAULT SM8205 ILED COUT Figure 2. LED Forward Current Controls Path Another important parameter to be aware of in the boost controller design, is the ripple current. The amount of ripple current through the LED string is equal to the output ripple voltage divided by LED AC resistance (RLED is provided by the LED manufacturer) plus the current sense and RON of the MOSFET resistor. The amount of allowable ripple current through the LED string is dependent upon the application and design’s discretion. The equation is shown as below: VOUT(RIPPLE) RLED + RON + RSN ΔILED = 9 www.samhop.com.tw Rev. 2.0 @ 2011/10 |
Numéro de pièce similaire - SM8205 |
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Description similaire - SM8205 |
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