16-Channel Constant-Current Latched LED Driver

with Open LED Detection and Dot Correction

A6285

10

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Figure 10 shows an example application with n cascaded A6285

devices connected to a controller. The maximum number of

cascaded devices depends on the application system and the data

transfer rate. The minimum data input transfer rate is calculated

as follows:

(3)

where:

n is the number of cascaded A6285 devices.

Operating Modes

The A6285 has two operating modes, deter-

mined by the MODE signal:

• On-Off mode (MODE = low)

• Dot Correction mode (MODE = high)

Fault Output, FAULT

The open-drain output FAULT is used

to report both of the fault flags, LOD and TSD. During normal

operating conditions, the internal transistor connected to the

FAULT pin is turned off. The voltage on FAULT is pulled up to

If an LOD or TSD condition is detected, the internal transistor is

turned on, and FAULT is pulled to PGND. Because FAULT is an

open-drain output, multiple ICs can be ORed together and pulled-

This reduces the number of signals needed to report faults.

To determine whether the fault is a TSD or an LOD, LOD can be

masked by setting O¯¯E¯ = high. However, it cannot be determined

if both a TSD and an LOD condition are present. The FAULT

Truth Table is shown on page 11.

Active Pull-up Enable, PE

The A6285 provides active

pull-ups on each output determined by the PE pin. When the

enabled. When the PE pin is tied to ground, the active pull-ups

are disabled. The Active Pull-up Enable is also current-limited to

2.8 mA typical, preventing possible damage to the device in the

event of a short-to-ground. This feature can eliminate ghosting in

multiplexing applications.

Undervoltage Lockout (UVLO) and Power-On Reset

(POR)

The A6285 includes an internal undervoltage lockout

circuit that disables the outputs in the event that the logic supply

voltage drops below a minimum acceptable level. This feature

prevents the display of erroneous information, a function neces-

sary for some critical applications. A Power-On Reset (POR)

is performed upon recovery of the logic supply voltage after a

UVLO event and at power-up. During POR, all internal shift

registers and latches are set to 0.

Thermal Shutdown Protection and Fault Flag (TSD)

The

A6285 provides thermal protection when the device is over-

heated, typically a result of excessive power being dissipated in

the outputs. If the junction temperature exceeds the threshold

A6285

SDI

SDO

OUT15

CLK

MODE

LE

REXT

FAULT

A6285

SDI

SDO

CLK

MODE

LE

REXT

FAULT

IC n

IC 1

5

SDI

PE

CLK

MODE

LE

FAULT

Controller

SDO

100 k

100 nF

V LED

V LED

VDD

100 nF

VDD

VDD

…

…

OE

OE

OE

OUT0

OUT15

PE

OUT0

VLED

VLED

Figure 10. Schematic of cascaded A6285 devices