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MAX6954 Datasheet(PDF) 19 Page - Maxim Integrated Products |
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MAX6954 Datasheet(HTML) 19 Page - Maxim Integrated Products |
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19 / 38 page  peak current ISEG, plus the 0.8V headroom required by the driver output stages. The MAX6954 correctly regu- lates ISEG with a supply voltage above this minimum voltage. If the supply drops below this minimum volt- age, the driver output stages may brown out, and be unable to regulate the current correctly. As the supply voltage drops further, the LED segment drive current becomes effectively limited by the output driver's on- resistance, and the LED drive current drops. The char- acteristics of each individual LED in a display digit are well matched, so the result is that the display intensity dims uniformly as supply voltage drops out of regula- tion and beyond. Computing Power Dissipation The upper limit for power dissipation (PD) for the MAX6954 is determined from the following equation: PD = (V+ x 35mA) + (V+ - VLED) (DUTY x ISEG x N) where: V+ = supply voltage DUTY = duty cycle set by intensity register N = number of segments driven (worst case is 17) VLED = LED forward voltage at ISEG ISEG = segment current set by RSET PD = Power dissipation, in mW if currents are in mA Dissipation example: ISEG = 30mA, N = 17, DUTY = 15/16, VLED = 2.4V at 30mA, V+ = 3.6V PD = 3.6V (35mA) + (3.6V - 2.4V)(15/16 x 30mA x 17) = 0.700W Thus, for a 36-pin SSOP package (TJA = 1 / 0.0118 = +85°C/W from Operating Ratings), the maximum allowed ambient temperature TA is given by: TJ(MAX) = TA + (PD x TJA) = +150°C = TA + (0.700 x +85°C/W) So TA = +90.5°C. Thus, the part can be operated safely at a maximum package temperature of +85°C. Power Supplies The MAX6954 operates from a single 2.7V to 5.5V power supply. Bypass the power supply to GND with a 0.1µF capacitor as close to the device as possible. Add a 47µF capacitor if the MAX6954 is not close to the board's input bulk decoupling capacitor. Terminating the Serial Interface The MAX6954 uses fixed voltage thresholds of 0.6V and 1.8V for the 4-wire interface inputs. These fixed thresholds allow the MAX6954 to be controlled by a host operating from a lower supply voltage than the MAX6954; for example, 2.5V. The fixed thresholds also reduce the logic input noise margin when operating the MAX6954 from a higher supply voltage, such as 5V. At higher supply voltages, it may be necessary to fit termi- nation components to the CLK, DIN, and CS inputs to avoid signal reflections that the MAX6954 could respond to as multiple transitions. Suitable termination components can be either a 33pF capacitor or 4.7k Ω resistor fitted from each of the CLK, DIN, and CS inputs to GND. 4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan ______________________________________________________________________________________ 19 |
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Descripción similar - MAX6954 |
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