MAX6977 Ver la hoja de datos (PDF) - Maxim Integrated

Número de pieza

componentes Descripción

Fabricante

MAX6977

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

Maxim Integrated 

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

LED Fault Detection

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 MAX6977 supply, V+. The

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

set for all eight outputs by an external resistor, RSET.

Initial Power-Up and Operation

An internal reset circuit clears the internal registers of

the MAX6977 on power-up. All outputs OUT0 to 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 MAX6977 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 MAX6977. The serial interface data

word length is 8 bits, D0–D7.

The functions of the five interface pins follow. DIN is the

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

pled on the rising edge of CLK (Table 1). Data is shift-

ed 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 MAX6977 8-bit shift register on its rising edge.

LE is the load input of the MAX6977 latch, which trans-

fers data from the MAX6977 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 rising edge of LE loads fault data into the MAX6977

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 to

OUT7 high impedance, without altering the contents of

the output latches, and low to enable outputs OUT0 to

OUT7 to follow the state of the output latches.

OE is independent of the operation of the serial inter-

face. Data can be shifted into the serial interface shift

register and latched regardless of the state of OE.

DOUT is the serial-data output, which shifts data out

from the MAX6977’s 8-bit shift register on the rising edge

of CLK. Data at DIN is propagated through the shift reg-

ister and appears at DOUT eight clock cycles later.

LED Fault Detection

The MAX6977 includes circuitry that detects open-cir-

cuit LEDs automatically. An open-circuit fault occurs

when an output is programmed to sink current but less

than about 50% of the programmed current flows.

Table 1. 4-Wire Serial Interface Truth Table

SERIAL CLOCK

DATA INPUT

INPUT

DIN CLK D0

SHIFT REGISTER

CONTENTS

LOAD

INPUT

D1 D2 … Dn-1 Dn LE

LATCH CONTENTS

BLANKING

INPUT

D0 D1 D2 … Dn-1 Dn

OE

OUTPUT CONTENTS

D0 D1 D2 … Dn-1 Dn

H

H R1 R2 … Rn-2 Rn-1 — — — — — — —

—

— —————

L

L R1 R2 … Rn-2 Rn-1 — — — — — — —

—

— —————

X

R0 R1 R2 … Rn-1 Rn — — — — — — —

—

— —————

—

—

X X X…X X

H R0 R1 R2 — Rn-1 Rn

—

— —————

—

—

P1 P2 P3 … Pn-1 Pn

L

P0 P1 P2 … Pn-1 Pn

L

P0 P1 P2 … Pn-1 Pn

—

— —————— — X X 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

_______________________________________________________________________________________ 7

Share Link: