Rainbow Electronics MAX6971 - Manual (Page 9)

Applications Information
Selecting External Component R SET to Set
LED Output Current
The MAX6971 uses an external resistor, R SET , to set the
LED current for outputs OUT0–OUT15. The minimum
allowed value of R
SET is 327.3 Ω, which sets the output
currents to 55mA. The maximum allowed value of R SET
is 5k Ω. The reference value, 360Ω, sets the output cur-
rents 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
MAX6971 is determined by 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 MAX6971 outputs driving LEDs at the
same time (maximum is 16)
V OUT = MAX6971 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 = 20mA, N = 16, DUTY = 1, V OUT = 2V, V+ =
5.25V
P D = (5.25V x 25mA) + (2V x 1 x 20mA x 16) = 0.77W
Thus, for a 24-pin TSSOP package (T JA = 1 / 0.0122 =
+82°C/W from the 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 + (0.77 x 82°C/W)
so T A = +86°C.
Overtemperature Cutoff
The MAX6971 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 is the load LEDs cycling between on and
off as the driver repeatedly overheats and cools, alter-
nately turning the LEDs off and then back on again.
Power-Supply Considerations
The MAX6971 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 MAX6971 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 MAX6971s. 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.
Chip Information
PROCESS: BiCMOS
MAX6971
16-Port, 36V Constant-Current LED Driver
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