Application Information Select External Component RSET Set LED Output Current MAX6968 uses external resistor RSET to set the LED current for output OUT0-OUT7. The minimum allowable value for RSET is 307.6 Ω, at which point the output current is set to 55 mA. The maximum allowable value for RSET is 1.5 kΩ. The reference value is 360Ω and the output current is set to 50mA. Use the following formula to set different output currents:
RSET = 18,000 / IOUT where IOUT is the required output current in mA.
Calculating Power Consumption The upper limit of the MAX6968 power (PD) is determined by:
PD = (V+ x I+) + (VOUT x DUTY x IOUT xN) where:
V+ = supply voltage I+ = N output absorbs IOUT LED drive current when operating current DUTY = PWM signal duty cycle added to OE terminal N = MAX6968 output port used to drive LED (maximum of 8) VOUT = The output voltage of the port when the MAX6968 drives the load LED IOUT = LED drive current set by RSET PD = power consumption, if the current is in mA, the unit is mW power calculation example:
IOUT = 47mA, N = 8, DUTY = 1, VOUT = 2V, V+ = 5.25VPD = (5.25V x 25mA) + (2V x 1 x 47mA x8) = 0.883W Therefore, for the 16-pin TSSOP package (according to Absolute MaximumRatings, TJA = 1 / 0.0213 = +46.95 °C / W), the maximum allowable ambient temperature TA is given by:
TJ(MAX) = TA + (PD x TJA) = +150°C=TA + (0.883 x 46.95°C/W) so TA = +108.5°C.
High Temperature Shutdown The MAX6968 includes an internal temperature sensor that turns off all outputs when the die temperature exceeds approximately +165°C. When the die temperature drops below approximately +140 °C, the output turns back on. The contents of the registers are unaffected, so when the power consumption of the driver is too high, the external performance is that the load LEDs are intermittently turned on and off as the driver repeats overheating and cooling, that is, the LEDs are alternately turned off and on.
Power Considerations The MAX6968 operates from a chip power supply V+ and one or more LED power supplies.
Bypass each supply to GND with a 0.1μF capacitor and place the capacitor as close as possible to the MAX6968. For static LED drivers, this is fine. For multi-chip cascading or PWM applications, for each additional 4 to 16 MAX6968 devices, each power supply requires an additional 4.7μF or larger electrolytic capacitor for decoupling. The required capacitance value depends on the LED load current, the PWM switching frequency, and the serial port speed. Insufficient V+ decoupling can cause timing problems. In addition, LED power supplies with large noise can affect the LED current regulation function.
For TSSOP packages, connect the bottom exposed pad to GND.
Chip Information TRANSISTOR COUNT: 2382PROCESS: BiCMOS
RSET = 18,000 / IOUT where IOUT is the required output current in mA.
Calculating Power Consumption The upper limit of the MAX6968 power (PD) is determined by:
PD = (V+ x I+) + (VOUT x DUTY x IOUT xN) where:
V+ = supply voltage I+ = N output absorbs IOUT LED drive current when operating current DUTY = PWM signal duty cycle added to OE terminal N = MAX6968 output port used to drive LED (maximum of 8) VOUT = The output voltage of the port when the MAX6968 drives the load LED IOUT = LED drive current set by RSET PD = power consumption, if the current is in mA, the unit is mW power calculation example:
IOUT = 47mA, N = 8, DUTY = 1, VOUT = 2V, V+ = 5.25VPD = (5.25V x 25mA) + (2V x 1 x 47mA x8) = 0.883W Therefore, for the 16-pin TSSOP package (according to Absolute MaximumRatings, TJA = 1 / 0.0213 = +46.95 °C / W), the maximum allowable ambient temperature TA is given by:
TJ(MAX) = TA + (PD x TJA) = +150°C=TA + (0.883 x 46.95°C/W) so TA = +108.5°C.
High Temperature Shutdown The MAX6968 includes an internal temperature sensor that turns off all outputs when the die temperature exceeds approximately +165°C. When the die temperature drops below approximately +140 °C, the output turns back on. The contents of the registers are unaffected, so when the power consumption of the driver is too high, the external performance is that the load LEDs are intermittently turned on and off as the driver repeats overheating and cooling, that is, the LEDs are alternately turned off and on.
Power Considerations The MAX6968 operates from a chip power supply V+ and one or more LED power supplies.
Bypass each supply to GND with a 0.1μF capacitor and place the capacitor as close as possible to the MAX6968. For static LED drivers, this is fine. For multi-chip cascading or PWM applications, for each additional 4 to 16 MAX6968 devices, each power supply requires an additional 4.7μF or larger electrolytic capacitor for decoupling. The required capacitance value depends on the LED load current, the PWM switching frequency, and the serial port speed. Insufficient V+ decoupling can cause timing problems. In addition, LED power supplies with large noise can affect the LED current regulation function.
For TSSOP packages, connect the bottom exposed pad to GND.
Chip Information TRANSISTOR COUNT: 2382PROCESS: BiCMOS
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