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Texas Instruments Improved Thermal Dissipation and Energy Efficiency for Peripheral Driving Application notes
Application Report
SLVA674 – October 2014
Improved Thermal Dissipation and Energy Efficiency for
Peripheral Driving with TPL7407L
James Lockridge, Andrew Leverette
ABSTRACT
The ULN2003A has long been a popular device used for driving high-current peripheral circuits from
microcontroller and control logic output signals. The ULN2003A consists of seven Darlington bipolar
transistors which sink current from the output to ground when a high logic signal is placed on the input.
Because the ULN2003A is based on bipolar Darlington transistor topology, it dissipates a considerable
amount of power even when it sinks small output currents.
The TPL7407L is a new peripheral driver that uses an N-channel MOSFET transistor on the output
instead of the bipolar Darlington pair. Because of the NMOS output, the TPL7407L can sink more current
to ground while dissipating less power and generating less heat which makes it an overall improved
device compared to the ULN2003A. This application note explains how the CMOS technology in the
TPL7407L improves power dissipation and thermal performance compared to the ULN2003A, including a
50% reduction in power consumption in typical use cases.
1
2
3
4
5
6
7
Contents
Benefits........................................................................................................................
Comparison ...................................................................................................................
Power Budgeting and Thermal Properties................................................................................
How to Use More Channels to Improve Power Dissipation ............................................................
Application ....................................................................................................................
Power Supply Recommendations .........................................................................................
Conclusion ....................................................................................................................
2
2
3
5
6
7
8
List of Figures
1
Schematic of Darlington Pair in Each Channel of the ULN2003A..................................................... 2
2
Schematic of NMOS in Each Channel of the TPL7407L ............................................................... 2
3
Output Voltage Versus Output Current for TPL7407L and ULN2003A ............................................... 3
4
Dissipated Power per Channel Versus Output Current for TPL7407L and ULN2003A ............................ 4
5
Dissipated Power Versus Seven Paralleled Channel Output Current for TPL7407L and ULN2003A ............ 4
6
Temperature Rise Versus 7 Paralleled Channel Output Current for TPL7407L and ULN2003A ................. 5
7
Air Conditioning System Using Two ULN2003A Chips to Drive Peripherals
8
9
........................................
Air-Conditioning System Using One TPL7407L Chip to Drive Peripherals ..........................................
D Package Maximum Collector Current Versus Duty Cycle at 70°C .................................................
6
7
8
List of Tables
1
Comparison Between ULN2003A and TPL7407L ....................................................................... 3
2
Total Power Dissipation and Number of Channels Used to Sink IT = 1 A ............................................ 6
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1
Benefits
1
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Benefits
The improvements of the TPL7407L over the ULN2003A:
• Improved power efficiency and lower leakage current
• Sink 600 mA / channel on the output
• Maximum input drive across all GPIO ranges (1.8 V–5.0 V)
• Dissipates less than half the power for currents less than 250 mA per channel due to lower VOL
• Wider temperature range, –40°C to 125°C
2
Comparison
Table 1 shows a side-by-side comparison of the ULN2003A and TPL7407L drivers. VOL is the voltage drop
from the output pin of the driver to ground. IL is the current the driver sinks on the output. In the case of the
ULN2003A, VOL is the collector-to-emitter voltage, referenced in the datasheet as voltage parameter
VCE(sat), and IL is the collector current per channel, IC (see Figure 1).
COM
OUT(1-7)
RB
+
IN(1-7)
IL
VOL
7.2 k
3k
í
E
Figure 1. Schematic of Darlington Pair in Each Channel of the ULN2003A
For the TPL7407L, VOL is the drain-to-source voltage referenced in the datasheet as voltage parameter
VDS, and IL is the drain current per channel, IDS (see Figure 2).
COM
Regulation
Circuitry
OUT(1-7)
+
50 k
IN(1-7)
IL
DRIVER
VOL
1M
í
Figure 2. Schematic of NMOS in Each Channel of the TPL7407L
2
Improved Thermal Dissipation and Energy Efficiency for Peripheral Driving
with TPL7407L
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Power Budgeting and Thermal Properties
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Table 1. Comparison Between ULN2003A and TPL7407L
ULN2003A
TPL7407L
Max Output voltage
50 V
40V
VOL
VOL at 100 mA ≥ 0.9 V (typ)
VOL at 200 mA ≥ 1 V (typ)
VOL at 100 mA = 0.2 V (typ)
VOL at 200 mA = 0.42 V (typ)
Output Off Current (IOFF)
100 uA / ch (max)
0.5 µA / ch (max)
Ambient Temperature
–20°C to 70°C (Standard)
–40°C to 105°C (I version)
–40°C to 125°C
Max Collector/Drain Current, IL
500 mA
600 mA
The parameters in Table 1 were taken from the data sheets of both the ULN2003A and the TPL7407L.
The table shows improved parameters of the TPL7407L over the ULN2003A such as: operating
temperature range, max output current, output voltage, and leakage current.
3
Power Budgeting and Thermal Properties
The key improvement of the TPL7407L over the ULN2003A is the reduced voltage drop from the output
pin to ground, VOL. This lower voltage drop means that the TPL7407L will dissipate less power than the
ULN2003A for the same amount of load current. Figure 3 shows the output voltage as a function of output
load current for both devices for ambient temperatures of 25°C and 70°C. The data used for the following
graphs was acquired using the SOIC package of both devices. The graph shows that the TPL7407L has a
significantly lower output voltage over temperature than the ULN2003A.
Figure 3. Output Voltage Versus Output Current for TPL7407L and ULN2003A
Figure 3 also shows that the output of the TPL7407L has the characteristics of a resistor. The output
voltage is linearly dependent on the load current, according to Ohm’s law. The ULN2003A does not
intercept the origin because one of the transistors in the Darlington pair will not saturate.
This zero-intercept operation of the TPL7407L translates to less power dissipated by the chip when
compared to the ULN2003A. The power dissipated per channel, PD(CH), is calculated by Equation 1.
PD(CH) = VOLIL
(1)
Because the output voltage of the TPL7407L is lower over the operating load current range, it will
dissipate less power than the ULN2003A. Figure 4 compares the power dissipated by the two devices for
a single channel.
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Power Budgeting and Thermal Properties
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Figure 4. Dissipated Power per Channel Versus Output Current for TPL7407L and ULN2003A
When driving larger current loads, multiple inputs and outputs may be tied together, or paralleled. Figure 5
shows the power dissipation of the two devices for an application where all seven outputs are paralleled to
sink current.
Figure 5. Dissipated Power Versus Seven Paralleled Channel Output Current for TPL7407L and
ULN2003A
The plot lines in Figure 5 are limited by the maximum current of each device based on the temperature of
operation. As the chip dissipates power, the temperature of the silicon junction, TJ, increases. The
absolute maximum junction temperature, TJ,MAX, that these chips can tolerate is 150°C. Equation 2 shows
the relationship between the junction temperature, the ambient temperature, TA, and the package thermal
impedance, θJA.
TJ = θJAPD(CH) + TA
(2)
When designing with thermal considerations, the temperature rise, ΔT, is calculated (Equation 3) to find
the maximum operating ambient temperature of a chip. The temperature rise is the change in temperature
that occurs between the silicon junction and the ambient temperature.
ΔT = θJAPD(CH)=TJ,MAX – TA
4
(3)
Improved Thermal Dissipation and Energy Efficiency for Peripheral Driving
with TPL7407L
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How to Use More Channels to Improve Power Dissipation
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A lower temperature rise means that the chip will generate less heat for the same amount of load current.
Figure 6 shows a comparison of the temperature rise for the TPL7407L and ULN2003A. The data used in
Figure 6 was obtained by using the data from Figure 5 with Equation 3. Figure 6 is a calculated
approximation of the actual operating characteristics. In practice, the chip increases the ambient
temperature around it, which changes the operating conditions.
