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A6262KLYTR-T Datasheet(PDF) 9 Page - Allegro MicroSystems |
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A6262KLYTR-T Datasheet(HTML) 9 Page - Allegro MicroSystems |
9 / 15 page Automotive LED Array Driver A6262 9 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com From these equations (and as illustrated in figure 5) it can be seen that, if the power in the A6262 is not limited, then it will increase as the supply voltage increases but the power in the LEDs will remain constant. Dissipation Limits There are two features limiting the power that can be dissipated by the A6262: thermal shutdown and thermal foldback. Thermal Shutdown If the thermal foldback feature is disabled by connecting the THTH pin to GND, or if the thermal resistance from the A6262 to the ambient environment is high, then the silicon temperature will rise to the thermal shutdown threshold and the current will be disabled. After the current is disabled the power dissipated will drop and the temperature will fall. When the temperature falls by the hysteresis of the thermal shutdown circuit, then the current will be re-enabled and the temperature will start to rise again. This cycle will repeat continuously until the ambient temperature drops or the A6262 is switched off. The period of this thermal shutdown cycle will depend on several electrical, mechanical, and thermal parameters, and could be from a few milliseconds to a few seconds. Thermal Foldback If there is a good thermal connection to the A6262, then the thermal foldback feature will have time to act. This will limit the silicon temperature by reducing the regulated current and therefore the dissipation. The thermal monitor will reduce the LED current as the tempera- ture of the A6262 increases above the thermal monitor activation temperature, TJM, as shown in figure 6. The figure shows the operation of the A6262 with 4 strings of 3 red LEDs, each string running at 50 mA. The forward voltage of each LED is 2.3 V and the graph shows the current as the supply voltage increases from 14 to 17 V. As the supply voltage increases, without the thermal foldback feature, the current would remain at 50 mA, as shown by the dashed line. The solid line shows the resulting current decrease as the thermal foldback feature acts. If the thermal foldback feature did not affect LED current, the current would increase the power dissipation and therefore the silicon temperature. The thermal foldback feature reduces power in the A6262 in order to limit the temperature increase, as shown in figure 7. The figure shows the operation of the A6262 under the same conditions as figure 6. That is, 4 strings of 3 red LEDs, each string running at 50 mA with each LED forward voltage at Figure 5. Power Dissipation versus Supply Voltage. 3.0 2.5 2.0 1.5 1.0 0.5 0 A6262 Power 70 80 90 110 100 Supply Voltage, VIN (V) 130 120 150 140 LED Power 4 Strings VLED = 6.9 V ILED = 50 mA Figure 6. LED current versus Supply Voltage. Figure 7. Junction Temperature versus Supply Voltage. 54 52 50 48 46 44 42 40 Without thermal monitor With thermal monitor 14.0 14.5 15.0 16.0 17.0 15.5 Supply Voltage, VIN (V) 16.5 4 Strings VLED = 6.9 V ILED = 50 mA TA = 50°C 130 125 120 115 110 105 100 Without thermal monitor With thermal monitor 14.0 14.5 15.0 16.0 17.0 15.5 Supply Voltage, VIN (V) 16.5 4 Strings VLED = 6.9 V ILED = 50 mA TA = 50°C |
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