Detailed Description

The MAX6978 LED driver comprises a 4-wire serial

interface driving eight constant-current sinking open-

drain output ports. The outputs drive LEDs in either sta-

tic or multiplex applications (Figure 1). The constant-

current outputs are guaranteed for current accuracy

not only with chip-supply voltage variations (5V ±10%

and 3V to 5.5V), but also over a realistic range of driver

output voltage drop (0.5V to 2.5V). The drivers use cur-

rent-sensing feedback circuitry (not simple current mir-

rors) to ensure very small current variations over the full

allowed range of output voltage (see the Typical

Operating Characteristics).

The 4-wire serial interface comprises an 8-bit shift reg-

ister and an 8-bit transparent latch. The shift register is

written through a clock input CLK and a data input DIN

and the data propagates to a data output DOUT. The

data output allows multiple drivers to be cascaded and

operated together. The contents of the 8-bit shift regis-

ter are loaded into the transparent latch through a

latch-enable input LE. The latch is transparent to the

shift register outputs when high, and latches the current

state on the falling edge of LE.

Each driver output is an open-drain constant-current

sink that should be connected to the cathode of either

a single LED or a series string of multiple LEDs. The

LED anode can be connected to a supply voltage of up

to 5.5V, independent of the MAX6978 supply, V+. The

constant-current capability is up to 55mA per output,

Initial Power-Up and Operation

An internal reset circuit clears the internal registers of

the MAX6978 on power-up. All outputs OUT0–OUT7,

therefore, initialize high impedance, and the interface

output DOUT initializes low, regardless of the initial

logic levels of the CLK, DIN, OE, and LE inputs.

4-Wire Serial Interface

The serial interface on the MAX6978 is a 4-wire serial

interface using four inputs (DIN, CLK, LE, OE) and a

data output (DOUT). This interface is used to write dis-

play data to the MAX6978. The serial-interface data

word length is 8 bits, D0–D7.

The functions of the five interface pins are as follows.

DIN is the serial-data input, and must be stable when it

is sampled on the rising edge of CLK. Data is shifted in,

MSB first. This means that data bit D7 is clocked in first,

followed by 7 more data bits, finishing with the LSB D0.

CLK is the serial-clock input, which shifts data at DIN

into the MAX6978 8-bit shift register on its rising edge.

LE is the latch load input of the MAX6978, which trans-

fers data from the MAX6978 8-bit shift register to its 8-bit

latch when LE is a high (transparent latch), and latches

the data on the LE falling edge of LE (Figure 2). The ris-

ing edge of LE loads fault data into the MAX6978 8-bit

shift register, replacing bits D6 and D5 with fault data.

This fault data is automatically shifted out through DOUT

when the next display data is shifted in.

The fourth input provides output-enable control of the

output drivers. OE is high to force outputs OUT0–OUT7

high impedance, without altering the contents of the

output latches, and low to enable outputs OUT0–OUT7

to follow the state of the output latches.

8-Port, 5.5V Constant-Current LED Driver with

LED Fault Detection and Watchdog

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7

Table 1. 4-Wire Serial-Interface Truth Table

SHIFT-REGISTER

CONTENTS

LATCH CONTENTS

OUTPUT CONTENTS

SERIAL

DATA

INPUT

DIN

CLOCK

INPUT

CLK

LOAD

INPUT

LE

BLANKING

INPUT

OE

H

—

——

—

—

——

—

—

————

—

L

—

——

—

—

——

—

—

————

—

X

—

——

—

—

——

—

—

————

—

——

XX

X

…

XX

H

—

—

————

—

——

L

L

…

——

——

——

—

—

—

XX

X

…

X

X

H

Hi-Z Hi-Z Hi-Z … Hi-Z Hi-Z

L = Low-logic level

H = High-logic level

X = Don’t care

P = Present state

R = Previous state