Datasheet
( )
J(MAX) A
(MAX)
JA
T T
PD
-
=
q
N
D OLi Li
i 1
P V I
=
= ´
å
ULN2803A
SLRS049G –FEBRUARY 1997–REVISED JANUARY 2015
www.ti.com
Typical Application (continued)
10.2.1 Design Requirements
For this design example, use the parameters listed in Table 1 as the input parameters.
Table 1. Design Parameters
DESIGN PARAMETER EXAMPLE VALUE
GPIO Voltage 3.3 V or 5.0 V
Coil Supply Voltage 12 V to 100 V
Number of Channels 8
Output Current (R
COIL
) 20 mA to 300 mA per channel
Duty Cycle 100%
10.2.2 Detailed Design Procedure
When using ULN2803A in a coil driving application, determine the following:
• Input Voltage Range
• Temperature Range
• Output & Drive Current
• Power Dissipation
10.2.2.1 Drive Current
The coil current is determined by the coil voltage (VSUP), coil resistance & output low voltage (V
OL
or V
CE(SAT)
).
I
COIL
= (V
SUP
– V
CE(SAT)
) / R
COIL
(1)
10.2.2.2 Output Low Voltage
The output low voltage (V
OL
) is the same thing as V
CE(SAT)
and can be determined by, Figure 1, Figure 2, or
Electrical Characteristics.
10.2.2.3 Power Dissipation & Temperature
The number of coils driven is dependent on the coil current and on-chip power dissipation. To determine the
number of coils possible, use the below equation to calculate ULN2803A on-chip power dissipation P
D
:
Where:
N is the number of channels active together.
V
OLi
is the OUT
i
pin voltage for the load current I
Li
. This is the same as V
CE(SAT)
(2)
In order to guarantee reliability of ULN2803A and the system the on-chip power dissipation must be lower that or
equal to the maximum allowable power dissipation (PD
(MAX)
) dictated by below equation Equation 3.
Where:
T
J(MAX)
is the target maximum junction temperature.
T
A
is the operating ambient temperature.
θ
JA
is the package junction to ambient thermal resistance. (3)
It is recommended to limit ULN2803A IC’s die junction temperature to less than 125°C. The IC junction
temperature is directly proportional to the on-chip power dissipation.
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