19-3578; Rev 2; 7/05
8-Port, 5.5V Constant-Current LED Driver with
LED Fault Detection and Watchdog
General Description
The MAX6978 serial-interfaced LED driver provides
eight open-drain, constant-current sinking LED driver
outputs rated at 5.5V. The MAX6978 operates from a 3V
to 5.5V supply. The MAX6978 supply and the LEDs’
supply can power up in any order. The constant-current
outputs are programmed together to up to 55mA using
a single external resistor. The MAX6978 operates with a
25Mb, industry-standard, 4-wire serial interface.
The MAX6978 includes circuitry that automatically
detects open-circuit LEDs. Fault status is loaded into the
serial-interface shift register when LE goes high and is
automatically shifted out on DOUT when the next data
transmission is shifted in.
The MAX6978 includes a fail-safe feature for safety-
related applications, which blanks the display if the ser-
ial interface fails. If the serial interface becomes
inactive for more than 1s, all driver output latches are
automatically cleared. This turns off all LEDs connected
to the outputs. The shift register data is not disturbed.
The outputs remain off until the driver output latches are
updated with data turning them on, so recovery is auto-
matic if the transmission failure is temporary. The
watchdog function requires no software change to the
application driving the MAX6978.
The MAX6978 uses the industry-standard shift-register-
plus-latch-type serial interface. The driver accepts data
shifted into an 8-bit shift register using data input DIN
and clock input CLK. Input data appears at the output
DOUT eight clock cycles later to allow cascading of
multiple MAX6978s. The latch-enable input LE loads
the 8 bits of shift-register data into an 8-bit output latch
to set which LEDs are on and which are off. The output-
enable input
OE
gates all eight outputs on and off, and
is fast enough to be used as a PWM input for LED
intensity control.
The MAX6978 is one of a family of 12 shift-register-plus-
latch-type LED drivers. The family includes 8-port and
16-port types, with 5.5V- or 36V-rated LED outputs, with
and without open-circuit LED detection and watchdog.
All versions operate from a 3V to 5.5V supply, and are
specified over the -40°C to +125°C temperature range.
o
3V to 5.5V Logic Supply
o
8 Constant-Current LED Outputs Rated at 5.5V
o
Up to 55mA Continuous Current per Output
o
Output Current Programmed by Single Resistor
o
3% Current Matching Between Outputs
o
6% Current Matching Between ICs
o
Watchdog Clears Display if Interface Fails
o
Reports Open-Circuit LED Faults
o
-40°C to +125°C Temperature Range
Features
o
25Mb Industry-Standard 4-Wire Serial Interface at 5V
MAX6978
Ordering Information
PART
MAX6978AUE
MAX6978APE
MAX6978AAE
TEMP RANGE
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
PIN-PACKAGE
16 TSSOP-EP*
16 PDIP
16 SSOP
*EP
= Exposed pad.
Typical Application Circuit and Selector Guide appear at
end of data sheet.
Pin Configuration
TOP VIEW
GND 1
DIN 2
CLK 3
LE 4
OUT0 5
OUT1 6
OUT2 7
OUT3 8
16 V+
15 SET
14 DOUT
Applications
Variable Message Signs
Marquee Displays
Point-of-Order Signs
Traffic Signs
Gaming Features
Architectural Lighting
MAX6978AUE
MAX6978APE
MAX6978AAE
13 OE
12 OUT7
11 OUT6
10 OUT5
9
OUT4
TSSOP/PDIP/SSOP
________________________________________________________________
Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
8-Port, 5.5V Constant-Current LED Driver with
LED Fault Detection and Watchdog
MAX6978
ABSOLUTE MAXIMUM RATINGS
Voltage (with respect to GND)
V+ ............................................................................-0.3V to +6V
OUT_ .......................................................................-0.3V to +6V
DIN, CLK, LE,
OE,
SET ................................-0.3V to (V+ + 0.3V)
DOUT Current ..................................................................±10mA
OUT_ Sink Current .............................................................60mA
Total GND Current ...........................................................480mA
Continuous Power Dissipation (T
A
= +70°C)
16-Pin SSOP (derate 7.1mW/°C above +70°C) ............571mW
16-Pin PDIP (derate 10.5mW/°C above +70°C)............842mW
16-Pin TSSOP (derate 21.3mW/°C over
T
A
= +70°C) ..............................................................1702mW
Operating Temperature Range .........................-40°C to +125°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(Typical
Operating Circuit,
V+ = 3V to 5.5V, T
A
= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at V+ = 5V, T
A
= +25°C.)
