MAX6979AUG+【PDF 9/12 页】IC LED DRIVER LINEAR 24-TSSOP

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16-Port, 5.5V Constant-Current LED Driver with

LED Fault Detection and Watchdog

Table 1. 4-Wire Serial-Interface Truth Table

SERIAL

DATA

INPUT

DIN

CLOCK

INPUT

CLK

D 0

SHIFT-REGISTER

CONTENTS

D 1 D 2 … D n-1 D n

LOAD

INPUT

LE

D 0

LATCH CONTENTS

D 1 D 2 … D n-1 D n

BLANKING

INPUT

OE

D 0

OUTPUT CONTENTS

D 1 D 2 … D n-1 D n

H

L

X

H

L

R 0

R 1

R 1

R 1

R 2

R 2

R 2

… R n-2 R n-1

… R n-2 R n-1

… R n-1 R n

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

X

X

X

…

X

X

H

R 0

R 1

R 2

— R n-1 R n

—

—

—

—

—

—

—

—

—

P 1

P 2

P 3

… P n-1 P n

L

P 0

P 1

P 2

… P n-1 P n

L

P 0

P 1

P 2

… P n - 1 P n

—

—

—

—

—

— — —

—

X

X

X

…

X

X

H

Hi-Z Hi-Z Hi-Z … Hi-Z Hi-Z

L = Low-logic level; H = High-logic level; X = Don’t care; P = Present state; R = Previous state

Computing Power Dissipation

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

MAX6979 is determined from 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 MAX6979 outputs driving LEDs at the

same time (maximum is 16)

V OUT = MAX6979 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 25mA) + (2V x 1 x 47mA x 16) = 1.776W

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 + (1.776 x 82°C/W)

so T A = +145.6°C.

Overtemperature Cutoff

The MAX6979 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 MAX6979 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 MAX6979 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 MAX6979s. 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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