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LM2794 Datasheet(PDF) 9 Page - Texas Instruments |
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LM2794 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 21 page VIN LM2795 SD Shutdown Control *Only on LM2795 10PA * LM2794, LM2795 www.ti.com SNVS168K – MAY 2004 – REVISED MAY 2004 Figure 2. Open-Drain Logic Shutdown Control The LM2794 has an active-low shutdown pin (LOW = shutdown, 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 aluminum electrolytic capacitors are generally not recommended for use with the LM2794/5 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 LM2794/5. 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 LM2794/5. Capacitors 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 capacitance 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 manufacturers. 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 ISET AND BRGT PINS An external resistor, RSET, 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 RSET. The current mirror circuitry matches the current through each LED to within 0.5%. Copyright © 2004, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2794 LM2795 |
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