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Texas Instruments High Accuracy Current Sense of Smart High Side Switches Application notes
Application Report
SLVAE08 – November 2019
High Accuracy Current Sense of Smart High Side
Switches
Shreyas Dmello
ABSTRACT
High accuracy current sense is a feature of various devices in the high side switch family of devices from
Texas Instruments. It is used as a diagnostic tool to provide high accuracy feedback to the user on the
current flowing through the switch by providing a low power solution to sense large load currents.
This application report introduces the implementation and use of the function for high side switches and
gives a guide on setting the sense voltages. It also provides user examples on how to leverage this
feature for different application scenarios such as detecting small changes in load currents without the
need for prior calibration.
The accuracy of these devices is pre-trimmed and does not require any further calibration by the user.
This allows high side switches to be used in applications where external calibration circuitry is not feasible
either due to increased system level costs or limited packaging space.
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Contents
Introduction ................................................................................................................... 2
Function Description ......................................................................................................... 2
Effect of Load Current on Sense Accuracy............................................................................... 4
Fault Indication and the SNS Mux ......................................................................................... 6
Application Example ......................................................................................................... 7
Summary .................................................................................................................... 10
References .................................................................................................................. 10
List of Figures
1
TPSxHxxx Current Sense Circuit .......................................................................................... 2
2
Current Sense Accuracy Characteristic................................................................................... 4
3
Current Sense Ratio as a Function of Load Current for TPS2HB08-Q1 ............................................. 5
4
TPS1H100 Fault Reporting ................................................................................................. 6
5
TPS2HB50 Fault Reporting
6
7
8
................................................................................................ 6
TPS4H160 LED Bank Load Circuit Diagram ............................................................................. 8
Resistor Sharing Network Among Multiple Devices ..................................................................... 9
Current Sensing in Low Duty, High Frequency Applications ......................................................... 10
List of Tables
1
TPSxHxxx Current Sense Accuracy Summary .......................................................................... 4
2
TPSxHBxx Current Sense Accuracy Summary .......................................................................... 5
3
TPS2HB08-Q1 Current Sense Ratio Variance
4
TPS2HB08 SNS Mux Outputs ............................................................................................. 6
5
LED Bank Load Circuit Parameters ....................................................................................... 8
6
Calculated Sense Voltage from LED Bank Loads ....................................................................... 9
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1
Introduction
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Trademarks
All trademarks are the property of their respective owners.
1
Introduction
High-side switches from Texas Instruments are robust devices that can be used in industrial, automotive,
and other power applications. They are rated at 40 V and have a wide range of on-resistance and multichannel configurations to meet the user's needs. The high accuracy current sense enables full diagnostics
including current limiting and various other protective features.
High accuracy current sensing allows for immediate feedback to the user on the status of the system. As a
diagnostic tool, it allows the user to monitor the current flowing into the load while maintaining the
efficiency of the system, especially in large load applications by providing a low power solution to monitor
high load currents. The current sense output can also be used in an analog circuit or can be parsed with
the help of an ADC and microcontroller to implement digital logic.
2
Function Description
2.1
TPSxHxxx Current Sense Circuit Description
A current mirror is a circuit that is specially designed to copy the current flowing through an active device.
Using two MOSFETs, the primary FET current is mirrored in the secondary FET. The primary current and
the copied current are related to each other by a fixed current gain ratio. This ratio is dependent on
physical characteristics of the device and cannot be changed by the user.
Figure 1 describes the construction of the current sense circuitry.
Figure 1. TPSxHxxx Current Sense Circuit
2
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Function Description
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A current mirror is integrated in TPSxHxxx devices using a MOSFET-based topology. This circuit is shared
between multiple channels and in this situation, the select pins (SELx) choose which channel is mirrored
through the sense circuitry. The use of a current mirror topology allows for immediate feedback to be
provided to the user without affecting the operation and the efficiency of the high power system. The
sense circuitry has low current flowing through it. This allows the use of a low power solution to sense
high load currents. The load current and the sense current are related to each other by a fixed gain factor
KCS or KSNS, which is referred to as the "Current Sense Ratio" in the device data sheet.
