MAX6978 Maxim Integrated Products, MAX6978 Datasheet - Page 9

• Manufacturer Part Number: MAX6978
• Description: 5.5V Constant-Current LED Driver
• Manufacturer: Maxim Integrated Products

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.
Selecting External Component R
The MAX6978 uses an external resistor R
LED current for outputs OUT0–OUT7. The minimum
allowed value of R
currents to 55mA. The maximum allowed value of R
is 1.5kΩ. The reference value, 360Ω, sets the output
currents to 50mA. To set a different output current, use
the formula:
where I
The upper limit for power dissipation (P
MAX6978 is determined from the following equation:
where:
OUT_
LE
OUT_
LE
CLK
OUT_
CLK
OUT_
8-Port, 5.5V Constant-Current LED Driver with
P
OUT
D
= (V+ x I+) + (V
is the desired output current in mA.
Applications Information
V+ = supply voltage
R
Computing Power Dissipation
_______________________________________________________________________________________
SET
SET
t
t
LRF
t
LRR
CRF
is 307.6Ω, which sets the output
= 18,000 / I
t
CRR
OUT
x DUTY x I
LED Output Current
LED Fault Detection and Watchdog
OUT
OUT
SET
SET
D
x N)
to set the
) for the
to Set
SET
I+ = operating supply current when sinking I
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
LED(s)
I
P
Dissipation example:
Thus, for a 16-pin TSSOP package (T
+46.95°C/W from the Absolute Maximum Ratings), the
maximum allowed ambient temperature T
so T
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.
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.
TRANSISTOR COUNT: 2382
PROCESS: BiCMOS
I
OUT
OUT
P
OUT
D
D
= power dissipation, in mW if currents are in mA
A
= (5.25V x 25mA) + (2V x 1 x 47mA x 8) = 0.883W
= LED drive current programmed by R
= 47mA, N = 8, DUTY = 1, V
= MAX6978 port output voltage when driving load
T
= +108.5°C.
J(MAX)
= T
A
Power-Supply Considerations
(0.883 x 46.95°C/W)
+ (P
D
Overtemperature Cutoff
x T
JA
Chip Information
) = +150°C = T
OUT
JA
= 2V, V+ = 5.25V
= 1 / 0.0213 =
A
is given by:
SET
A
OUT
+
LED
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