Figure 6. Temperature Rise Versus 7 Paralleled Channel Output Current for TPL7407L and ULN2003A
Although the thermal impedance of the ULN2003A (73°C/W) is lower than the TPL7407L (91.9°C/W), the
TPL7407L dissipates such low power by comparison that its temperature rise is lower for the operating
range of the ULN2003A. This lower temperature rise means that less heat will be generated by the chip.
4
How to Use More Channels to Improve Power Dissipation
As shown briefly in the previous section, the channels of the TPL7407L may be paralleled to drive loads
beyond the single channel maximum current. For example, a 1-A load can be driven by 2 to 7 paralleled
channels. The current through each channel is found by Equation 4 where IT is the total current being sunk
through the paralleled channels, IOL/CH is the current which each channel sinks, and N is the number
channels used to sink that current.
IL/CH = IT/N
(4)
When choosing the number of channels to sink a large current, the designer must be conscientious of
ambient temperature and power dissipation. One channel on the TPL7407L can sink its maximum
specification of 600 mA at 25°C so long as no other channels are sinking current. As more channels sink
current, the junction temperature increases, which decreases the individual current capacity of each
channel. The datasheet specifies that the TPL7407L can sink 2 A using all the pins. That is a maximum of
286 mA per channel.
However, there are power dissipation benefits for increasing the number of channels used to sink the load
current. This is because the output voltage is dependent on the load current of the TPL7407L. Figure 4
shows that as the current through each channel is reduced, the output voltage decreases proportionally.
V
(
)
The total power dissipated by chip, PT, is dependent on the output voltage, OL at IL / CH , at a specific
output current, IL/CH, as shown in Equation 5.
PT = V
IT = V
IL/CHN
OL at I
OL at I
(
L/CH
)
(
L/CH
)
(5)
When more channels are used, a less current is sunk by each channel. When less current is sunk through
each channel, the total power dissipated by the chip decreases for the same total load current. Table 2
shows a comparison of the total power dissipated by the chip when different numbers of channels are
used. The total current sunk by the chip is 1 A at 25°C. The values in the table were calculated from the
results shown in Figure 4.
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Application
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Table 2. Total Power Dissipation and Number of Channels Used to Sink IT = 1 A
N
IL/CH(MAX) (mA)
PT (W) for ULN2003A
PT (W) for TPL7407L
2
500
1.30
0.899
3
333
1.10
0.568
4
250
1.01
0.422
5
200
0.959
0.335
6
167
0.924
0.278
7
143
0.899
0.238
As Table 2 shows, when the TPL7407L uses only two channels to sink 1 A of current, it dissipates the
same power as seven channels of the ULN2003A. Paralleling more channels of the TPL7407L decreases
its dissipated power even further below the lowest power dissipated by the ULN2003A. With lower power
dissipated by the TPL7407L, more channels may be used to control more peripherals which can reduce
the number of chips used per board from two ULN2003A chips to one TPL7407L chip. Another example of
this BOM cost reduction can be found in Section 5.
5
Application
The ULN2003A and the TPL7407L are designed to sink large current loads that most logic or control
devices cannot normally handle. Inductive devices such as solenoids, relays, and stepper motors require a
large current to energize their coils quickly. One such system that uses multiple kinds of inductive devices
controlled by a microcontroller is an air-conditioning system. Figure 7 shows a simplified block diagram of
an air conditioning system using ULN2003A chips to drive various peripherals. The peripherals considered
in this particular system are relays that power a blower motor, and a unipolar stepper motor that controls a
valve or external louver.
Channels not utilized due to
thermal and current limitations
ULN2003A
Temperature
Sensor
x
x
Relay
Blower
x
Relay
Relay
Stepper
ULN2003A
Louver/
Valve
Channels not utilized due to
thermal and current limitations
Central & Fan
Control MCU
LED Indicators
Misc.
