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MIC2293C-34YML Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC2293C-34YML Datasheet(HTML) 9 Page - Micrel Semiconductor |
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9 / 10 page  Micrel, Inc. MIC2292/93C September 2009 9 M9999-091509-A 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 MIC2292/93C 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 MIC2292/93C 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 MIC2292/93C 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 MIC2292/93C has three product options in the 8-pin MLF ® with overvoltage protection, OVP. The extra pins of the 8-pin MLF ® package allow the use of a dedicated OVP monitor with options for 15V 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, and accordingly, smallest size and lowest cost. The OVP will clamp the output voltage to within the specified limits. VIN EN GND SW FB OUT VIN Figure 3. MLF ® Package OVP Circuit Start-Up and Inrush Current During start-up, inrush current of approximately double the nominal current flows to set up the inductor current and the voltage on the output capacitor. If the inrush current needs to be limited, a soft-start circuit similar to Figure 4 could be implemented. The soft-start capacitor, Css, provides over-drive to the FB pin at start-up, resulting in gradual increase of switch duty cycle and limited inrush current. VIN EN 10k 2200pF SW OUT FB CSS R GND VIN Figure 4. One of Soft-Start Circuit |
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