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Texas Instruments Designing Overcurrent protection for TPL7407L Peripheral Driver Application notes
Application Report
SLLA352 – November 2014
Designing Overcurrent Protection for the TPL7407L
Peripheral Driver
Aditya Ambardar ........................................................................................... Standard Linear and Logic
Ronald Michallick
Ryan Land
ABSTRACT
Microcontroller units (MCUs) are generally limited with lower drive on the I/Os. Most of the applications
requiring MCU boards to drive high-current loads need discrete implementation and additional
components, which use a lot of board space. This board space is called a peripheral driver section. The
TPL7407L device is an integrated peripheral driver with seven channel drivers inside. The TPL7407L
device is a higher performance version of popular ULN2003 drivers. The TPL7407L device does not have
overcurrent protection. For markets such as industrial and automotive, short and overcurrent protection
features are imperative for a device. This application report shows some examples of designing with the
TPL7407L device for high-current applications, including the overcurrent protection for the circuit without
the use of external current sense.
1
2
3
4
Contents
Designing a Typical Application ...........................................................................................
How to Calculate the Maximum Current Draw from TPL7407L .......................................................
Overcurrent Protection for the Channels .................................................................................
3.1
Using Multiple, Cost-Effective BJTs .............................................................................
3.2
Using Open-Drain Comparator (LM2903) and Reference (TLV431) .........................................
3.3
Self-Protecting TPL7407L .........................................................................................
Conclusion ...................................................................................................................
2
3
4
4
6
7
8
List of Figures
1
LED Driving Using TPL7407L .............................................................................................. 2
2
Relay Drive Example ........................................................................................................ 3
3
Overcurrent Protection Using a BJT
4
5
6
......................................................................................
TINA Simulation Results for Overcurrent Conditions ...................................................................
Overcurrent Protection Using LM2903 and TLV431 ....................................................................
Without any External Active Component Internal Channel used for Protection .....................................
4
5
7
8
TINA-TI is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
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1
Designing a Typical Application
1
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Designing a Typical Application
Figure 1 shows a typical LED-drive application with one channel connected.
VCOM
LED1
U1
TPL74074L
+
Input
R1
26.7
IN1
OUT1
IN2
OUT2
IN3
OUT3
IN4
OUT4
IN5
OUT5
IN6
OUT6
IN7
OUT7
GND
+
V1 12
COM
VCOM
Figure 1. LED Driving Using TPL7407L
For the LED drive application, the following values are assumed:
• Current through the LED, ILED = 100 mA (high-brightness LED)
• VCOM = 12 V
To design with the TPL7407L device, use Equation 1 to calculate the series resistance.
(VCOM VF ) (12 3)
R1 RFET
90 :
I LED
0.1
where
•
RFET is the channel FET resistance of the TPL7407L device.
(1)
See the TPL7407L data sheet for the specified FET resistance (SLRS066).
The maximum voltage drop at 100 mA is 320 mV, which implies that the maximum RFET value is equal to
V / I = 3.2 Ω. Use the standard value of 86.8 Ω for R1: R1 = 90 – 3.2 Ω = 86.8 Ω.
Assuming that all channels are driving LEDs each with a current of 100 mA, use Equation 2 to calculate
the total power dissipation on the package.
PD(tot) TPL7407L
2
ILED
RFET u 7
0.12 u 3.2 u 7
0.224 W
(2)
Use Equation 3 to calculate the temperature rise for the device (ΔT).
'T RTJA u PD 91.9 u 0.224 20qC
(3)
RθJA describes the rate of increase in device temperature as the device dissipates power. See the
TPL7407L data sheet for the specified thermal information (SLRS066). For the previous example, the
SOIC package was used with a junction-to-ambient thermal resistance (RθJA) of 91.9°C/W.
Assuming an ambient temperature of 80°C, the junction temperature of the device would be 100°C, which
is less than the maximum junction temperature of about 150°C allowed for the device.