Accurate current sensing can be used as a diagnostic tool in circuit design. The goal of the current sense
circuit is to translate the load current into a voltage value that can be measured by sourcing a sense
current through an external sense resistor. In the case of a fault, there exists a feedback loop that sources
a steady fault current or pulls up to a fault voltage depending on which device is used. Therefore, the
switch is able to delineate between fault operation and normal operation. Given the best-in-class accuracy
with TI’s high-side switch portfolio, further calibration of the sense circuitry is not needed, saving
expensive production line test costs and complexity.
2.2
Choosing RSNS
An external resistor must be connected to the CS pin or the SNS pin of the high-side switch. Using the
chosen resistor, the user is able to get voltage values in a range that can be parsed depending on the
allowed voltages of the ADC or MCU. This voltage value is directly proportional to the amount of load
current flowing through the switch and can be used to implement an off chip fault controller or feedback
circuitry in the form of a current meter.
A resistance value has to be chosen so that the entire load current range can be sensed by the MCU. A
larger resistor has the potential to have greater accuracy since the voltage range gained is larger but the
designer has to note that the fault condition can cause the sense voltage to reach levels that damage the
MCU. To mitigate this, a clamping circuit with a zener diode or a voltage divider circuit can be used if
necessary.
RSNS = VSNS ÷ ISNS = (VSNS × KSNS) ÷ IOUT
(1)
Equation 1 describes the calculations that are performed to choose RSNS. Here, VSNS is the nominal voltage
value chosen by the user for the ADC. This resistor is placed between the SNS pin or CS pin and system
ground.
To achieve the most accurate current sense value, it is recommended to apply filtering to the SNS output.
There are two methods of filtering:
• Low-Pass RC filter between the SNS pin and the ADC input. Figure 1 illustrates this filter. The typical
range for the capacitor CSNS is 100 pF to 10 nF. The typical value for the protection resistor RPROT is 15
kΩ. The designer must select a CSNS capacitor value based on system requirements. A larger value
provides improved filtering while a smaller value allows for faster transient response.
• The ADC and MCU can also be used for filtering. It is recommended that the ADC collects several
measurements of the SNS output. The median value of this data set must be considered as the most
accurate result. By performing this median calculation, the MCU is able to filter out any noise or outlier
data.
For example, if a 3.3 V ADC is used and the nominal load current of the system is 1 A, Equation 2
describes the calculation for RSNS on the TPS1H100-Q1. A 3 V linear range is chosen to get good current
sense resolution.
RSNS = (3 ×500) ÷ 1 = 1.5 kΩ
(2)
The fault current for the TPS1H100-Q1 is 10 mA. With the chosen resistance, the fault voltage is 15 V.
Thus, a zener clamp of 3.3 V and adequate power rating is used to set the fault voltage at 3.3 V, which is
within specification for the ADC.
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Effect of Load Current on Sense Accuracy
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3
Effect of Load Current on Sense Accuracy
3.1
Sense Accuracy Trend Over Load Current
As load currents are decreased, the accuracy of the current mirror in the sense circuitry decreases. Even
though certain devices are trimmed to different load current ranges, this trend is present in all TPSxHxxx
devices. The accuracy for different load current ranges is documented in the data sheet and tends to be
lower for low IOUT. Figure 2 broadly describes the accuracy loss at lower load currents. The load current
and accuracy range are device specific and are stated on the data sheet of every high-side switch.
Figure 2. Current Sense Accuracy Characteristic
When running the device at lower load currents, concessions need to be made due to the lower sense
accuracy. The sense pin on these devices act as fault indicators when a fault is triggered. The pin sources
a fault current, ISNSFH, which help differentiate between normal device operation and fault operation. Some
issues that can arise due to the loss of accuracy include incorrect ISNSFH sourcing and a current limit
overshoot. As such, it is recommended that the designer chooses the high-side switch that is both capable
of driving the desired IOUT and also meets the current accuracy requirements of the system. Table 1
summarizes the current sense accuracy range of TI high-side switches at various current loads. Detailed
accuracy information is given in the respective data sheets.
