MAX16807_07 Datasheet, PDF (18/21 Pages) Maxim Integrated Products – Integrated 8-Channel LED Drivers with Switch-Mode Boost and SEPIC Controller

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.
Selecting External Component
R_SET to Set LED Output Current
The MAX16807/MAX16808 use an external resistor,
R_SET, to set the LED current for outputs OUT0–OUT7.
The minimum allowed value of R_SET is 330Ω, which
sets the output currents to 55mA. The maximum
allowed value of R_SET is 5kΩ (I_{OUT_} = 3.6mA) and max-
imum allowed capacitance at SET is 100pF.
Use the following formula to set the output current:
R_SET
=
18,000
I_OUT _
where I_{OUT_} is the desired output current in milliamps
and the value for R_SET is in ohms.
Overtemperature Cutoff
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.
Stand-Alone Operation
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.
LED Dimming
PWM Dimming
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.
LED Current Amplitude Adjustment
Using an analog or digital potentiometer as R_SET allows
for LED current amplitude adjustment and linear dimming.
Computing Power Dissipation
Use the following equation to estimate the upper limit
power dissipation (PD) for the MAX16807/MAX16808:
⎡
i=7
⎤
PD = DUTY x _⎢(V + x I+) + ∑ V_OUTi x I_OUTi⎥ + (V_CC x I_CC)
⎣⎢
i=0
⎦⎥
where:
V+ = supply voltage
I+ = operating supply current
DUTY = PWM duty cycle applied to OE
V_OUTi = MAX16807/MAX16808 port output voltage
when driving load LED(s)
I_OUTi = LED drive current programmed by R_SET
PD = power dissipation.
PCB Layout Guidelines
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.
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