2
Designing Overcurrent Protection for the TPL7407L Peripheral Driver
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How to Calculate the Maximum Current Draw from TPL7407L
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The power dissipation for the TPL7407 device in the previous design example can be compared with the
performance of the ULN2003 device. The VCE value for the output node of the ULN2003 device is 0.9 V.
Use Equation 4 to calculate the power dissipation.
PD(tot) ULN2003 VCE(sat) u ILED u 7 0.9 u 0.1 u 7 0.63 W
(4)
As is evident Equation 2 and Equation 3, the ULN2003 power dissipation is 2.8 times more than that of
the TPL7407L device. For the ULN2003 device, the delta increase in temperature is about 45.9°C which
yields an IC junction temperature of approximately 125.9°C, which is about 25.9°C more than the
TPL7407 device.
2
How to Calculate the Maximum Current Draw from TPL7407L
Some applications may require that more current is driven from the device. The output-current drive
capability can be increased by shorting multiple outputs together.
Assuming all the outputs are short for the device, use Equation 1, Equation 2, and Equation 3 to calculate
the maximum current from the device. Figure 2(a) shows an example.
(a) Single High-Current Relay
(b) Multiple Relays
VCOM
VCOM
Relay
Relay
U1
TPL74074L
+
Input
VCOM
U1
TPL74074L
IN1
OUT1
IN2
OUT2
+
Input
IN1
OUT1
IN2
OUT2
IN3
OUT3
IN4
OUT4
IN5
OUT5
IN3
OUT3
IN4
OUT4
IN5
OUT5
IN6
OUT6
IN6
OUT6
IN7
OUT7
IN7
OUT7
GND
+
Input
GND
COM
VCOM
Relay
COM
VCOM
Figure 2. Relay Drive Example
The maximum allowed junction temperature for the TPL7407L device is 150°C. Suppose a stable ambient
temperature for the system is about 40°C. The total temperature rise is given by 150°C – 40°C = 110°C.
Use Equation 5 to calculate the maximum temperature dissipation for the SOIC package.
110 91.9 u PD
(5)
PD
110
1.2 W
91.1
(6)
Assuming all the channels are shorted together at the output, the effective on-resistance for the FETs of
the device is RFET / 7.
PD(tot)
2
IO
u RFET
Ÿ 1.2 W
7
2
Imax
u 3.2
7
where
•
Imax for the TPL7407L device with all output channels shorted = 1.62 A at 40°C ambient.
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(7)
3
Overcurrent Protection for the Channels
3
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Overcurrent Protection for the Channels
CAUTION
To avoid device malfunction or damage, protect the device from an over current
condition by either limiting the current from the source or by adding overcurrent
protection.
Using overcurrent protection is imperative for the peripheral drivers in some applications if the load is not
controllable and short circuits could occur.
One of the advantages of the TPL7407L device over the industry-popular Darlington-pair peripheral
drivers is that because of the internal NMOSFETs, implementing overcurrent protection is easy and cost
effective. The following section describes two ways to implement overcurrent protection.
3.1
Using Multiple, Cost-Effective BJTs
The protection can be implemented using external, minimum discrete components (such as bipolarjunction transistors [BJTs] as shown in Figure 3.
