Rainbow Electronics MAX6983 - Manual (Page 9)

Applications Information
Selecting External Component R SET to Set
LED Output Current
The MAX6983 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
MAX6983 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 MAX6983 outputs driving LEDs at the
same time (maximum is 16)
V OUT = MAX6983 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 = +85°C.
Overtemperature Cutoff
The MAX6983 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 affect-
ed, so when a driver is overdissipating, the external
symptom is 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 MAX6983 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 MAX6983 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 MAX6983s. 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.
MAX6983
16-Port, 36V Constant-Current LED Driver with
LED Fault Detection and Watchdog
_______________________________________________________________________________________ 9
L = Low-logic level; H = High-logic level; X = Don’t care; P = Present state; R = Previous state.
SHIFT REGISTER
CONTENTS
LATCH CONTENTS OUTPUT CONTENTS
SERIAL
DATA
INPUT
DIN
CLOCK
INPUT
CLK
D 0 D 1 D 2 … D n-1 D n
LOAD
INPUT
LE
D 0 D 1 D 2 … D n-1 D n
BLANKING
INPUT
OE D 0 D 1 D 2 … D n-1 D n
H H R 1 R 2 … R n-2 R n-1 — — — — — — — — — — — — — —
L L R 1 R 2 … R n-2 R n-1 — — — — — — — — — — — — — —
X R
0
R
1
R
2
… R
n-1
R
n
— — — — — — — — — — — — — —
— — X X X … X X H R 0 R 1 R 2 R n-1 R n — — — — — — —
— — P 1 P 2 P 3 … P n-1 P n L P 0 P 1 P 2 … P n-1 P n L P 0 P 1 P 2 … P n-1 P n
— — — — — — — — — X X X … X X H Hi-Z Hi-Z Hi-Z … Hi-Z Hi-Z
Table 1. 4-Wire Serial-Interface Truth Table