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LM27966SQ Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM27966SQ Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 13 page Application Information (Continued) MAXIMUM OUTPUT CURRENT, MAXIMUM LED VOLTAGE, MINIMUM INPUT VOLTAGE The LM27966 can drive 6 LEDs at 30mA each (Main Display and D AUX) from an input voltage as low as 3.2V, so long as the LEDs have a forward voltage of 3.6V or less (room temperature). The statement above is a simple example of the LED drive capabilities of the LM27966. The statement contains the key application parameters that are required to validate an LED- drive design using the LM27966: LED current (I LEDx), num- ber of active LEDs (N x), LED forward voltage (VLED), and minimum input voltage (V IN-MIN). The equation below can be used to estimate the maximum output current capability of the LM27966: I LED_MAX = [(1.5 x VIN)-VLED -(IDAUX xROUT)] / [(N MAIN xROUT)+kHR] (eq. 1) I LED_MAX = [(1.5 x VIN )-VLED -(IDAUX x 2.75 Ω)] / [(N MAIN x 2.75 Ω)+k HR] I DAUX is the additional current that could be delivered to the AUX LED. R OUT – Output resistance. This parameter models the inter- nal losses of the charge pump that result in voltage droop at the pump output P 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 LM27966 is typically 2.75 Ω (V IN = 3.6V, T A = 25˚C). In equation form: V POUT =(1.5xVIN)– [NMAINxILED-MAIN xROUT] (eq. 2) k HR – Headroom constant. This parameter models the mini- mum voltage required to be present across the current sources for them to regulate properly. This minimum voltage is proportional to the programmed LED current, so the con- stant has units of mV/mA. The typical k HR of the LM27966 is 8mV/mA. In equation form: (V POUT –VLEDx) > kHR xILEDx (eq. 3) Typical Headroom Constant Value k HR = 8mV/mA 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 LEDx. 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 an LED with a lower forward voltage. Excessive power dissipation may also limit output current capability of an application. Total Output Current Capability The maximum output current that can be drawn from the LM27966 is 180mA. Each driver bank has a maximum allot- ted current per Dx sink that must not be exceeded. MAXIMUM Dx CURRENT 30mA The 180mA load can be distributed in many different con- figurations. Special care must be taken when running the LM27966 at the maximum output current to ensure proper functionality. PARALLEL CONNECTED AND UNUSED OUTPUTS Outputs D1-5 may be connected together to drive one or two LEDs at higher currents. In such a configuration, all five parallel current sinks (Main Display) of equal value can drive a single LED. The LED current programmed for Main Display should be chosen so that the current through each of the outputs is programmed to 20% of the total desired LED current. For example, if 60mA is the desired drive current for a single LED, R SET should be selected such that the current through each of the current sink inputs is 12mA. Connecting the outputs in parallel does not affect internal operation of the LM27966 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 5-LED application circuit. Main Display utilizes LED forward voltage sensing circuitry on each Dxx pin to optimize the charge-pump gain for maxi- mum efficiency. Due to the nature of the sensing circuitry, it is not recommended to leave any of the Dx (D1-D4) pins unused if either diode bank is going to be used during normal operation. Leaving Dx pins unconnected will force the charge-pump into 1.5x mode over the entire V IN range ne- gating any efficiency gain that could be achieve by switching to 1x mode at higher input voltages. If D5 is not used, it is recommended that the driver pin be grounded and the general purpose register bit EN-D5 be set to 0 to ensure proper gain transitions. Care must be taken when selecting the proper R SET value. The current on any Dx pin must not exceed the maximum current rating for any given current sink pin. POWER EFFICIENCY The 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 1.5x/1x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). The efficiency of the LM27966 can be predicted as follows: P LEDTOTAL =(VLED-MAIN xNMAIN xILED-MAIN)+ (V LED-AUX xILED-AUX) P IN =VIN xIIN P IN =VIN x (GAIN x ILEDTOTAL +IQ) E=(P LEDTOTAL ÷PIN) It is also worth noting that efficiency 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. POWER DISSIPATION The power dissipation (P DISS) and junction temperature (TJ) can be approximated with the equations below. P IN is the power generated by the 1.5x/1x charge pump, P LED is the power consumed by the LEDs, T A is the ambient tempera- ture, and θ JA is the junction-to-ambient thermal resistance for the LLP-24 package. V IN is the input voltage to the LM27966, V LED is the nominal LED forward voltage, N is the number of LEDs and I LED is the programmed LED current. P DISS =PIN -PLEDA P DISS= (GAIN x VIN xILEDA )-(VLEDA xNA xILEDA)- (V LED xIDAUX) www.national.com 11 |
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