MAX16808 Maxim Integrated Products, MAX16808 Datasheet - Page 18

• Manufacturer Part Number: MAX16808
• Description: (MAX16807 / MAX16808) Integrated 8-Channel LED Drivers
• Manufacturer: Maxim Integrated Products

Integrated 8-Channel LED Drivers with
Switch-Mode Boost and SEPIC Controller
data have been clocked into the shift register(s), and
then DOUT outputs data bit D7. A typical fault-detect-
ing application tests all the shifted out data. Bits D0–D4
and D7 are checked against the originally transmitted
data to check data-link integrity. Bits D5 and D6 are
checked first to see that they contain the same data
(validating the status), and second, whether faults are
reported or not by the actual logic level.
The MAX16807/MAX16808 use an external resistor,
R
The minimum allowed value of R
sets the output currents to 55mA. The maximum
allowed value of R
imum allowed capacitance at SET is 100pF.
Use the following formula to set the output current:
where I
and the value for R
The MAX16807/MAX16808 contain an internal tempera-
ture sensor that turns off all outputs when the die temper-
ature exceeds +165°C. The outputs are enabled again
when the die temperature drops below +140°C. Register
contents are not affected, so when a driver is overdissi-
pating, the external symptom is the load LEDs cycling on
and off as the driver repeatedly overheats and cools,
alternately turning itself off and then back on again.
In stand-alone operation, the MAX16807/MAX16808
does not use the 4-wire interface (see the Typical
Operating Circuits ). Connect DIN and LE to V+ provide
at least 8 external clock pulses to CLK to enable 8 out-
puts. This startup pulse sequence can be provided
either using an external clock or the PWM signal. The
external clock can also be generated using the signal
at RTCT and an external comparator.
18
SET
______________________________________________________________________________________
, to set the LED current for outputs OUT0–OUT7.
OUT_
is the desired output current in milliamps
R
Selecting External Component
SET
SET
R
SET
SET
to Set LED Output Current
is 5kΩ (I
is in ohms.
=
Overtemperature Cutoff
Stand-Alone Operation
18 000
I
OUT
,
OUT_
_
SET
= 3.6mA) and max-
is 330Ω, which
All the output channels can be dimmed simultaneously
by applying a PWM signal (50Hz to 30kHz) to OE. This
allows for a wide range of dimming up to a 5000:1 ratio.
Each channel can be independently turned on and off
using a 4-wire serial interface. The dimming is propor-
tional to the PWM duty cycle.
Using an analog or digital potentiometer as R
for LED current amplitude adjustment and linear dimming.
Use the following equation to estimate the upper limit
power dissipation (PD) for the MAX16807/MAX16808:
where:
V+ = supply voltage
I+ = operating supply current
DUTY = PWM duty cycle applied to OE
V
when driving load LED(s)
I
PD = power dissipation.
Careful PCB layout is critical to achieve low switching
losses and clean, stable operation. Use a multilayer
board whenever possible for better noise immunity.
Protect sensitive analog grounds by using a star
ground configuration. Minimize ground noise by con-
necting AGND, PGND, the input bypass-capacitor
ground lead, and the output-filter ground lead to a sin-
gle point (star ground configuration). Also, minimize
trace lengths to reduce stray capacitance, trace resis-
tance, and radiated noise. The trace between the out-
put voltage-divider and the FB pin must be kept short,
as well as the trace between AGND and PGND.
OUTi
PD
OUTi
=
= LED drive current programmed by R
= MAX16807/MAX16808 port output voltage
DUTY x
LED Current Amplitude Adjustment
⎡
⎢
⎢
⎣
(
V
Computing Power Dissipation
+
x I
+ +
)
PCB Layout Guidelines
i
i
=
=
∑
7
V
0
OUTi
LED Dimming
x I
PWM Dimming
OUTi
⎤
⎥
⎥
⎦
+
SET
(
V
SET
CC
x I
allows
CC
)