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LM2796TLX Arkusz danych(PDF) 6 Page - National Semiconductor (TI) |
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LM2796TLX Arkusz danych(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page Circuit Description OVERVIEW The LM2796 is primarily intended for Lithium-Ion battery driven white-LED drive applications, and is well suited to drive white LEDs that are used for backlighting small-format displays. The part has seven matched constant-current out- puts, each capable of driving up to 20mA (or more) through white LEDs. The well-matched current sources ensure the current through all the LEDs is virtually identical. This keeps brightness of all LEDs matched to near perfection so that they can provide a consistent backlight over the entire dis- play. The core of the LM2796 is a 1.5x/1x dual-mode charge pump. The input of the charge pump is connected to the V IN pin. The recommended input voltage range of the LM2796 is 2.7V to 5.5V. The output of the charge pump is the P OUT pin ( “Pump OUTput”). The output voltage of the charge pump is unregulated and varies with input voltage and load current. The charge pump operates in the 1.5x mode when the input voltage is below 4.75V (typ.). In this mode, the input-to- output voltage gain of the charge pump is 1.5, and the voltage at the output of the charge pump will be approxi- mately 1.5x the input voltage (V(P OUT) ≈ 1.5*V IN ). When in the 1.5x mode, the charge pump provides the voltage boost that is required to drive white LEDs from a Li-Ion battery. (White LEDs typically have a forward voltage in the range of 3.3V to 4.0V. A Li-Ion battery typically is not considered to be fully discharged until the battery voltage falls to 3.0V (ap- prox.) ) The charge pump operates in the 1x mode when the input voltage is above 4.75V (typ.). In these conditions, voltage boost is not required to drive the LEDs, so the charge pump merely passes the input voltage to P OUT (V(POUT) ≈ V IN). This reduces the input current and the power dissipation of the LM2796 when the input voltage is high. The matched current outputs are generated with a precision current mirror that is biased off the charge pump output. Matched currents are ensured with the use of tightly matched internal devices and internal mismatch cancellation circuitry. Top-side current drive allows LEDs to be connected between each current output and GND, simplifying PWB routing and connectivity. There are seven regulated current outputs. These seven outputs are split into two groups, a group of 4 outputs and a group of 3 outputs. There is an ON/OFF control pin for each group. The DC current through the LEDs is programmed with an external resistor. Changing currents on-the-fly can be achieved with the use of digital pulse (PWM) signals. ENABLE PINS: EN, ENA, ENB The LM2796 has 3 enable pins. All three are active-high logic (HIGH = ON). There are internal pull-down resistors (300k Ω typ.) that are connected internally between each of the enable pins and GND. The EN pin is the master enable pin for the part. When voltage on this pin is low (<0.5V), the part is in shutdown mode. All internal circuitry is OFF and the part consumes very little supply current when the LM2796 is shutdown. When the voltage on the EN pin is high (>1.1V), the part is active. The charge pump is ON, and it is possible to turn on the output currents to drive the LEDs. ENA and ENB are used to turn the output currents ON and OFF. ENA activates/deactivates the four group-A outputs (D1A-D4A). ENB activates/deactivates the three group-B outputs (D1B-D3B). SETTING LED CURRENTS The output currents of the LM2796 can be set to a desired value simply by connecting an appropriately sized resistor (R SET) between the ISET pin of the LM2796 and GND. The output currents (LED currents) are proportional to the current that flows out of the I SET pin. The output currents are a factor of 100 greater than the I SET current. The feedback loop of an internal amplifier sets the voltage of the I SET pin to 1.25V (typ.). Placing a resistor between I SET and GND programs the I SET current, and thus the LED currents. The statements above are simplified in the equations below: I Dxx = 100 x(VSET /RSET) R SET = 100 x (1.25V / IDxx) Maximum Output Current, Maximum LED Voltage, Minimum Input Voltage The LM2796 can drive 7 LEDs at 15mA each from an input voltage as low as 3.0V, 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 LM2796. The statement contains the key application parameters that are required to validate an LED- drive design using the LM2796: LED current (I LED), number of active LEDs (N), LED forward voltage (V LED), and mini- mum input voltage (V IN-MIN). The equation below can be used to estimate the total output current capability of the LM2796: I LED_MAX = ((1.5 x VIN)-VLED)/((NxROUT)+kHR) (eq. 1) I LED_MAX = ((1.5 x VIN )-VLED) / ((N x 2.7 Ω) + 22mV/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 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 LM2796 is typically 2.7 Ω (V IN = 3.0V, T A = 25˚C). In equation form: V POUT = 1.5xVIN – NxILEDxROUT (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 LM2796 is 22mV/mA. In equation form: (V POUT –VLED) > kHRxILED (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 an LED with a lower forward voltage. Excessive power dissipation may also limit output current capability of an application. Soft Start The LM2796 contains internal soft-start circuitry to limit input inrush currents when the part is enabled. Soft start is imple- mented internally with a controlled turn-on of the internal voltage reference. During soft start, the current through the www.national.com 6 |
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