MAX6977 查看數據表（PDF） - Maxim Integrated

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MAX6977

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

Maxim Integrated 

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

LED Fault Detection

Open circuits are checked just after the rising edge of

LE, when new LED data is loaded into the output latch-

es from the shift register(s). If one or more output port is

detected with an open-circuit fault, then the 2 bits, D6

and D5, in the serial interface shift register are both set

high. If no open-circuit faults are detected, then D6 and

D5 are both cleared low. The data in other 6-bit posi-

tions in the serial interface shift register is not altered.

Fault status is automatically shifted out on DOUT when

the next data transmission is shifted in. LE is normally

taken high after all 8 bits of new LED data have been

clocked into the shift register, and so at that time,

DOUT is outputting data bit D7. On the next two rising

edges of CLK, the 2 fault status bits, D6 and D5, are

clocked out in that order, followed by the remaining 5

unchanged data bits D4 to D0.

A typical fault-detecting application tests all the shifted-

out data. Bits D0–D4 and D7 are checked against the

originally transmitted data to check data link integrity.

Bits D5 and D6 are checked first to see that they con-

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.

Applications Information

Selecting External Component RSET to

Set LED Output Current

The MAX6977 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 = 18000 / IOUT

where IOUT is the desired output current in mA.

Computing Power Dissipation

The upper limit for power dissipation (PD) for the

MAX6977 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 MAX6977 outputs driving LEDs at the

same time (maximum is 8)

VOUT = MAX6977 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 = 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 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 MAX6977 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.

Power-Supply Considerations

The MAX6977 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 MAX6977 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 MAX6977s. The

necessary capacitance depends on the LED load cur-

rent, PWM switching frequency, and serial interface

speed. Inadequate V+ decoupling may 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

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