MAX6969EVKIT+【PDF 9/12 页】KIT EVAL FOR MAX6969

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16-Port, 5.5V Constant-Current LED Driver

Applications Information

Selecting External Component R SET to Set

LED Output Current

The MAX6969 uses an external resistor, R SET , to set the

LED current for outputs OUT0–OUT15. The minimum

allowed value of R SET is 327.3 ? , which sets the output

currents to 55mA. The maximum allowed value of R SET

is 1.5k ? . The reference value, 360 ? , sets the output

currents to 50mA. To set a different output current, use

Overtemperature Cutoff

The MAX6969 contains an internal temperature sensor

that turns off all outputs when the die temperature

exceeds approximately +165°C. The outputs are

enabled again when the die temperature drops below

approximately +140°C. Register contents are not

affected, so when a driver is overdissipating, the exter-

nal symptom is the load LEDs cycling between on and

off as the driver repeatedly overheats and cools, alter-

nately turning the LEDs off and then back on again.

the formula:

R SET = 18,000 / I OUT

Power-Supply Considerations

The MAX6969 operates with a chip supply V+, and one

where I OUT is the desired output current in mA.

Computing Power Dissipation

The upper limit for power dissipation (P D ) for the

MAX6969 is determined by the following equation:

P D = (V+ x I+) + (V OUT x DUTY x I OUT x N)

where:

V+ = supply voltage

I+ = operating supply current when sinking I OUT LED

drive current into N outputs

DUTY = PWM duty cycle applied to OE

N = number of MAX6969 outputs driving LEDs at the

same time (maximum is 16)

V OUT = MAX6969 port output voltage when driving load

LED(s)

I OUT = LED drive current programmed by R SET

P D = power dissipation, in mW if currents are in mA

Dissipation example:

I OUT = 47mA, N = 16, DUTY = 1, V OUT = 2V, V+ =

5.25V

P D = (5.25V x 50mA) + (2V x 1 x 47mA x 16) = 1.767W

Thus, for a 24-pin TSSOP package (T JA = 1 / 0.0122 =

+82°C/W from the Absolute Maximum Ratings ), the

maximum allowed ambient temperature T A is given by:

T J(MAX) = T A + (P D x T JA ) = +150°C =

T A + (1.767 x 82°C/W)

so T A = +145.6°C.

or more LED supplies. Bypass each supply to GND

with a 0.1μF capacitor as close to the MAX6969 as pos-

sible. This is normally adequate for static LED driving.

For multiplex or PWM applications, it is necessary to

add an additional bulk electrolytic capacitor of 4.7μF or

more to each supply for every 4 to 16 MAX6969s. The

necessary capacitance depends on the LED load cur-

rent, PWM switching frequency, and serial-interface

speed. Inadequate V+ decoupling can cause timing

problems, and very noisy LED supplies can affect LED

current regulation.

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