MAX6979 查看數據表（PDF） - Maxim Integrated

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MAX6979

16-Port, 5.5V Constant-Current LED Driver with LED Fault Detection and Watchdog

Maxim Integrated 

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

LED Fault Detection and Watchdog

LE

tLRF

OUT_

LE

tLRR

OUT_

CLK

tCRF

OUT_

CLK

tCRR

OUT_

Figure 3. LE and CLK to OUT_ Timing

Watchdog

The MAX6979 includes a watchdog circuit that monitors

the CLK, DIN, and LE inputs. If there is no transition on

all of these inputs for nominally 1s, then the output latch-

es are cleared and outputs OUT0–OUT15 go high

impedance like the initial power-up condition. This turns

off all LEDs connected to the outputs. The shift-register

data does not change, just the output-latch data.

The watchdog is intended to be used as a fail-safe fea-

ture for applications, which prefer a blank display to an

incorrect display if the serial interface fails. When the

watchdog triggers, the outputs remain off until the dri-

ver output latches are updated with data turning them

on. Recovery is therefore automatic if the transmission

failure is temporary, because the MAX6979 does not

lock up in the watchdog timeout state. The MAX6979

operates correctly when the serial interface is next acti-

vated, and the watchdog circuit is reset and starts

monitoring the serial interface again. The watchdog

function requires no software change to the application

driving the MAX6979.

LED Fault Detection

The MAX6979 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.

Open circuits are checked just after the falling edge of

OE

LE

CLK

DOUT

Figure 4. Fault Timing Diagram

D15 D14 D13 D12

FAULT STATUS BITS

OE. The fault data is latched on the rising edge of LE

and is shifted out when new LED data is loaded into the

output latches from the shift register. If one or more out-

put ports are detected with an open-circuit fault, the

D14 and D13 bits of DOUT go high. If no open-circuit

faults are detected, the D14 and D13 are set to low.

The data in the other 14 bit positions in DOUT are not

altered.

Fault status is shifted out on DOUT when the next data

transmission is shifted in after the rising edge of LE. LE

is normally taken high after all 16 bits of new LED data

have been clocked into the shift register(s), and then

DOUT outputs data bit D15. On the next two rising

edges of CLK, the 2 fault status bits, D14 and D13, are

clocked out in that order, followed by the remaining 13

unchanged data bits D12 to D0.

A typical fault-detecting application tests all the-shifted

out data. Bits D0–D12 and D15 are checked against

the originally transmitted data to check data-link integri-

ty. Bits D13 and D14 are checked first to see that they

contain the same data (validating the status) and sec-

ond, whether faults are reported or not by the actual

logic level.

Figure 4 is the LE and CLK to OUT_ timing diagram.

Applications Information

Selecting External Component RSET to

Set LED Output Current

The MAX6979 uses an external resistor RSET to set the

LED current for outputs OUT0–OUT15. The minimum

allowed value of RSET is 327.3Ω, which sets the output

currents to 55mA. The maximum allowed value of RSET

is 1.5kΩ. The reference value, 360Ω, sets the output

currents to 50mA. To set a different output current, use

the formula:

RSET = 18,000/IOUT

where IOUT is the desired output current in mA.

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