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A6278ELWTR-T Datasheet(PDF) 10 Page - Allegro MicroSystems |
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A6278ELWTR-T Datasheet(HTML) 10 Page - Allegro MicroSystems |
10 / 18 page Serial-Input, Constant-Current Latched LED Drivers with Open LED Detection A6278 and A6279 10 Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com Load Supply Voltage (VLED) These devices are designed to operate with driver voltage drops (VCE) of 0.7 to 3V, with an LED forward voltage, VF, of 1.2 to 4.0 V. If higher voltages are dropped across the driver, package power dissipation will increase significantly. To mini- mize package power dissipation, it is recommended to use the lowest possible load supply voltage, VLED, or to set any series voltage dropping, VDROP, according to the following formula: VDROP = VLED – VF – VCE, with VDROP = IO×RDROP for a single driver or for a Zener diode (VZ), or for a series string of diodes (approximately 0.7 V per diode) for a group of drivers (see figure 3). If the available volt- age source, VLED, will cause unacceptable power dissipation and series resistors or diodes are undesirable, a voltage regulator can be used to provide supply voltages. For reference, typical LED forward voltages are: LED Type VF (V) White 3.5 to 4.0 Blue 3.0 to 4.0 Green 1.8 to 2.2 Yellow 2.0 to 2.1 Amber 1.9 to 2.65 Red 1.6 to 2.25 Infrared 1.2 to 1.5 Pattern Layout This device has a common logic ground and power ground terminal, GND. For the LP package, the GND pin should be tied to the exposed metal pad, EP, allowing the ground plane copper to be used to dissipate heat. If the ground pattern layout contains large common mode resistance, and the voltage between the system ground and the LATCH ENABLE, OUTPUT ENABLE, or CLOCK terminals exceeds 2.5 V (because of switching noise), these devices may not work properly. Package Power Dissipation (PD) The maximum allowable package power dissipation based on package type is determined by: PD(max) = (150 – TA) / RθJA, where RθJA is the thermal resistance of the package, determined experimentally. Power dissipation levels based on the package are shown in the Package Thermal Characteristics section (see page 14). The actual package power dissipation is determined by: PD(act) = DC ×(VCE ×IO×16) + (VDD×IDD), where DC is the duty cycle. The value 16 represents the maxi- mum number of available device outputs for the A6279, used for the worst-case scenario (displaying all 16 LEDs; this would be 8 for the A6278). When the load suppy voltage, VLED, is greater than 3 to 5 V, and PD(act) > PD(max), an external voltage reducer (VDROP) must be used (see figure 3). Reducing the percent duty cycle, DC, will also reduce power dis- sipation. Typical results are shown on the following pages. Application Information VLED VDROP VF VCE VLED VDROP VF VCE VLED VDROP VF VCE Figure 3. Typical appplications for voltage drops |
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