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8-Port, 5.5V Constant-Current LED Driver with
LED Fault Detection and Watchdog
I+ = operating supply current when sinking I OUT LED
LE
OUT_
LE
OUT_
CLK
t LRF
t LRR
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)
V OUT = MAX6978 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
OUT_
t CRF
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 the Absolute Maximum Ratings ), the
CLK
t CRR
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.
Applications Information
Selecting External Component R SET to Set
LED Output Current
The MAX6978 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 = 18,000 / 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
MAX6978 is determined from the following equation:
P D = (V+ x I+) + (V OUT x DUTY x I OUT x N)
where:
V+ = supply voltage
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 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.
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
_______________________________________________________________________________________
9
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MAX6979AUG+ 功能描述:LED照明驱动器 16Port 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
MAX6979AUG+T 功能描述:LED照明驱动器 16Port 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
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