Table 1. TPSxHxxx Current Sense Accuracy Summary
DEVICE
TPS1H100-Q1
TPS2H160-Q1
TPS4H160-Q1
TPS2H000-Q1
4
LOAD CURRENT
CURRENT SENSE ACCURACY
IOUT ≥ 5 mA
±80%
IOUT ≥ 50 mA
±7%
IOUT ≥ 1 A
±3%
IOUT ≥ 5 mA
±85%
IOUT ≥ 50 mA
±8%
IOUT ≥ 0.5 A
±3%
IOUT ≥ 5 mA
±65%
IOUT ≥ 50 mA
±8%
IOUT ≥ 0.5 A
±3%
IOUT ≥ 1 mA
±50%
IOUT ≥ 5 mA
±10%
IOUT ≥ 100 mA
±2.5%
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Effect of Load Current on Sense Accuracy
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Table 1. TPSxHxxx Current Sense Accuracy Summary (continued)
DEVICE
LOAD CURRENT
CURRENT SENSE ACCURACY
IOUT ≥ 1 mA
±70%
IOUT ≥ 5 mA
±15%
IOUT ≥ 100 mA
±3%
TPS4H000-Q1
Table 2. TPSxHBxx Current Sense Accuracy Summary
3.2
Sense Ratio Variance
As seen in Section 3.1, the load current flowing through the device has a direct effect on the current sense
accuracy. Texas Instruments specs and trims a KCS value for all high-side switches at a fixed load current.
In addition to this specification, there is also a ISNSI specification that describes the sense current under
various load current values. This can be found under Current Sense Characteristics in the Electrical
Characteristics table of the data sheet. Table 3 describes the sense current variation (ISNSI) for TPS2HB08Q1. The nominal KSNS specified for this device is 5000, which is specified at 1 A of load current.
Table 3. TPS2HB08-Q1 Current Sense Ratio Variance
LOAD CURRENT
SENSE CURRENT
EFFECTIVE KSNS
IOUT = 10 A
ISNSI = 2.020 mA
4950
IOUT = 3 A
ISNSI = 0.606 mA
4950
IOUT = 1 A
ISNSI = 0.200 mA
5000
IOUT = 300 mA
ISNSI = 0.059 mA
5085
IOUT = 100 mA
ISNSI = 0.0185 mA
5405
IOUT = 50 mA
ISNSI = 0.0084 mA
5952
To calculate the sense ratio at a load current that has not been specified in the data sheet, linear
interpolation can be used. A straight line can be drawn between the two closest specified current loads
that bound the application load current. Figure 3 describes the linear interpolation between specified load
currents for TPS2HB08-Q1.
Current Sense Ratio as a Funtion of Load Current
6000
5900
5800
Sense Ratio
5700
5600
5500
5400
5300
5200
5100
5000
4900
0
1
2
3
4
5
6
IOUT (A)
7
8
9
10
CSRa
Figure 3. Current Sense Ratio as a Function of Load Current for TPS2HB08-Q1
For a load current of x = 2 A, a straight line between the data at 1 A and 3 A can be drawn. Using
Equation 3, the current sense ratio can be found.
(y - y1) = ((y2 - y1)/(x2 - x1)) * (x - x1)
(y - 5000) = ((4950 - 5000)/(3 - 1)) * (2 - 1)
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(3)
(4)
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Fault Indication and the SNS Mux
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y = KSNS = 4975
4
(5)
Fault Indication and the SNS Mux
TPSxHBxx high-side switches are a subfamily of TPSxHxxx devices. While they provide similar benefits
and protection, they are built with a different architecture. The main difference between TPSxHBxx
devices and the rest of the TPSxHxxx family is the range of on-resistance offered as well as the
implementation of the external current limiting circuit. TPSxHxxx devices also report a fault on the SNS pin
with an internal pullup that pulls the voltage to VCS(H). This differs from TPSxHBxx devices that output a
fixed current during fault operation.