Sensors
Figure 7. Air Conditioning System Using Two ULN2003A Chips to Drive Peripherals
Typical air-conditioning systems require multiple ULN2003A chips to drive these and other peripherals in
their systems. As mentioned in the previous section, when multiple channels are used, the current
capacity of each channel decreases because of the heat generated by power dissipation in the chip. As
shown in Figure 4, the TPL7407L dissipates much less power than the ULN2003A. Since less power is
dissipated, less heat is generated, which allows the current capacity of the TPL7407L to be much larger.
With a larger current capacity, two ULN2003A chips may be substituted for one TPL7407L (Figure 8).
6
Improved Thermal Dissipation and Energy Efficiency for Peripheral Driving
with TPL7407L
Copyright © 2014, Texas Instruments Incorporated
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Power Supply Recommendations
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x
x
Blower
x
Relay
Relay
Temperature
Sensor
Relay
TPL7407L
Central and Fan
Control MCU
LED Indicators
Misc.
Sensors
Stepper
Louver/
Valve
Figure 8. Air-Conditioning System Using One TPL7407L Chip to Drive Peripherals
When one TPL7407L is used in place of two ULN2003A chips, less heat is generated by the overall
circuit. This is important, especially for designs that experience a wide range of temperatures. Other
advantages of using one TPL7407L chip include: reduced BOM cost, less leakage/standby-current, and
less circuit board space required.
6
Power Supply Recommendations
The COM pin is the power supply pin of this device to power the gate drive circuitry. This ensures full
drive potential with any GPIO above 1.5 V. The gate-drive circuitry is based on low voltage CMOS
transistors that can only handle a max gate voltage of 7 V. An integrated LDO reduces the COM voltage
of 8.5 V to 40 V to a regulated voltage of 7 V. Though 8.5 V minimum is recommended for Vcom, the part
will still function with a reduced COM voltage with a reduced gate drive voltage and a resulting higher
Rdson.
To prevent overvoltage on the internal LDO output due to a line transient on the COM pin, the COM pin
must be limited to below 3.5 V/µs. Faster slew-rate (or hot-plug) may cause damage to the internal gatedriving circuitry due to the LDO's inability to clamp a fast input transient fast enough. Since most modern
power supplies are loaded by capacitors > 10 µF, this should not be of any concern. It is recommended to
use a bypass capacitor that will limit the slew rate to below 0.5 V/µs.
Figure 9 is a great example where repetitive slew rates may occur on the Vcom pin. Whenever a Zener
diode is used between Vcom and the motor supply, the Vcom pin will slew from the coil supply to a voltage
that is the sum of the Zener voltage and the coil supply when there is a flyback event. Depending on the
coil inductance and resistance, this can be very rapid.
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Conclusion
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Channel Current De-Rating @70°C (D Package)
0.55 A
0.50 A
Maximum Current per Channel
0.45 A
0.40 A
0.35 A
N=1
N=2
0.30 A
N=3
0.25 A
N=4
0.20 A
N=5
0.15 A
N=6
N=7
0.10 A
0.05 A
0.00 A
0%
10%
20%
30%
40%
50%
60%
Duty Cycle
70%
80%
90%
100%
Figure 9. D Package Maximum Collector Current Versus Duty Cycle at 70°C
In summary, whenever the COM pin may experience a slew rate greater than 0.5 V/µs a capacitor must
be added to limit the slew to < 0.5 V/µs.
7
Conclusion
The TPL7407L offers many improvements in designs over the ULN2003A. The TPL7407L has lower
output voltage, lower power dissipation, less heat generation, and increased output current capability. It is
pin-for-pin compatible with the popular ULN2003A and can reduce bill-of-materials costs and board space
in high-current peripheral-driving applications.
8
Improved Thermal Dissipation and Energy Efficiency for Peripheral Driving
with TPL7407L
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