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LM2792LDX-L Fiches technique(PDF) 8 Page - National Semiconductor (TI) |
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LM2792LDX-L Fiches technique(HTML) 8 Page - National Semiconductor (TI) |
8 / 10 page Application Information (Continued) be used for most applications or a surface mount diode such as BAT54-series and MA2J704 can be used to reduce the circuit size.Table 2 below lists suggested schottky diode manufactures. TABLE 2. Diode Manufactures Manufacturer Contact website ON Semiconductor (800) 344 3860 www.onsemi.com Phillips Semiconductors (800) 234 7381 www.philipssemi- conductor.com Panasonic Semiconductors (408) 945 5622 www.panasonic.com LED SELECTION The LM2792 is designed to drive LEDs with a forward volt- age of about 3.0V to 4.0V or higher. The typical and maxi- mum V F depends highly on the manufacturer and their tech- nology. Table 3 lists two suggested manufactures and example part numbers. Each supplier makes many LEDs that work well with the LM2792. The LEDs suggested below are in a surface mount package and TOPLED or SIDEVIEW configuration with a maximum forward current of 20mA. These diodes also come in SIDELED or SIDEVIEW configu- ration and various chromaticity groups. For applications that demand color and brigthness matching, care must be taken to select LEDs from the same chromaticity group. Forward current matching is assured over the LED process variations due to the constant current output of the LM2792. For best fit selection for an application, consult the manufacturer for detailed information. TABLE 3. White LED Selection: Component Manufacture Contact LWT673/LWT67C Osram www.osram-os.com NSCW100/NSCW215 Nichia www.nichia.com I SETPIN An external resistor, R SET, sets the mirror current that is required to provide a constant current through the LEDs. The current through R SET and the LED is set by the internal current mirror circuitry with a ratio of 25:1 The currents through each LED are matched within 0.3%. R SET should be chosen not to exceed the maximum current delivery capa- bility of the device. Table 3 shows a list of R SET values when maximun BRGT=3V is applied. For other BRGT voltages, R SET can be calculated using this formula: R SET = ((0.42*BRGT) / ISET)*25 TABLE 4. R SETSelections ( when BRGT pin = 3V maximum) I LED per LED R SET 15mA 2.1K Ω 10mA 3.15K Ω 5mA 6.3K Ω BRGT PIN The BRGT pin can be used to smoothly vary the brightness of the LEDs. In the LM2792, current on BRGT is connected to an internal resistor divider which gives a factor of 0.42 (see Figure 1) . This voltage is fed to the operational ampli- fier that controls the current through the mirror resistor R SET. The nominal range on BRGT is 0V to 3V. This means some current must be provided on the BRGT pin or no current will flow through the LEDs. The LM2792 can provide an infinite ratio, from fully off (essentially zero cur- rent) to the maximum current set by the R SET resistor. Care must be taken to prevent voltages on BRGT that cause LED current to exceed a total of 34mA. Although this will not cause damage to the IC, it will not meet the guaranteed specifications listed in the Electrical Characteristics. Calculation of LED Current When Using BRGT : V IN = 3.6V R SET = 1800 Ω I SET = ((VBRGT * (0.42) / RSET )* 25 I SET = ((2.75*(0.42)) / 1800 )*25 = 16mA Note that making V BRGT = 0V results in ISET ∼ = 0mA BRIGHTNESS CONTROL USING PWM Brigthness control can be implemented by pulsing a signal at the SD pin. The recommended signal should be between 100Hz to 1kHz. If the operating PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. Likewise, if frequency is much higher, brightness in the LEDs will not be linear. When a PWM signal is used to drive the SD pin of the LM2792, connect BRGT pin to a maximun of 3V to ensure the widest range. Similarly, the voltage at the BRGT pin can be set higher than 3V without damage to the IC, it will not increase the brigthness of the LED significantly. R SET value is selected using the above I SET equation as if BRGT pin is used. The brigthness is controlled by increasing and de- creasing the duty cycle of the PWM. Zero duty cycle will turn off the brigthness and a 50% duty cycle waveform produces an average current of 7.5mA if R SET is set to produce a maximum LED current of 15mA. So the LED current varies linearly with the duty cycle. PARALLEL DX OUTPUTS FOR INCREASED CURRENT DRIVE Outputs D1 and D2 may be connected together to drive a single LED. In such a configuration, two parallel current sources of equal value drive the single LED. R SET and V BRGT should be chosen so that the current through each of the outputs is programmed to 50% of the total desired LED current. For example, if 30mA is the desired drive current for the single LED, R SET and VBRGT should be selected so that the current through each of the outputs is 15mA. Connecting the outputs in parallel does not affect internal operation of the LM2792 and has no impact on the Electrical Character- istics and limits previously presented. The available Dx out- put current, maximum Dx voltage, and all other specifica- tions provided in the Electrical Characteristics table apply to this parallel output configuration, just as they do to the standard 2-LED application circuit. THERMAL PROTECTION The LM2792 has internal thermal protection circuitry to dis- able the charge pump if the junction temperature exceeds 150˚C. This feature will protect the device from damage due to excessive power dissipation. The device will recover and operate normally when the junction temperature falls below the maximum operating junction temperature of 100˚C. It is www.national.com 8 |
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