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A6210GEUTR-T Datasheet(PDF) 5 Page - Allegro MicroSystems |
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A6210GEUTR-T Datasheet(HTML) 5 Page - Allegro MicroSystems |
5 / 11 page 3 A, 2 MHz Buck-Regulating LED Driver A6210 5 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com Basic Operation The A6210 is a buck regulator that utilizes valley current mode control. The on-time is set by the amount of current that flows into the TON pin. This is determined by the value of the TON resistor chosen (R1 in the Functional Block diagram) and the magnitude of the input voltage, VIN. Under a specific set of conditions, an on-time can be set that then dictates the switching frequency. This switching frequency remains reasonably con- stant throughout load and line conditions as the on-time varies inversely with the input voltage. At the beginning of the switching cycle, the buck switch is turned on for a fixed period that is determined by the current flowing into TON. Once the current comparator trips, a one-shot mono- stable, the On Timer, is reset, turning off the switch. The current through the inductor then decays. This current is sensed through the external sense resistor (R2), and then compared against the current-demand signal. After the current through the sense resis- tor decreases to the valley of the current-demand signal, the On Timer is set to turn the buck switch back on again and the cycle is repeated. Disable/Enable The regulator is enabled by pulling the DIS pin low. To disable the regulator, the DIS pin can simply be discon- nected (open circuit). Shutdown The regulator is disabled in the event of either an overtemperature event, or an undervoltage on VIN (VINUV) or on an internal housekeeping supply. As soon as any of the above faults have been removed and assuming DIS = 0, the output is restored. Switch On Time and Switching Frequency The switch on-time effectively determines the operating frequency of the converter. To minimize the size of the power inductor and input filtering it is recommended to run with as high a frequency as possible. The MOSFET drivers are optimized to minimize switching losses. An important consideration in selecting the switching frequency is to ensure that the on time (60 ns) and off time (350 ns) limita- tions are not reached under extreme conditions: • the minimum on time occurs at maximum input voltage • the maximum off time occurs at minimum input voltage The following table takes into account the above maximum off time figure and outlines the typical switching frequencies that can be achieved for a given number of LEDs and input voltage. Note that it is highly recommended that worst case values are used when considering any design. Switching Frequency (MHz) Input Voltage 12 V 24 V 36 V Quantity of LEDs LED Span Voltage (V) Quantity of LEDs LED Span Voltage (V) Quantity of LEDs LED Span Voltage (V) 2.0 1 3.5 2 7.0 3 10.5 1.7 1 3.5 3 10.5 4 14.0 1.0 2 7.0 4 14.0 6 21.0 0.300 3 10.5 6 21.0 9 31.5 The switch on time is programmed by the current flowing into the TON pin. The current is determined by the input voltage, VIN, and the resistor, R1. The on time, Ton, can be found: Ton = . VIN × 2.05 × 1010 10 × 10–9 R1 + (1) To calculate the actual switching frequency, fsw, the Ton from the above calculation can be used in conjunction with the transfer function of the converter, as follows: fSW = . × VOUT + Vf VIN + Vf 1 Ton (2) A simplified approach to selecting the Ton resistor (R1), to accomplish an approximate switching frequency, can be found from the following formula: R1 fSW = . VIN × 2.05 × 1010 (3) Figure 1 illustrates a range of switching frequencies that can be achieved with a given resistor and LED voltage. Each LED is assumed to have a voltage drop of 3.5 V. High Brightness LED Driving The A6210 can be configured as a very simple, low cost, high brightness LED driver. The solution can drive high brightness LEDs up to more than 3 A, while achieving very high efficien- cies, in excess of 90%. The solution uses valley current mode control. This architecture is optimized for high switching frequencies, allowing the use Functional Description |
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