(Note 1)
PARAMETER
Operating Supply Voltage
Output Voltage
Standby Current
(Interface Idle, All Output Ports
High Impedance, R
SET
= 360Ω)
Standby Current
(Interface Running, All Output
Ports High Impedance,
R
SET
= 360Ω)
Supply Current
(Interface Idle, All Output Ports
Active Low, R
SET
= 360Ω)
Input High Voltage
DIN, CLK, LE,
OE
Input Low Voltage
DIN, CLK, LE,
OE
Hysteresis Voltage
DIN, CLK, LE,
OE
Input Leakage Current
DIN, CLK, LE,
OE
Output-High Voltage DOUT
Output Low Voltage
Output Current
OUT_
Output Leakage Current OUT_
Watchdog Timeout
SYMBOL
V+
V
OUT
I
+
All logic inputs at V+ or GND, DOUT
unloaded
4.5
CONDITIONS
MIN
3.0
TYP
MAX
5.5
5.5
5.6
UNITS
V
V
mA
I
+
f
CLK
= 5MHz,
OE
= V+, DIN and LE = V+
or GND, DOUT unloaded
4.7
6
mA
I
+
All logic inputs at V+ or GND,
DOUT unloaded
0.7 V+
10
25
mA
V
IH
V
IL
∆V
I
I
IH
, I
IL
V
OH
V
OL
I
OUT
I
LEAK
t
WD
I
SOURCE
= 4mA
I
SINK
= 4mA
V+ = 3V to 5.5V, V
OUT
= 0.8V to 2.5V,
R
SET
= 360Ω
OE
= V+, V
OUT
= V+
V
0.3 V+
0.8
V
V
+1
µA
V
0.5
V
mA
µA
s
-1
V+
- 0.5V
42
50
56
1
1
2
_______________________________________________________________________________________
8-Port, 5.5V Constant-Current LED Driver with
LED Fault Detection and Watchdog
5V TIMING CHARACTERISTICS
(Typical
Operating Circuit,
V+ = 4.5V to 5.5V, T
A
= T
MIN
to T
MAX
, unless otherwise noted.) (Note 1)
PARAMETER
CLK Clock Period
CLK Pulse-Width High
CLK Pulse-Width Low
DIN Setup Time
DIN Hold Time
DOUT Propagation Delay
DOUT Rise and Fall Time
LE Pulse-Width High
LE Pulse Width (Fault-Detection
Data Valid)
LE Setup Time
LE Rising to OUT_ Rising Delay
LE Rising to OUT_ Falling Delay
CLK Rising to OUT_ Rising Delay
CLK Rising to OUT_ Falling Delay
OE
Rising to OUT_ Rising Delay
OE
Falling to OUT_ Falling Delay
LED Output OUT_ Turn-On Fall
Time
LED Output OUT_ Turn-Off Rise
Time
SYMBOL
t
CP
t
CH
t
CL
t
DS
t
DH
t
DO
t
DR
, t
DF
t
LW
t
LF
t
LS
t
LRR
t
LRF
t
CRR
t
CRF
t
OEH
t
OEL
t
f
t
r
80% to 20%
20% to 80%
(Note 2)
(Note 2)
(Note 2)
(Note 2)
C
DOUT
= 10pF, 20% to 80%
20
20
10
100
280
100
310
100
300
200
120
CONDITIONS
MIN
40
19
19
4
8
12
32
10
TYP
MAX
UNITS
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
MAX6978
_______________________________________________________________________________________
3
8-Port, 5.5V Constant-Current LED Driver with
LED Fault Detection and Watchdog
MAX6978
3.3V TIMING CHARACTERISTICS
(Typical
Operating Circuit,
V+ = 3V to 5.5V, T
A
= T
MIN
to T
MAX
, unless otherwise noted.) (Note 1)
PARAMETER
CLK Clock Period
CLK Pulse-Width High
CLK Pulse-Width Low
DIN Setup Time
DIN Hold Time
DOUT Propagation Delay
DOUT Rise and Fall Time
LE Pulse-Width High
LE Pulse Width (Fault-Detection
Data Valid)
LE Setup Time
LE Rising to OUT_ Rising Delay
LE Rising to OUT_ Falling Delay
CLK Rising to OUT_ Rising Delay
CLK Rising to OUT_ Falling Delay
OE
Rising to OUT_ Rising Delay
OE
Falling to OUT_ Falling Delay
LED Output OUT_ Turn-On Fall
Time
LED Output OUT_ Turn-Off Rise
Time
t
OEH
t
OEL
t
f
t
r
80% to 20%
20% to 80%
t
LW
t
LW
t
LS
SYMBOL
t
CP
t
CH
t
CL
t
DS
t
DH
t
DO
C
DOUT
= 10pF, 20% to 80%
20
20
15
100
310
100
330
100
330
200
120
CONDITIONS
MIN
52
24
24
4
8
12
48
10
TYP
MAX
UNITS
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
Note 1:
All parameters tested at T
A
= +25°C. Specifications over temperature are guaranteed by design.
Note 2:
See Figure 3.