(a) Single Input, Single Channel
VCOM
Current 1-7
Current 6-7
LED1
U1
TPL74074L
R9
10 k
+
Input
R8
1k
R12
100 k
R1
48
IN1
OUT1
IN2
OUT2
IN3
OUT3
IN4
OUT4
IN5
OUT5
IN6
OUT6
IN7
OUT7
GND
C1
1 nF
LED2
R2
48
R3
48
R4
48
R5
48
R6
48
Drop 6-7
R10
2
High Load
T1
IRFZ44
+
VCOM
COM
VCOM
R11
100 k
R7
10 k
+
V1 12
T3
MMBT3904LT1
(b) Multiple Inputs, Multiple Channels
VCOM
LED1
LED2
+
Input
Relay
U1
TPL74074L
R9
100 k
+
C2
1 nF
Input
R12
100 k
IN1
OUT1
IN2
OUT2
IN3
OUT3
IN4
OUT4
IN5
OUT5
IN6
OUT6
IN7
OUT7
GND
C1
1 nF
R1
200
R2
200
R6
48
R3
50
VCOM
COM
VCOM
+
+
Input
R8
100 k
V1 12
R4
1k
R7
1k
R11
100 k
T4
MMBT3904LT1
T3
MMBT3904LT1
Figure 3. Overcurrent Protection Using a BJT
4
Designing Overcurrent Protection for the TPL7407L Peripheral Driver
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Overcurrent Protection for the Channels
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The circuits in Figure 3 use the VBE (base emitter voltage) of transistor T3 and T4 as a reference to turn off
the respective input of the device if an overcurrent condition occurs at the output. Because the driver is no
longer a Darlington pair but a MOSFET device, the drop across the resistance of the FETs can be
measured to protect the device. BJT T3 in Figure 3(a) and T3 and T4 in Figure 3(b) both turn on when the
drop across the driving FET increases and crosses the VBE voltage to turn off the corresponding inputs.
The T1 transistor in Figure 3(a) is used to simulate a high-load condition only and does not appear in the
actual application circuit as shown in Figure 3(b). Use Equation 8 when BJT will turn on to calculate the
overcurrent through the channel.
§R
·
IOC u ¨ FET ¸
N
©
¹
VBE
where
•
•
•
•
VBE is the base emitter voltage of the BJT.
IOC is the overcurrent.
RFET is the typical on-resistance.
N is the number of channels shorted together.
(8)
The user can use the device to parallel the appropriate number of channels. Use Equation 9 to calculate
the maximum overcurrent (IOC).
§ N ·
2
IOC VBE u ¨
667 mA
¸ 0.7 u
2.1
© RFET ¹
(9)
The overcurrent is based on the value of VBE so it varies with the temperature. The value of RFET according
to data sheet varies from 2.1 to 3.25 Ω. The VBE variation must also be considered when designing with
this type of protection. For example, if the application operates with an ambient temperature of –40°C to
85°C, the VBE variation should be considered to ensure that the protection does not trigger while in normalmode operation. For the maximum current calculation, the minimum resistance of RFET is considered in
Equation 9.
Figure 4 shows the results based on TINA-TI™ simulation software. The explanation of the waveforms
shown in the simulation for circuit shown in Figure 3(a) follows:
Current 1-7 — is the measurement on all the currents through channels 1 through 7 of the device.
Current 6-7 — is the current measured through channels 6 and 7.
Drop 6-7 — is the input voltage for the T3 transistor to sense overcurrent condition.
High Load — is a dummy load which simulates the overcurrent for channels 6 and 7.
Input — is the signal for turning all the channels on simultaneously.
6
Current 1-7
0
4
Current 6-7
0
20
Drop 6-7
0
10
High Load
0
4
Input
0
0
10
20
30
40
50
Time (ms)
Figure 4. TINA Simulation Results for Overcurrent Conditions
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Overcurrent Protection for the Channels
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Figure 4 shows, that during normal operation, currents 6-7 is in the nominal range of 240 mA, but when a
simulated high load goes high, a spike occurs on currents 6-7. The circuit reacts in about 10 to 20 µs
which causes a current spike to occur. When the high load is active and an overcurrent is detected by T3,
the input IN6 and IN7 are pulled low, resulting in a 0-A current through channels 6 and 7. This method
does not allow flexibility when selecting the overcurrent value because it is dependent on the VBE of the
BJT and the internal resistance of the TPL7407 FETs.