The SNS pin outputs all the necessary diagnostic information depending on how the SNS Mux is
configured with the SELx pins. The following faults are communicated via the SNS output:
• Switch shutdown, due to:
– Thermal shutdown
– Current limit
• Active current limiting
• Open-load/VOUT shorted-to-battery
Open-load and short-to-battery faults are also indicated when the switch is off and DIAG_EN is high.
There is an integrated comparator that checks the voltage on the out pin against an internal open-load
detection threshold, VOL.
• If VOL > VOUT, then there is no fault.
• If VOL < VOUT, then SNS reports a fault.
The SNS pin only reports a fault on the selected channel with the SNS Mux. Irrespective of the nature of
the fault, it is reported by sourcing a fault current, ISNSFH, from the SNS pin. Table 4 describes the outputs
of the SNS Mux of the TPS2HB08-Q1 high-side switch. Refer to the device-specific data sheet for other
TPSxHBxx devices for their respective SNS Mux outputs. Version B TPSxHxxx devices do not have
dedicated status and diagnostic pins. They report faults through the CS pin in a similar method to
TPSxHBxx devices.
Table 4. TPS2HB08 SNS Mux Outputs
INPUTS
OUTPUTS
DIA_EN
SEL1
SEL2
FAULT DETECT
SNS
0
X
X
X
High-Z
1
0
0
0
CH1 Current
1
0
1
0
CH2 Current
1
1
0
0
Device Temperature
1
1
1
0
N/A
1
0
0
1
ISNSFH
1
0
1
1
ISNSFH
1
1
0
1
Device Temperature
1
1
1
1
N/A
Figure 4 and Figure 5 describes the fault reporting of the TPS1H100-Q1 and the TPS2HB50, respectively.
These graphs describe the fault current loop that the device uses to report a fault. It is recommended to
view the device data sheet to confirm the range of the parameters. Considerations such as zener diode
clamps must be used if these voltages reach values that are too high for any external circuitry or
microcontroller.
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Application Example
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Figure 4. TPS1H100 Fault Reporting
5
Application Example
5.1
LED Driver Accuracy
Figure 5. TPS2HB50 Fault Reporting
Modern lighting solutions are trending toward more LED use. These low power devices have similar
luminosity to that of conventional lighting solutions but consume significantly less power. Industrial and
automotive sectors are also moving toward replacing incandescent and halogen lamps with LEDs. Since
LEDs require very low currents to function, current sense accuracy becomes paramount in being able to
accurately sense and protect from fault and open load events.
The high side driver topology is popular in the market since it has short to ground and can drive parallel
loads. Hence, Texas Instruments offers high-side switches that combine these benefits with high accuracy
current sense in devices that have a range of on-resistance and output channel configurations. This
versatility allows the designer to choose the switch that best fits for the application.
A common application for high-side switches in the automotive and industrial sector is to drive a bank of
LEDs connected in parallel. The high current sense accuracy allows the system to recognize a fault of one
LED in a large bank, hence, the system can recognize small load current changes. Devices such as the
TPS4H160 and TPS4H000 can be used to drive LED banks. Figure 6 shows an example using the
TPS4H160.
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Application Example
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Figure 6. TPS4H160 LED Bank Load Circuit Diagram
The accuracy of the current sense feature is dependent on the current pulled by the load and is bounded
by the value stated in the data sheet. This bound describes the largest current sense inaccuracy of the
switch. ICS then passes through a chosen resistor, RCS, and VCS is parsed through an ADC. Table 5
describes the system constants.
