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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 R SET to
Set LED Output Current
The MAX6977 uses an external resistor R SET to set the
LED current for outputs OUT0 – OUT7. The minimum
allowed value of R SET is 307.6 ? , which sets the output
currents to 55mA. The maximum allowed value of R SET
is 1.5k ? . The reference value, 360 ? , sets the output
currents to 50mA. To set a different output current, use
the formula:
R SET = 18000 / I OUT
where I OUT is the desired output current in mA.
Computing Power Dissipation
The upper limit for power dissipation (P D ) for the
MAX6977 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 MAX6977 outputs driving LEDs at the
same time (maximum is 8)
V OUT = MAX6977 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 = 8, DUTY = 1, V OUT = 2V, V+ = 5.25V
P D = (5.25V x 25mA) + (2V x 1 x 47mA x 8) = 0.883W
Thus, for a 16-pin TSSOP package (T JA = 1 / 0.0213 =
+46.95 ° C/W from 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 +
(0.883 x 46.95 ° C/W)
so T A = +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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相关代理商/技术参数
MAX6977AAE+T
功能描述:LED照明驱动器 8-Port 5.5V Constant Current LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX6977AAE-T
功能描述:LED显示驱动器 RoHS:否 制造商:Micrel 数位数量:5 片段数量: 安装风格:SMD/SMT 封装 / 箱体:PLCC-44 工作电源电压:4.75 V to 11 V 最大电源电流:10 mA 最大工作温度:+ 85 C 最小工作温度:- 40 C 封装:Tube
MAX6977AEE
制造商:Maxim Integrated Products 功能描述:8-PORT, 5.5V CONSTANT-CURRENT LED DRIVER WITH - Rail/Tube
MAX6977APE
功能描述:LED照明驱动器 8-Port 5.5V Constant Current LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX6977APE+
功能描述:LED照明驱动器 8-Port 5.5V Constant Current LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX6977AUE
功能描述:LED照明驱动器 8-Port 5.5V Constant Current LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX6977AUE+
功能描述:LED照明驱动器 8-Port 5.5V Constant Current LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX6977AUE+T
功能描述:LED照明驱动器 8-Port 5.5V Constant Current LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N