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LM2793LDX датащи(PDF) 7 Page - National Semiconductor (TI) |
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LM2793LDX датащи(HTML) 7 Page - National Semiconductor (TI) |
7 / 10 page Application Information CIRCUIT DESCRIPTION The LM2793 is a 1.5x CMOS charge pump that provides two matched constant current outputs for driving up to 16mA through high forward voltage drop White LEDs from Li-Ion battery sources. The device has two regulated current sources connected to the output of the device’s 1.5x loosely regulated charge pump (P OUT). The device’s loosely- regulated charge pump has both open loop and closed loop modes of operation. When the device is in open loop, the voltage at P OUT is 1.5 times the voltage at the input. When the device is in closed loop, the voltage at P OUT is loosely regulated to 4.9V (typ.). To set the LED drive current, the device uses the voltage applied to the dual function shutdown-brightness pin (SD-BRGT) to set a reference cur- rent. This reference current is then multiplied and mirrored to each current output. The LED brightness can be controlled by both analog and/or digital methods. The digital technique uses a PWM (Pulse Width Modulation) signal applied to the SD-BRGT pin. The analog technique applies an analog volt- age in the range of 0.7V to 2.75 to the SD-BRGT pin to vary the LED current (see Shutdown and Brightness Control). SOFT START LM2793 includes a soft start function to reduce the inrush currents and high peak current during power up of the de- vice. Soft start is implemented internally by ramping the reference voltage more slowly than the applied voltage. During soft start, the switch resistances limit the inrush cur- rent used to charge the flying and hold capacitors. SHUTDOWN AND BRIGHTNESS CONTROL The LM2793 has an active-low dual function shutdown- brightness control pin, SD-BRGT. A voltage higher than 0.65V (typ.) on SD-BRGT will put the LM2793 in active mode. Applying a voltage below 0.35V (typ.) on the SD- BRGT pin will turn off the device, reducing the quiescent current to 3µA (typ.). The LM2793 has the ability to adjust LED brightness by applying an analog voltage or a PWM signal to the SD- BRGT pin. For constant brightness or analog brightness control, continue with “Analog brightness control” below. Otherwise go to “Brightness control using PWM”. 1. Analog brightness control The current for the dual LED outputs can be adjusted by varying the voltage on the SD-BRGT pin. The typical range for adjusting LED brightness is between 0.7 and 2.75V. Figure 1 shows how the current changes with respect to the voltage applied to SD-BRGT. If full bright- ness (16mA) is desired, the voltage on SD-BRGT should be greater than 2.75V (typ.) but not more than V IN. 2. Brightness control using PWM Increasing and decreasing the duty cycle of the PWM signal controls the LED brightness. Zero duty cycle will turn off the LEDs and a 50% duty cycle will result in an average I LED being half of the maximum LED current. The recommended frequency range for the PWM signal is between 100Hz and 1KHz. If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. If the frequency is much higher than 1kHz, brightness in the LEDs will not adjust linearly with duty cycle due to the 30µs (typ.) start-up time of the device. The voltage level for the PWM signal should be greater than 2.75V (typ.) but not exceed the voltage on V IN. CAPACITOR SELECTION The LM2793 requires 4 external capacitors for proper opera- tion. Surface-mount multi-layer ceramic capacitors are rec- ommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and alumi- num electrolytic capacitors are generally not recommended for use with the LM2793 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2793. These capacitors have tight capacitance tolerance (as good as ±10%), hold their value over temperature (X7R: ±15% over −55˚C to 125˚C; X5R: ±15% over −55˚C to 85˚C), and typically have little voltage coefficient. Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2793. Capacitors with these temperature characteristics typically have wide ca- pacitance tolerance (+80%, −20%), vary significantly over temperature (Y5V: +22%, −82% over −30˚C to +85˚C range; Z5U: +22%, −56% over +10˚C to +85˚C range), and have poor voltage coefficients. Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2793. Table 1 lists suggested capaci- tor suppliers for the typical application circuit. TABLE 1. Ceramic Capacitor Manufacturers Manufacturer Contact TDK www.component.tdk.com Murata www.murata.com Taiyo Yuden www.t-yuden.com LED SELECTION The LM2793 is designed to drive LEDs with a forward volt- age of about 3.0V to 4.0V. The typical and maximum diode forward voltage depends highly on the manufacturer and their technology. Table 2 lists two suggested manufacturers. Forward current matching is assured over the LED process variations due to the constant current output of the LM2793. 20063607 FIGURE 1. LED Current vs. V SD-BRGT 2 LEDs, V DX = 3.6V, VIN = 3.6V www.national.com 7 |
Аналогичный номер детали - LM2793LDX |
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Аналогичное описание - LM2793LDX |
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