Datenblatt-Suchmaschine für elektronische Bauteile |
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SM8237 Datenblatt(PDF) 10 Page - SamHop Microelectronics Corp. |
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SM8237 Datenblatt(HTML) 10 Page - SamHop Microelectronics Corp. |
10 / 14 page SAMHOP Microelectronics Corp. 1.2MHz STEP-UP CONVERTER APPLICATIONS INFORMATION LED current control The LED current is controlled by the feedback resistor. The feedback reference is 95mV. The LED current is 95 mV/R1. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The formula and table for R1 selection are shown below. (1) R1 = 95 mV / ILED ILED (mA) R1 (Ω) 5 19.1 10 9.53 12 7.87 15 6.34 20 4.75 SM8237 Table 1. R1 resistor value selection Figure 2. LED driver with open-circuit protection SM8237 Page 9 V.1.3 Aug 28,2007 Open-circuit protection In the cases of output circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The SM8237 will then switch at a high duty cycle resulting in a high output voltage, which may cause the SW pin voltage to exceed its maximum 36V rating. A zener diode can be used at the output to limit the voltage on the SW pin (figure 2). The zener voltage should be larger than the maximum forward voltage of the LED string. The current rating zener should be larger than 0.1mA. Dimming control There are four different types of dimming control circuits: 1. Using a PWM signal to SHDN pin With the PWM signal applied to the SHDN pin, the SM8237 is turned on or off by the PWM signal. The LEDs operate at either zero or full current. The average LED current increases proportionally with the duty cycle of the PWM signal. A 0% duty cycle will turn off the SM8237 and corresponds to zero full LED current. A 100% duty cycle corresponds to full current. The typical frequency range of the PWM signal is 1kHz to 10kHz. The magnitude of the PWM signal should be higher than the minimum SHDN voltage high. Setting the output voltage for DC to DC Converter Set the output voltage by selecting the resistive voltage divider ratio. Using 51 KΩ for the High-side resistor R1 of the voltage divider. Determine the Low-side resistor R2 according to the following formula: R1 · VFB Vout - VFB R2 = Where Vout is the output voltage for R1= 51 KΩ and VFB = 0.095 V, then R2 (KΩ) = 4.845 / Vout -0.095 Please see the typical application on page 1. |
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Ähnliche Beschreibung - SM8237 |
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