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LM27952SDX Scheda tecnica(PDF) 5 Page - National Semiconductor (TI) |
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LM27952SDX Scheda tecnica(HTML) 5 Page - National Semiconductor (TI) |
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5 / 8 page  Application Information CIRCUIT DESCRIPTION The LM27952 is an adaptive 1.5x/1x CMOS charge pump, optimized for driving white LEDs used in backlighting small- format displays. It provides four constant current inputs ca- pable of sinking up to 30mA through each LED. The well- matched current sinks ensure the current through all the LEDs are virtually identical, providing a uniform brightness across the entire display. Each LED is driven from V OUT and connected to one of the four current sinks. LED drive current is programmed by connecting a resistor, R SET, to the current set pin, ISET. LED brightness is adjusted by applying a Pulse Width Modulated (PWM) signal to the dedicated PWM input pin. CHARGE PUMP The input to the 1.5x/1x charge pump is connected to the V IN pin, and the loosely regulated output of the charge pump is connected to the V OUT pin. The recommended input voltage range of the LM27952 is 3.0V to 5.5V. The device’s loosely- regulated charge pump has both open loop and closed loop modes of operation. When the device is in open loop, the voltage at V OUT is equal to the gain times the voltage at the input. When the device is in closed loop, the voltage at V OUT is loosely regulated to 4.5V (typ.). The charge pump gain transitions are actively selected to maintain regulation based on LED forward voltage and load requirements. This allows the charge pump to stay in the most efficient gain (1x) over as much of the input voltage range as possible, reducing the power consumed from the battery. SOFT START The LM27952 contains internal soft-start circuitry to limit input inrush currents when the part is enabled. Soft start is implemented internally with a controlled turn-on of the inter- nal voltage reference. Due to the soft-start circuitry, startup time of the LM27952 is approximately 330µs (typ.). ENABLE AND PWM PINS The LM27952 has 2 logic control pins. Both pins are active- high logic (HIGH = ON). There is an internal pull-down resistor (150k Ω typ.) connected between the enable pin (EN) and GND. There is no pull-up or pull-down connected to the Pulse Width Modulated (PWM) pin. The EN pin is the master enable pin for the part. When the voltage on this pin is low (<0.4V), the part is in shutdown mode. In this mode, all internal circuitry is OFF and the part consumes very little supply current (<1µA typ.). When the voltage on the EN pin is high (>1.0V), the part will activate the charge pump and regulate the output voltage to its nominal value. The PWM pin serves as a dedicated logic input for LED brightness control. When the voltage on this pin is low (<0.4V), the current sinks will be turned off and no current will flow through the LEDs. When the voltage on this pin is high (>1.0V), the currents sinks will turn on and regulate to the current level set by the resistor connected to the I SET pin. SETTING LED CURRENTS The current through the four LEDs connected to D 1-4 can be set to a desired level simply by connecting an appropriately sized resistor (R SET) between the ISET pin of the LM27952 and GND. The LED currents are proportional to the current that flows out of the I SET pin and are a factor of 200 times greater than the I SET current. The feedback loop of an inter- nal amplifier sets the voltage of the I SET pin to 1.25V (typ.). The statements above are simplified in the equations below: I Dx = 200 x(VSET /RSET) R SET = 200 x (1.25V / IDx) ADJUSTING LED BRIGHTNESS (PWM control) Perceived LED brightness can be adjusted using a PWM control signal on the LM27952 PWM logic input pin, turning the current sources ON and OFF at a rate faster than per- ceptible by the eye. When this is done, the total brightness perceived is proportional to the duty cycle (D) of the PWM signal (D = the percentage of time that the LED is on in every PWM cycle). A simple example: if the LEDs are driven at 15mA each with a PWM signal that has a 50% duty cycle, perceived LED brightness will be about half as bright as compared to when the LEDs are driven continuously with 15mA. The minimum recommended PWM frequency is 100Hz. Fre- quencies below this may be visibly noticeable as flicker or blinking. The maximum recommended PWM frequency is 1kHz. Frequencies above this may cause interference with internal current driver circuitry and/or noise in the audible range. Due to the regulation control loop, the maximum frequency and minimum duty cycle applied to the PWM pin should be chosen such that the minimum ON time is no less than 30µs in duration. If a PWM signal is applied to the EN pin instead, the maximum frequency and minimum duty cycle should be chosen to accommodate both the LM27952 startup time (330µs typ.) and the 30µs control loop delay. The preferred method to adjust brightness is to keep the master EN voltage ON continuously and apply a PWM signal to the dedicated PWM input pin. The benefit of this type of connection can be best understood with a contrary example. When a PWM signal is connected to the master enable (EN) pin, the charge pump repeatedly turns on and off. Every time the charge pump turns on, there is an inrush of current as the capacitances, both internal and external, are recharged. This inrush current results in a current spike and a voltage dip at the input of the part. By only applying the PWM signal to PWM logic input pin, the charge pump continuously stays on, resulting in much lower input noise. In cases where a PWM signal must be connected to the EN pin, measures can be taken to reduce the magnitude of the charge-pump turn-on transient response. More input capaci- tance, series resistors and/or ferrite beads may provide ben- efits. If the current spikes and voltage dips can be tolerated, connecting the PWM signal to the EN pin does provide a benefit of lower supply current consumption. When the PWM signal to the EN pin is low, the LM27952 will be shutdown and input current will only be a few micro-amps. This results in a lower time-averaged input current than the prior sugges- tion, where EN is kept on continuously. MAXIMUM OUTPUT CURRENT, MAXIMUM LED VOLTAGE, MINIMUM INPUT VOLTAGE The LM27952 can drive 4 LEDs at 30mA each from an input voltage as low as 3.0V, so long as the LEDs have a forward voltage of 3.5V or less (room temperature). The statement above is a simple example of the LED drive capabilities of the LM27952. The statement contains key application parameters required to validate an LED-drive design using the LM27952: LED current (I LED), number of active LEDs (N), LED forward voltage (V LED), and minimum input voltage (V IN-MIN). www.national.com 5 |
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