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MIC2289C-34BML Datasheet(PDF) 7 Page - Micrel Semiconductor |
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MIC2289C-34BML Datasheet(HTML) 7 Page - Micrel Semiconductor |
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7 / 10 page  Micrel, Inc. MIC2289C September 2006 7 M9999-090706 Dimming Control There are two techniques for dimming control. One is PWM dimming, and the other is continuous dimming. 1. PWM dimming control is implemented by applying a PWM signal on EN pin as shown in Figure 1. The MIC2289C is turned on and off by the PWM signal. With this method, the LEDs operate with either zero or full current. The average LED current is increased proportionally to the duty-cycle of the PWM signal. This technique has high-efficiency because the IC and the LEDs consume no current during the off cycle of the PWM signal. Typical frequency should be between 100Hz and 10kHz. 2. Continuous dimming control is implemented by applying a DC control voltage to the FB pin of the MIC2289C through a series resistor as shown in Figure 2. The LED current is decreased proportionally with the amplitude of the control voltage. The LED intensity (current) can be dynamically varied applying a DC voltage to the FB pin. The DC voltage can come from a DAC signal, or a filtered PWM signal. The advantage of this approach is that a high frequency PWM signal (>10kHz) can be used to control LED intensity. PWM VIN EN SW FB OUT GND VIN Figure 1. PWM Dimming Method VIN EN SW FB OUT 5.11k 49.9k GND DC VIN Equivalent Figure 2. Continuous Dimming Open-Circuit Protection If the LEDs are disconnected from the circuit, or in case an LED fails open, the sense resistor will pull the FB pin to ground. This will cause the MIC2289C to switch with a high duty-cycle, resulting in output overvoltage. This may cause the SW pin voltage to exceed its maximum voltage rating, possibly damaging the IC and the external components. To ensure the highest level of protection, the MIC2289C has 3 product options in the 2mm × 2mm MLF ®-8 with overvoltage protection, OVP. The extra pins of the 2mm × 2mm MLF ®-8 package allow a dedicated OVP monitor with options for 15V, 24V, or 34V (see Figure 3). The reason for the three OVP levels is to let users choose the suitable level of OVP for their application. For example, a 3-LED application would typically see an output voltage of no more than 12V, so a 15V OVP option would offer a suitable level of protection. This allows the user to select the output diode and capacitor with the lowest voltage ratings, therefore smallest size and lowest cost. The OVP will clamp the output voltage to within the specified limits. For the Thin SOT-23-5 package, an OVP pin is not available. An external zener diode can be connected from the output of the converter to FB pin as shown in Figure 4 to implement similar protection. VIN EN GND SW FB OUT VIN Figure 3. MLF ® Package OVP Circuit VIN EN SW FB GND VIN 5.11k Figure 4. Thin SOT-23 Package OVP Circuit |
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Similar Description - MIC2289C-34BML |
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