Typical Operating Characteristics
(T
A
= +25°C, unless otherwise noted.)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
(INTERFACE IDLE, ALL OUTPUTS
OFF, R
SET
= 720Ω)
MAX6978 toc01
SUPPLY CURRENT vs. SUPPLY VOLTAGE
(INTERFACE IDLE, ALL OUTPUTS
OFF, R
SET
= 360Ω)
MAX6978 toc02
SUPPLY CURRENT vs. SUPPLY VOLTAGE
(INTERFACE IDLE, ALL OUTPUTS
ON, R
SET
= 720Ω)
T
A
= +85°C
MAX6978 toc03
3.3
3.2
T
A
= +125°C
SUPPLY CURRENT (mA)
3.1
3.0
2.9
2.8
2.7
2.6
T
A
= -40°C
2.5
3.0
3.5
4.0
4.5
5.0
T
A
= +25°C
T
A
= +85°C
5.2
5.1
5.0
SUPPLY CURRENT (mA)
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
T
A
= -40°C
T
A
= +25°C
T
A
= +125°C
T
A
= +85°C
15
T
A
= +125°C
12
SUPPLY CURRENT (mA)
9
6
T
A
= -40°C
3
T
A
= +25°C
0
3.0
3.5
4.0
4.5
5.0
5.5
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
5.5
SUPPLY VOLTAGE (V)
4
_______________________________________________________________________________________
8-Port, 5.5V Constant-Current LED Driver with
LED Fault Detection and Watchdog
MAX6978
Typical Operating Characteristics (continued)
(T
A
= +25°C, unless otherwise noted.)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
(INTERFACE IDLE, ALL OUTPUTS
ON, R
SET
= 360Ω)
T
A
= +125°C
12
SUPPLY CURRENT (mA)
T
A
= +85°C
MAX6978 toc04
PORT OUTPUT CURRENT vs. PORT OUTPUT
VOLTAGE (R
SET
= 720Ω, V+ = 3.3V)
MAX6978 toc05
PORT OUTPUT CURRENT vs. PORT OUTPUT
VOLTAGE (R
SET
= 360Ω, V+ = 3.3V)
MAX6978 toc06
15
30
25
T
A
= -40°C
20
15
10
5
0
T
A
= +25°C
T
A
= +85°C
T
A
= +125°C
60
50
40
30
20
10
0
T
A
= -40°C
T
A
= +25°C
T
A
= +85°C
T
A
= +125°C
9
T
A
= -40°C
T
A
= +25°C
6
3
0
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
PORT OUTPUT CURRENT (mA)
PORT OUTPUT CURRENT (mA)
0
0.5
1.0
1.5
2.0
2.5
3.0
0
0.5
1.0
1.5
2.0
2.5
3.0
PORT OUTPUT VOLTAGE (V)
PORT OUTPUT VOLTAGE (V)
PORT OUTPUT CURRENT vs. PORT OUTPUT
VOLTAGE (R
SET
= 720Ω, V+ = 5.0V)
MAX6978 toc07
PORT OUTPUT CURRENT vs. PORT OUTPUT
VOLTAGE (R
SET
= 360Ω, V+ = 5.0V)
MAX6978 toc08
PORT OUTPUT CURRENT vs. SUPPLY
VOLTAGE (R
SET
= 720Ω, V
OUT
= 2V)
MAX6978 toc09
30
25
T
A
= -40°C
20
15
10
5
0
0
0.5
1.0
1.5
2.0
2.5
T
A
= +25°C
T
A
= +85°C
T
A
= +125°C
60
50
T
A
= -40°C
40
30
20
10
0
T
A
= +25°C
T
A
= +85°C
T
A
= +125°C
26.0
PORT OUTPUT CURRENT (mA)
PORT OUTPUT CURRENT (mA)
PORT OUTPUT CURRENT (mA)
25.5
T
A
= +125°C
25.0
T
A
= +85°C
T
A
= +25°C
24.5
T
A
= -40°C
3.0
24.0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.0
3.5
4.0
4.5
5.0
5.5
PORT OUTPUT VOLTAGE (V)
SUPPLY VOLTAGE (V)
PORT OUTPUT VOLTAGE (V)
PORT OUTPUT CURRENT vs. SUPPLY
VOLTAGE (R
SET
= 360Ω, V
OUT
= 2V)
MAX6978 toc10
PORT OUTPUT CURRENT
vs. R
SET
RESISTANCE (V+ = 5.0V)
MAX6978 toc11
52
50
PORT OUTPUT CURRENT (mA)
PORT OUTPUT CURRENT (mA)
51
T
A
= +125°C
50
T
A
= +85°C
40
30
20
T
A
= +25°C
49
T
A
= -40°C
10
48
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
0
0
2000
4000
6000
8000
10,000
R
SET
RESISTANCE (Ω)
_______________________________________________________________________________________
5
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