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LM27952 Folha de dados(PDF) 6 Page - National Semiconductor (TI) |
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LM27952 Folha de dados(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page Application Information (Continued) The equation below can be used to estimate the total output current capability of the LM27952: I LED_MAX = ((1.5 x VIN)-VLED)/((NxROUT)+kHR) (eq. 1) I LED_MAX = ((1.5 x VIN )-VLED) / ((N x 3.3 Ω) + 12mV/mA) R OUT – Output resistance. This parameter models the inter- nal losses of the charge pump that result in voltage droop at the pump output V OUT. Since the magnitude of the voltage droop is proportional to the total output current of the charge pump, the loss parameter is modeled as a resistance. The output resistance of the LM27952 is typically 3.3 Ω (V IN = 3.0V, T A = 25˚C). In equation form: V VOUT =1.5xVIN – NxILED xROUT (eq. 2) k HR – Headroom constant. This parameter models the mini- mum voltage required across the current sinks for proper regulation. This minimum voltage is proportional to the pro- grammed LED current, so the constant has units of mV/mA. The typical k HR of the LM27952 is 12mV/mA. In equation form: (V VOUT –VLED) > kHR xILED (eq. 3) The "I LED-MAX" equation (eq. 1) is obtained from combining the R OUT equation (eq. 2) with the kHR equation (eq. 3) and solving for I LED. Maximum LED current is highly dependent on minimum input voltage and LED forward voltage. Output current capability can be increased by raising the minimum input voltage of the application, or by selecting LEDs with a lower forward voltage. Excessive power dissipation may also limit output current capability of an application. CAPACITOR SELECTION The LM27952 requires 4 external capacitors for proper op- eration. Surface-mount multi-layer ceramic capacitors are recommended. 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 LM27952 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 LM27952. These capacitors have tight capacitance toler- ance (as good as ±10%) and hold their value over tempera- ture (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 LM27952. 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 LM27952. The voltage rating of the output capacitor should be 10V or more. All other capacitors should have a voltage rating at or above the maximum input voltage of the application. 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 such a configuration, all four parallel current sinks of equal value drive the single LED. The LED current programmed should be chosen so that the current through each of the outputs is programmed to 25% of the total desired LED current. For example, if 60mA 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 15mA. Similarly, if two LEDs are to be driven by pairing up the D 1-4 inputs (i.e D1-2,D3-4), 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 LM27952 and has no impact on the Electri- cal Characteristics and limits previously presented. The available diode output current, maximum diode voltage, and all other specifications provided in the Electrical Character- istics table apply to this parallel output configuration, just as they do to the standard 4-LED application circuit. POWER EFFICIENCY Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (PLED) to the power drawn at the input of the part (PIN). With a 1.5x/1x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). For a simple approxima- tion, the current consumed by internal circuitry can be ne- glected and the efficiency of the LM27952 can be predicted as follows: P LED =NxVLED xILED P IN =VIN xIIN P IN =VIN x (GainxNxILED +IQ) E=(P LED ÷PIN) Neglecting I Q will result in a slightly higher efficiency predic- tion, but this impact will be no more than a few percentage points when several LEDs are driven at full power. It is also worth noting that efficiency as defined here is in part depen- dent 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 analy- sis, it is recommended that power consumed by the circuit (V IN xIIN) be evaluated rather than power efficiency. THERMAL PROTECTION Internal thermal protection circuitry disables the LM27952 when the junction temperature exceeds 150˚C (typ.). This feature protects the device from being damaged by high die temperatures that might otherwise result from excessive power dissipation. The device will recover and operate nor- mally when the junction temperature falls below 140˚C (typ.). It is important that the board layout provide good thermal conduction to keep the junction temperature within the speci- fied operating ratings. POWER DISSIPATION The power dissipation (P DISSIPATION) and junction tempera- ture (T J) can be approximated with the equations below. PIN is the power generated by the 1.5x/1x charge pump, P LED is the power consumed by the LEDs, T Ais the ambient tem- perature, and θ JA is the junction-to-ambient thermal resis- tance for the LLP-14 package. V IN is the input voltage to the LM27952, V LED is the nominal LED forward voltage, and I LED is the programmed LED current. P DISSIPATION =PIN -PLED = [Gain x V IN x(4xILED)]−(VLED x4xILED) T J =TA +(PDISSIPATION x θ JA) www.national.com 6 |
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