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LM2754 датащи(PDF) 9 Page - National Semiconductor (TI) |
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LM2754 датащи(HTML) 9 Page - National Semiconductor (TI) |
9 / 11 page Application Information (Continued) signal on the TX pin, the LED current level returns to the Flash current level set by R SET2. The TX pin responds to the typical logic High (>1.2V) and logic Low (<0.4V) signal levels. Flash Timeout is not active during the TX mode operation. SEL PIN Connecting the SEL pin to a logic Low (<0.4V) signal places the device in normal operation, with all 4 current sinks active. To accommodate Flash LED modules with only 3 LEDs, place a logic High (>1.2V) signal on the SEL pin to disable the current sink D 4. If only 3 current sinks are used, the 200mA per current sink recommendation still applies, and the maximum Flash current will be 600mA. Connect D 4 to V OUT when the logic in the SEL pin is High. Optional use of the SEL pin is to reduce the LED current used for Torch or Flash by 25% for high battery load conditions. SETTING LED CURRENTS The current through the LEDs connected to D 1-4 can be set simply by connecting an appropriately sized resistor (R SETx) between the I SET1 pin of the LM2754 and GND for Torch mode and the I SET2 pin and GND for Flash Mode. The LED currents are proportional to the current that flows out of the I SETx pin and are a factor of approximately 800 times greater than the I SETx current. The feedback loop of an internal amplifier sets the voltage of the I SET pin to 1.25V (typ.). The statements above are simplified in the equations below: I Dx = 800 x(VSET /RSET) R SET = 800 x (1.25V / IDx) The maximum recommended current through each current sink is 100mA during Torch mode and 200mA during Flash mode. Maximum recommended total Flash current with all 4 current sinks used is 800mA (max 200mA per current sink). Using the part in conditions where the junction temperature might rise above the rated maximum requires that the oper- ating ranges and/or conditions be de-rated. The printed cir- cuit board also must be carefully laid out to account for high thermal dissipation in the part. PARALLEL DX OUTPUTS FOR INCREASED CURRENT DRIVE Outputs D 1-4 may be connected together to drive a one or two LEDs at higher currents. In applications using a single LED, all four parallel current sinks of equal value drive the single LED. For this type of configuration, the LED current should be programmed so that the current through each of the outputs is 25% of the total desired LED current. For example, if 200mA is the desired drive current for the single LED, R SET should be selected such that the current through each of the current sink inputs is 50mA. Similarly, if two LEDs are to be driven by pairing up the D 1-4 inputs (i.e D1-2, D 3-4), RSET should be selected such that the current through each current sink input is 50% of the desired LED current. Connecting the outputs in parallel does not affect internal operation of the LM2754 and has no impact on the Electrical Characteristics and limits previously presented. The avail- able diode output current, maximum diode voltage, and all other specifications provided in the Electrical Characteristics table apply to this parallel output configuration, just as they do to the standard 4-LED application circuit. Maximum recommended LED current for any configuration is 200mA per current sink, and 800mA total. For situations where only 3 current sinks will be used for the application, see the SEL PIN operation section. CAPACITOR SELECTION The LM2754 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 <20m Ω typ.). Tantalum capacitors, OS-CON capacitors, and alumi- num electrolytic capacitors are not recommended for use with the LM2754 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 LM2754. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over temperature (X7R: ±15% over -55˚C to 125˚C; X5R: ±15% over -55˚C to 85˚C). Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2754. Ca- pacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30˚C to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C range). 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 capaci- tors to fail to meet the minimum capacitance requirements of the LM2754. The voltage rating of the output capacitor should be 10V or more. For example, a 10V 0603 4.7µF output capacitor (TDK C1608X5R1A475) is acceptable for use with the LM2754, as long as the capacitance on the output does not fall below a minimum of 3µF in the intended application. All other capaci- tors should have a voltage rating at or above the maximum input voltage of the application and should have a minimum capacitance of 1µF. POWER EFFICIENCY Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (P LED) to the power drawn at the input of the part (P IN). With a 1x/1.5x/2x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). The efficiency of the LM2754 can be predicted as follows: P LED =NxVLED xILED P IN =VIN xIIN P IN =VIN x (GainxNxILED +IQ) E=(P LED ÷PIN) For a simple approximation, the current consumed by inter- nal circuitry (I Q) can be neglected, and the resulting effi- ciency will become: E=V LED ÷(VIN x Gain) Neglecting I Q will result in a slightly higher efficiency predic- tion, but this impact will be negligible due to the value of I Q being very low compared to the typical Torch and Flash current levels (100-800mA). It is also worth noting that effi- ciency as defined here is in part dependent on LED voltage. Variation in LED voltage does not affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced analysis, it is recommended that power consumed by the circuit (V IN xIIN) be evaluated rather than power efficiency. www.national.com 9 |
Аналогичный номер детали - LM2754 |
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Аналогичное описание - LM2754 |
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