A6278 ALLEGRO [Allegro MicroSystems], A6278 Datasheet - Page 10

• Manufacturer Part Number: A6278
• Description: Serial-Input Constant-Current Latched LED Drivers with Open LED Detection
• Manufacturer: ALLEGRO [Allegro MicroSystems]

A6278
A6279
Load Supply Voltage (V
These devices are designed to operate with driver voltage
drops (V
1.2 to 4.0 V. If higher voltages are dropped across the driver,
package power dissipation will increase significantly. To mini-
mize package power dissipation, it is recommended to use the
lowest possible load supply voltage, V
voltage dropping, V
with V
(V
diode) for a group of drivers (see figure 3). If the available volt-
age source, V
series resistors or diodes are undesirable, a voltage regulator can
be used to provide supply voltages.
For reference, typical LED forward voltages are:
Z
), or for a series string of diodes (approximately 0.7 V per
DROP
CE
) of 0.7 to 3V, with an LED forward voltage, V
= I
LED
O
and
× R
, will cause unacceptable power dissipation and
LED Type
V
Infrared
Amber
DROP
DROP
Green
Yellow
White
Blue
Red
DROP
, according to the following formula:
for a single driver or for a Zener diode
= V
LED
LED
)
– V
V
F
1.9 to 2.65
1.6 to 2.25
DROP
LED
V
3.5 to 4.0
3.0 to 4.0
1.8 to 2.2
2.0 to 2.1
1.2 to 1.5
V
LED
– V
Figure 3. Typical appplications for voltage drops
V
V
CE
F
F
, or to set any series
(V)
CE ,
Application Information
Serial-Input, Constant-Current Latched
LED Drivers with Open LED Detection
V
DROP
V
V
LED
F
V
CE
, of
F
Pattern Layout
This device has a common logic ground and power ground
terminal, GND. For the LP package, the GND pin should be tied
to the exposed metal pad, EP, allowing the ground plane copper
to be used to dissipate heat. If the ground pattern layout contains
large common mode resistance, and the voltage between the
system ground and the LATCH ENABLE, OUTPUT ENABLE,
or CLOCK terminals exceeds 2.5 V (because of switching noise),
these devices may not work properly.
Package Power Dissipation (P
The maximum allowable package power dissipation based on
package type is determined by:
where R
experimentally. Power dissipation levels based on the package
are shown in the Package Thermal Characteristics section (see
page 14).
The actual package power dissipation is determined by:
where DC is the duty cycle. The value 16 represents the maxi-
mum number of available device outputs for the A6279, used for
the worst-case scenario (displaying all 16 LEDs; this would be 8
for the A6278).
When the load suppy voltage, V
P
used (see figure 3).
Reducing the percent duty cycle, DC, will also reduce power dis-
sipation. Typical results are shown on the following pages.
D(act)
V
DROP
V
> P
V
LED
θJA
V
CE
F
D(max)
P
is the thermal resistance of the package, determined
D(act)
, an external voltage reducer (V
= DC × (V
P
D(max)
= (150 – T
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
CE
× I
LED
O
× 16) + (V
D
, is greater than 3 to 5 V, and
)
A
) / R
θJA
DD
,
DROP
× I
DD
) must be
) ,
10