Table 5. LED Bank Load Circuit Parameters
INPUTS
VALUE
ADCsupply
5V
RCS
2000 Ω
Rvariance
3%
KCS
300
ADCvariance
2%
ADCleakage
3.5 uA
ADCLSB, tolerance
4 LSB
Inominal, 1 LED
60 mA
Using this bound, the load current sense can be calculated by Equation 6.
IOUT = Inominal × (1 - KCS, tolerance ÷ 100)
(6)
The voltage present on the CS pin is also dependant on the accuracy of the resistor used. When this
voltage is parsed by the ADC, the value read is further affected by its inaccuracy range and the leakage
current of the ADC. Equation 7 describes the calculation of the voltage read by the ADC.
VCS, read = (IOUT ÷ (KCS × (RCS × (1 - Rvariance ÷ 100)))) - (ADCleakage ÷ (1000 × (RCS × (1 - Rvariance ÷ 100))))
(7)
The voltage difference between normal operation and fault operation needs to be large enough for the
ADC to recognize. The difference must result in an LSB drop greater than the LSB tolerance of the ADC.
The resolution of the ADC is given by Equation 8.
ADCresolution = (ADCsupply × (1+(ADCvariance ÷ 100))) ÷ (210 - 1) × 1000 = 4.99 mV/step
(8)
Table 6 summarizes these calculation for various LED loads in parallel.
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Application Example
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Table 6. Calculated Sense Voltage from LED Bank Loads
KCS,
TOLERANCE
8%
4%
LED LOAD
INOMINAL
IREAL
1 LED
60 mA
55.2 mA
VCS,
350 mV
2 LEDs
120 mA
110.4 mA
707 mV
5 LEDs
300 mA
288 mA
1.856 V
6 LEDs
360 mA
345.6 mA
2.228 V
READ
ΔVCS,
READ
LSB DROP
357 mV
71 LSBs
372 mV
74 LSBs
Hence, high side switches from Texas Instruments have the current sense accuracy to distinguish small
changes in the load current flowing through the circuit and this change can be correctly reported by a
standard ADC.
5.2
Sense Resistor Sharing
In an effort to reduce BOM costs, a microcontroller can be used to run more than one load. This reduces
the number of passive components required in the system, thereby reducing package space as well. The
microcontroller may not have enough ADC terminals to support a current sense report on every load.
Multiple high side switches can use the same sense resistor as shown in Figure 7.
Figure 7. Resistor Sharing Network Among Multiple Devices
Certain considerations need to be taken if sense resistor sharing is used. Different load currents per
channel result in different ADC ranges during current sense. Larger load currents on one channel can
result in voltages that are unsafe for the ADC terminal. It is recommended that the sense resistor be
chosen for the largest expected load current in the shared network.
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Application Example
5.3
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Low Duty, High Frequency Switching
Some high power applications can require high frequency switching of the high side switch with a low duty
cycle PWM signal. The benefit of using a PWM input allows high power loads to dissipate energy and
provides precise control of the circuit. These applications require a fast settling of the SNS output to be
read accurately by an ADC. All TPSxHxxx devices have the sense circuitry settling time defined in their
respective data sheets.
Figure 8 describes a 250 Hz, 5% duty cycle PWM. This signal results in an on-time of only 200 µs. The
ADC can sample the SNS output after the defined settling time, tsettle.
Figure 8. Current Sensing in Low Duty, High Frequency Applications
6
Summary
Texas Instruments offers high-side switches with accurate current sensing abilities that can be used as a
protective and diagnostic feature. These devices allow the user to customize the voltage output with the
help of an external sense resistor.
Current sensing is useful for designers in power applications since it can provide immediate feedback to
the designer about the workings of the circuit. As such, this feature benefits designers by eliminating the
need of an external sense circuitry by integrating it into the switch. The current sense circuitry is pretrimmed and calibrated to a high accuracy, allowing easy integration into user designs.
7
References
•
10
Texas Instruments, Adjustable Current Limit of Smart Power Switches Application Report
High Accuracy Current Sense of Smart High Side Switches
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