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A6268KLPTR-T Datasheet(PDF) 9 Page - Allegro MicroSystems |
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A6268KLPTR-T Datasheet(HTML) 9 Page - Allegro MicroSystems |
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9 / 19 page  Automotive High Current LED Controller A6268 9 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com LF Single diode forward voltage reference input. Measures the forward voltage of the first LED. This value is used as a reference against the voltage on the LA pin to detect possible shorted LEDs in the LED string. Circuit Operation Converter A constant frequency, current mode control scheme is used to regulate the current through the LEDs. There are two control loops within the regulator. The inner loop formed by the amplifier, AS (see the Functional Block Diagram for AS, AC, AE, and AL), comparator, AC, and the RS bistable, controls the induc- tor current as measured through the switch by the switch sense resistor, RSS. The outer loop including the amplifier, AL, and the integrating error amplifier, AE, controls the average LED current by provid- ing a setpoint reference for the inner loop. The LED current is measured by the LED sense resistor, RSL, and compared to the internal reference current to produce an integrated error signal at the output of AE. This error signal sets the average amount of energy required from the inductor by the LEDs. The average inductor energy transferred to the LEDs is defined by the average inductor current as determined by the inner control loop. The inner loop establishes the average inductor current by controlling the peak switch current on a cycle-by-cycle basis. Because the relationship between peak current and average cur- rent is non-linear, depending on the duty cycle, the reference level for the peak switch current is modified by a slope generator. This compensation reduces the peak switch current measurement by a small amount as the duty cycle increases (refer to figure 1). The slope compensation also removes the instability inherent in a fixed frequency current control scheme. The control loops work together as follows: the switch current, sensed by the switch current sense resistor, RSS, is compared to the LED current error signal. As the LED current increases the output of AE will reduce, reducing the peak switch current and thus the current delivered to the LEDs. As the LED current decreases the output of AE increases, increasing the peak switch current and thus increasing the current delivered to the LEDs. Under some conditions, especially when the LED current is set to a low value, the energy required in the inductor may result in the inductor current dropping to zero for part of each cycle. This is known as discontinuous mode operation, and results in some low frequency ripple. The average LED current, however, remains regulated down to zero. In discontinuous mode, when the induc- tor current drops to zero, the voltage at the drain of the external MOSFET rings, due to the resonant LC circuit formed by the inductor, and the switch and diode capacitance. This ringing is low frequency and is not harmful. Switch Current Limit The switch current is measured by the switch sense resistor, RSS, and the switch sense amplifier, AS (see the Functional Block Diagram). The input limit of the sense amplifier, VIDS, and the maximum switch current, ISMAX, define the maximum value of the sense resistor as: RSS = VIDS / ISMAX (1) This defines the maximum measurable value of the switch (and inductor) current. The maximum switch current is modulated by the on-time of the switch. An internal slope compensation signal is subtracted from the voltage sense signal to produce a peak sense voltage which effectively defines the current limit. This signal is applied at a rate of –16 mV/ μs starting with no contribution (t = 0 μs) at the beginning of each switching cycle. Figure 1 illustrates how the peak sense voltage (typical values) changes over a period of 3 μs. For example, the maximum current (typical) through the switch at t = 1.5 μs (D = 50%) would be 145 mV/RSS, however, if the switch remained on for a further 1 μs, the maximum current through the switch would be 129 mV/RSS. 200 150 100 50 0 01 Period (μs) 23 Figure 1. Slope compensation for peak switch current control. |
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