3.2
Using Open-Drain Comparator (LM2903) and Reference (TLV431)
Figure 5 shows a more flexible and precise method of the short-circuit protection. This circuit overcomes
the limitation of the previously mentioned circuits in terms of the flexibility and precision of the reference
used for the implementation.
The LM2903 device, in combination with the TLV431 device, can be used to protect the most vulnerable
output loads on the TPL7407L device in a precise manner.
The TLV431 reference voltage of 1.25 V is further divided with the resistance to achieve any reference
voltage allowing user to program protection for any value of output current.
When the voltage across the internal OUT FET exceeds the reference voltage, the LM2903 device, which
is an open-drain output, mimics the circuit mentioned in Section 3.1 by pulling the input to ground.
Use Equation 10 to calculate the overcurrent protection for implementation in Figure 5.
Vref u R10
N
IOC
u
(R5 R10) RFET
where
•
•
•
•
IOC is the overcurrent
Vref is the voltage reference which is 1.25 V for the TLV431
N is the number of channels parallel
RFET is the on resistance for internal FETs
(10)
For Figure 5, outputs 3, 4, and 5 are shorted together and therefore the overcurrent can be calculated
using Equation 10 as shown in Equation 11.
1.25 u 100
3
IOC
892 mA
u
(100 100) 2.1
(11)
Depending on the selected reference source, such as the TLV431 device, the output overcurrent
variations are minimal in this case when compared to the BJT implementations previously discussed.
6
Designing Overcurrent Protection for the TPL7407L Peripheral Driver
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Overcurrent Protection for the Channels
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VCOM
LED1
LED2
+
Input
Relay
U1
TPL74074L
R9
100 k
+
C2
1 nF
Input
R12
100 k
IN1
OUT1
IN2
OUT2
IN3
OUT3
IN4
OUT4
IN5
OUT5
IN6
OUT6
IN7
OUT7
GND
C1
1 nF
R1
200
C3
10 µF
Input
R6
48
R3
50
COM
+
R2
200
VCOM
R8
100 k
VCOM
U2
LM2903
R11
100 k
+
+ +
R7
1k
R4
10 k
V1 12
R5
100 k
+
R10
100 k
V2 12
TLV431
T3
MMBT3904LT1
Figure 5. Overcurrent Protection Using LM2903 and TLV431
3.3
Self-Protecting TPL7407L
The TPL7407L device has input nodes with a high input-level (VIH) threshold of about 1.5 V. This option
can be used if only short-circuit protection is required. Consider using Equation 12 to calculate the peak
current through each OUT pin (IOC).
IOC
§ N ·
VIH u ¨
¸
© RFET ¹
1.5 u
1
2.1
where
•
IOC = 714 mA
(12)
Figure 6 shows the method to implement self-protection, where channel 5 is used similar to external BJT
and pulls down the IN6 and IN7 pins to protect the output channels 6 and 7.
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Conclusion
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VCOM
LED1
U1
TPL74074L
R9
10 k
+
Input
R12
100 k
IN1
OUT1
IN2
OUT2
IN3
OUT3
IN4
OUT4
IN5
OUT5
IN6
OUT6
IN7
OUT7
GND
C1
1 nF
R1
48
LED2
R2
48
R3
48
R4
48
VCOM
COM
VCOM
R6
48
VO
R11
0k
+
V1 12
R7
10 k
Figure 6. Without any External Active Component Internal Channel used for Protection
4
Conclusion
To drive high-current applications, consider the thermal characteristics of the package used.
To protect the device from an overcurrent and short-circuit event, carefully selecting the most appropriate
and affordable option based on the tradeoff for cost and performance is recommended.
For cost-constrained applications, use the self-protection option or use a bipolar-junction transistor. For
applications requiring precise control of overcurrent and short-circuit protections, using the LM2903 device
in conjunction with a precision reference such as the TLV431 device is recommended
8
Designing Overcurrent Protection for the TPL7407L Peripheral Driver
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