MAX6968 MAXIM [Maxim Integrated Products], MAX6968 Datasheet - Page 9

• Manufacturer Part Number: MAX6968
• Description: 16-Port, 36V Constant-Current LED Driver
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX6971 uses an external resistor, R
LED current for outputs OUT0–OUT15. The minimum
allowed value of R
currents to 55mA. The maximum allowed value of R
is 5kΩ. The reference value, 360Ω, sets the output cur-
rents to 50mA. To set a different output current, use the
formula:
where I
The upper limit for power dissipation (P
MAX6971 is determined by the following equation:
where:
V+ = supply voltage
I+ = operating supply current when sinking I
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
LED(s)
I
P
Dissipation example:
Thus, for a 24-pin TSSOP package (T
+82°C/W from the Absolute Maximum Ratings), the
maximum allowed ambient temperature T
so T
Selecting External Component R
OUT
P
OUT
D
D
I
OUT
= power dissipation, in mW if currents are in mA
A
= (5.25V x 25mA) + (2V x 1 x 20mA x 16) = 0.77W
= LED drive current programmed by R
= MAX6971 port output voltage when driving load
= +86°C.
P
OUT
D
= 20mA, N = 16, DUTY = 1, V
T
= (V+ x I+) + (V
J(MAX)
is the desired output current in mA.
16-Port, 36V Constant-Current LED Driver
Applications Information
= T
R
Computing Power Dissipation
_______________________________________________________________________________________
SET
SET
A
T
+ (P
A
is 327.3Ω, which sets the output
= 18,000 / I
+ (0.77 x 82°C/W)
5.25V
OUT
D
x T
x DUTY x I
LED Output Current
JA
) = +150°C =
OUT
OUT
JA
OUT
= 1 / 0.0122 =
A
SET
= 2V, V+ =
SET
is given by:
SET
D
, to set the
x N)
OUT
) for the
to Set
LED
SET
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.
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.
PROCESS: BiCMOS
Power-Supply Considerations
Overtemperature Cutoff
Chip Information
9