Integrated 8-Channel LED Drivers with

Switch-Mode Boost and SEPIC Controller

18

______________________________________________________________________________________

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

The MAX16807/MAX16808 use an external resistor,

sets the output currents to 55mA. The maximum

imum allowed capacitance at SET is 100pF.

Use the following formula to set the output current:

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

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:

where:

V+ = supply voltage

I+ = operating supply current

DUTY = PWM duty cycle applied to OE

when driving load LED(s)

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.

PD

DUTY x

V

x I

V

x I

V

x I

OUTi

i

i

OUTi

CC

CC

(

)

(

)

=+

+ +

∑

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥

+

=

=

0

7

R

I

SET

OUT

,

_

=

18 000