MAX6970 查看數據表（PDF） - Maxim Integrated

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MAX6970

8-Port, 36V Constant-Current LED Driver

Maxim Integrated 

8-Port, 36V Constant-Current LED Driver

LE

tLRF

OUT_

LE

tLRR

OUT_

CLK

tCRF

OUT_

CLK

tCRR

OUT_

Figure 3. LE and CLK to OUT_ Timing

Applications Information

Selecting External Component RSET to Set

LED Output Current

The MAX6970 uses an external resistor RSET to set the

LED current for outputs OUT0 to OUT7. The minimum

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

currents up to 55mA. The reference value, 360Ω, sets

the output currents 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

MAX6970 is determined from 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 MAX6970 outputs driving LEDs at the

same time (maximum is eight)

VOUT = MAX6970 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 = 50mA, N = 8, DUTY = 1, VOUT = 2V, V+ = 5.25V

PD = (5.25V x 25mA) + (2V x 1 x 50mA x 8) = 0.931W.

Thus, for a 16-pin TSSOP package (TJA = 1 / 0.0213 =

+46.95°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 + (0.931 x

46.95°C/W)

so TA = +106.3°C.

Overtemperature Cutoff

The MAX6970 contains an internal temperature sensor

that turns off all outputs when the die temperature

exceeds approximately +165°C. The outputs are re-

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 MAX6970 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 MAX6970 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 four to 16 MAX6970s.

The necessary capacitance depends on the LED load

current, 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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