MAX6978 Ver la hoja de datos (PDF) - Maxim Integrated

Número de pieza

componentes Descripción

Fabricante

MAX6978

8-Port, 5.5V Constant-Current LED Driver with LED Fault Detection and Watchdog

Maxim Integrated 

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

LED Fault Detection and Watchdog

LE

tLRF

OUT_

I+ = operating supply current when sinking IOUT LED

drive current into N outputs

DUTY = PWM duty cycle applied to OE

N = number of MAX6978 outputs driving LEDs at the

same time (maximum is 8)

LE

tLRR

OUT_

VOUT = MAX6978 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:

CLK

tCRF

OUT_

CLK

tCRR

OUT_

Figure 3. LE and CLK to OUT_ Timing

tain the same data (validating the status), and second

whether faults are reported by the actual logic level.

The fault status bit identifies which driver chip has at

least one open-circuit LED. It is possible to determine

how many LEDs driven by a chip are at fault by running

eight test patterns, each of which lights a different sin-

gle LED. The fault status bit then effectively identifies

only one output at a time.

IOUT = 47mA, N = 8, DUTY = 1, VOUT = 2V, V+ = 5.25V

PD = (5.25V x 25mA) + (2V x 1 x 47mA x 8) = 0.883W

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 +

(0.883 x 46.95°C/W)

so TA = +108.5°C.

Overtemperature Cutoff

The MAX6978 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 will be the load LEDs cycling between on

and off as the driver repeatedly overheats and cools,

alternately turning the LEDs off and then back on again.

Applications Information

Selecting External Component RSET to Set

LED Output Current

The MAX6978 uses an external resistor RSET to set the

LED current for outputs OUT0–OUT7. The minimum

allowed value of RSET is 307.6Ω, which sets the output

currents to 55mA. The maximum allowed value of RSET

is 1.5kΩ. 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

MAX6978 is determined from the following equation:

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

where:

Power-Supply Considerations

The MAX6978 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 MAX6978 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 MAX6978s. 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.

For the TSSOP version, connect the underside exposed

pad to GND.

Chip Information

TRANSISTOR COUNT: 2382

PROCESS: BiCMOS

V+ = supply voltage

_______________________________________________________________________________________ 9

Share Link: