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LM2794 датащи(PDF) 11 Page - National Semiconductor (TI) |
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LM2794 датащи(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page Application Information (Continued) I LED CURRENT SELECTION PROCEDURES The following procedures illustrate how to set and adjust output current levels. For constant brightness or analog brightness control, go to “Brightness control using BRGT”. Otherwise refer to “Brightness control using PWM”. Brightness Control Using PWM 1. Set the BRGT pin to 0V. 2. Determine the maximum desired I LED current. Use the I LED equation to calculate RSET by setting BRGT to 0V or use Table 3 to select a value for R SET when BRGT equals 0V. 3. Brightness control can be implemented by pulsing a signal at the SD pin. LED brightness is proportional to the duty cycle (D) of the PWM signal. For linear bright- ness control over the full duty cycle adjustment range, the PWM frequency (f) should be limited to accommo- date the turn-on time (T ON = 100µs) of the device. D x (1/f) > T ON f MAX =DMIN ÷TON If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. For the LM2794, zero duty cycle will turn off the LEDs and a 50% duty cycle will result in an average I LED being half of the programmed LED current. For example, if R SET is set to program 15mA, a 50% duty cycle will result in an average I LED of 7.5mA. For the LM2795 however, 100% duty cycle will turn off the LEDs and a 50% duty cycle will result in an average I LED being half the programmed LED current. Brightness Control Using BRGT 1. Choose the maximum I LED desired and determine the max voltage to be applied to the BRGT pin. For constant brightness, set BRGT to a fixed voltage between 0V to 3V. 2. Use Table 3 to determine the value of R SET required or use the I LED equation above to calculate RSET. 3. Use Table 4 as a reference for the dimming profile of the LEDs, when BRGT ranges from 0V to 3V. TABLE 3. R SET Values LED Current BRGT 5mA 10mA 15mA 20mA 0.0V 374 Ω 187 Ω 124 Ω 93.1 Ω 0.5V 768 Ω 383 Ω 255 Ω 191 Ω 1.0V 1.15K Ω 576 Ω 383 Ω 287 Ω 1.5V 1.54K Ω 768 Ω 511 Ω 383 Ω 2.0V 1.91K Ω 953 Ω 624 Ω 475 Ω LED Current BRGT 5mA 10mA 15mA 20mA 2.5V 2.32K Ω 1.15K Ω 768 Ω 576 Ω 3.0V 2.67K Ω 1.33K Ω 909 Ω 665 Ω RSET values are rounded off to the nearest 1% standard values. TABLE 4. LED Current R SET Values BRGT 2.67K Ω 1.33K Ω 909 Ω 665 Ω 0.0V 0.7mA 1.4mA 2.1mA 2.8mA 0.5V 1.4mA 2.9mA 4.2mA 5.7mA 1.0V 2.1mA 4.3mA 6.3mA 8.6mA 1.5V 2.9mA 5.8mA 8.4mA 11.5mA 2.0V 3.6mA 7.2mA 10.5mA 14.4mA 2.5V 4.3mA 8.7mA 12.7mA 17.3mA 3.0V 5.0mA 10.1mA 14.8mA 20.2mA CHARGE PUMP OUTPUT (P OUT) The LM2794/5 charge pump is an unregulated switched capacitor converter with a gain of 1.5. The voltage at the output of the pump (the P OUT pin) is nominally 1.5 x VIN. This rail can be used to deliver additional current to other circuitry. Figure 2 shows how to connect additional LEDs to P OUT.A ballast resistor sets the current through each LED, and LED current matching is dependent on the LED forward voltage matching. Because of this, LEDs driven by P OUT are recom- mended for functions where brightness matching is not criti- cal, such as keypad backlighting. Since P OUT is unregulated, driving LEDs directly off POUT is usually practical only with a fixed input voltage. If the input voltage is not fixed (Li-Ion battery, for example), using a linear regulator between the P OUT pin and the LEDs is recommended. National Semiconductor’s LP3985-4.5V low- dropout linear regulator is a good choice for such an appli- cation. The voltage at P OUT is dependent on the input voltage supplied to the LM2794/5, the total LM2794/5 output current, and the output resistance (R OUT) of the LM2794/5 charge pump. Output resistance is a model of the switching losses of the charge pump. Resistances of the internal charge pump switches (MOS transistors) are a primary component of the LM2794/5 output resistance. Typical LM2794/5 output resistance is 3.0 Ω. For worst-case design calculations, using an output resistance of 3.5 Ω is recommended. (Worst-case recommendation accounts for parameter shifts from part-to- part variation and applies over the full operating temperature range). www.national.com 11 |
Аналогичный номер детали - LM2794 |
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Аналогичное описание - LM2794 |
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