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LM27953TLX Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM27953TLX Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Block Diagram 20128003 Circuit Description OVERVIEW The LM27953 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 four 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. CHARGE PUMP The core of the LM27953 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 LM27953 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 LM27953 when the input voltage is high. REGULATED CURRENT OUTPUTS 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. There are four regulated common anode current outputs. There is an ON/OFF control pin for the group (EN). 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 PIN: EN The LM27953 has an enable pin that has active-high logic (HIGH = ON). There is an internal pull-down resistor (300k Ω typ.) that is connected internally between the enable pin and GND. www.national.com 8 |
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