MAX6971 Ver la hoja de datos (PDF) - Maxim Integrated

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MAX6971

16-Port, 36V Constant-Current LED Driver

Maxim Integrated 

16-Port, 36V Constant-Current LED Driver

Applications Information

Selecting External Component RSET to Set

LED Output Current

The MAX6971 uses an external resistor, RSET, to set the

LED current for outputs OUT0–OUT15. The minimum

allowed value of RSET is 327.3Ω, which sets the output

currents to 55mA. The maximum allowed value of RSET

is 5kΩ. The reference value, 360Ω, sets the output cur-

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

formula:

RSET = 18,000 / IOUT

where IOUT is the desired output current in mA.

Computing Power Dissipation

The upper limit for power dissipation (PD) for the

MAX6971 is determined by the following equation:

PD = (V+ x I+) + (VOUT x DUTY x IOUT x N)

where:

V+ = supply voltage

I+ = operating supply current when sinking IOUT LED

drive current into N outputs

DUTY = PWM duty cycle applied to OE

N = number of MAX6971 outputs driving LEDs at the

same time (maximum is 16)

VOUT = MAX6971 port output voltage when driving load

LED(s)

IOUT = LED drive current programmed by RSET

PD = power dissipation, in mW if currents are in mA

Dissipation example:

IOUT = 20mA, N = 16, DUTY = 1, VOUT = 2V, V+ =

5.25V

PD = (5.25V x 25mA) + (2V x 1 x 20mA x 16) = 0.77W

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

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

maximum allowed ambient temperature TA is given by:

TJ(MAX) = TA + (PD x TJA) = +150°C =

TA + (0.77 x 82°C/W)

so TA = +86°C.

Overtemperature Cutoff

The MAX6971 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.

Power-Supply Considerations

The MAX6971 operates with a chip supply V+, and one

or more LED supplies. Bypass each supply to GND

with a 0.1µF capacitor as close to the MAX6971 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 MAX6971s. 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.

PROCESS: BiCMOS

Chip Information

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