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LM2794TLX Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM2794TLX Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 16 page Application Information (Continued) SHUTDOWN MODE The shutdown pin (SD) disables the part and reduces the quiescent current to 2.3µA (typ.). The LM2795 has an active-high shutdown pin (HIGH = shutdown, LOW = operating). An internal pull-up is con- nected between SD and V IN of the LM2795. This allows the use of open-drain logic control of the LM2795 shutdown, as shown in Figure 1. The LM2795 SD pin can also be driven with a rail-to-rail CMOS logic signal. The LM2794 has an active-low shutdown pin (LOW = shut- down, HIGH = operating). The LM2794 SD pin can be driven with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic, etc). There is no internal pull-up or pull-down on the SD pin of the LM2794. CAPACITOR SELECTION The LM2794/5 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. 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 LM2794/5 due to their high ESR, as com- pared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2794/5. These capacitors have tight capacitance toler- ance (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. Capaci- tors with Y5V or Z5U temperature characteristic are gener- ally not recommended for use with the LM2794/5. Capaci- tors with these temperature characteristics typically have wide capacitance 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 capaci- tance requirements of the LM2794/5. Table 1 lists suggested capacitor 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 LM2794/5 is designed to drive LEDs with a forward voltage 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 manufac- turers. Forward current matching is assured over the LED process variations due to the constant current output of the LM2794/5. TABLE 2. White LED Selection Manufacturer Contact Osram www.osram-os.com Nichia www.nichia.com I SET AND BRGT PINS An external resistor, R SET, is connected to the ISET pin to set the current to be mirrored in each of the LED outputs. The internal current mirror sets each LED output current with a 10:1 ratio to the current through R SET. The current mirror circuitry matches the current through each LED to within 0.5%. In addition to R SET, a voltage may be applied to the VBRGT pin to vary the LED current. By adjusting current with the Brightness pin (BRGT), the brightness of the LEDs can be smoothly varied. Applying a voltage on BRGT between 0 to 3 volts will linearly vary the LED current. Voltages above 3V do not increase the LED current any further. The voltage on the V BRGT pin is fed into an internal resistor network with a ratio of 0.385. The resulting voltage is then summed with a measured offset voltage of 0.188V, which comes from the reference voltage being fed through a resistor network (See Functional Block Diagram). The brightness control circuitry then uses the summed voltage to control the voltage across R SET.An equation for approximating the LED current is: 20028540 20028536 FIGURE 1. Open-Drain Logic Shutdown Control www.national.com 10 |
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