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LM2795 Datenblatt(PDF) 11 Page - Texas Instruments |
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LM2795 Datenblatt(HTML) 11 Page - Texas Instruments |
11 / 24 page ¸¸ ¹ · ¨¨ © § ¸¸ ¹ · ¨¨ © § Amps 1 10 * 188 BRGT V * + 385 . 0 . 0 SET R * o MirrorRati OFFSET V + BRGT V 385 . 0 * R SET LED = I LED = I LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 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%. In addition to RSET, 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 VBRGT 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 RSET. An equation for approximating the LED current is: ILED 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. Copyright © 2002–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2794 LM2795 |
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