Modern Power Switches Zero In on Accurate Current Sensing

Modern Power Switches Zero In on Accurate Current Sensing



STMicroelectronics recently introduced a new generation of automotive intelligent switches that use an internal A/D converter to provide digital information about the load current. In this article, we’ll take a look at the necessity of having power switches with accurate current sensing and see how modern switches implement this functionality. 

 

New high-side drivers

ST says the new high-side switches are the first on the market to include digital current sensing as part of its all-digital on-chip diagnostic features. Image used courtesy of STMicroelectronics
 

Power Switches Require Protection Functions

High-side switches are commonly used in a wide variety of automotive and industrial applications. When delivering power to a load, it is important to implement the required set of diagnostic and failure-prevention mechanisms.

For example, one primary concern is a short circuit from the switch output to the system ground. If the wire harness that distributes the power accidentally contacts the car chassis, the switch should shut off the output power immediately before damage is caused to the switch, wire harness, and the power supply.   

To ensure robust protection against such fault conditions, modern intelligent power switches include several different diagnostic and failure detection functions such as over-voltage, under-voltage, over-temperature, over-load, and short protection.

 

Accurate Current Sensing

In some applications, we need to accurately measure the load current. For instance, in an automotive seat heater, the generated heat is determined by the current that flows through the switch. In order to precisely control the heat, a closed-loop system must monitor the load current and make the required adjustments. High precision current sensing also enables torque control for motor control applications.

Accurate current sensing allows us to detect open load conditions when driving a bank of LEDs connected in parallel. A sufficiently accurate current sensing mechanism enables detecting the fault condition of one LED being open in a large bank of LEDs.

 

Analog Current Sensing

Most intelligent power switches with current sensing capability provide an analog signal proportional to the load current. The following figure shows the analog current sensing mechanism of the TPS1H100-Q1 from Texas Instruments.

 

A TPSxHxxx current sense circuit

A TPSxHxxx current sense circuit. Image used courtesy of Texas Instruments
 

In this case, a current mirror topology is used to create a current proportional to the output current, IOUT, in the “Sense MOSFET” branch. This copied current is delivered to the sense resistor, RSNS, through the SNS pin. Therefore, the voltage across the sense resistor is proportional to the load current.

The RC low-pass filter between the SNS pin and the ADC input removes the unwanted high-frequency components of the sense resistor voltage. The filtered output is then delivered to the A/D converter of the MCU that controls the power switch. The ratio of the output current to the sense current, ISNS, is given in the device datasheet and hence, the MCU can easily determine the load current.   

 

Digital Current Sensing

STMicroelectronics has recently introduced the VN9D30Q100F and VN9D5D20FN high-side drivers that can output the digital value of the load current. The functional block diagram of the VN9D30Q100F is shown below.

VN9D30Q100F

Block diagram of the VN9D30Q100F. Image used courtesy of STMicroelectronics
 

As you can see, the device uses an internal ADC to digitize the output current value. The incorporated ADC is a 10-bit SAR A/D converter. Since the VN9D30Q100F has six different channels, an analog multiplexer is used to connect the channel being monitored to the amplifier and ADC block.

An SPI interface is used to send the digitized information to the MCU. Integrating the ADC into the switch package enables resource-saving on the MCU side and reduces the number of external components. In this way, the new device can reduce the MCU workload as well as I/O usage.

STMicroelectronics claims that the new devices are the first intelligent power switches on the market that offer digital current sensing. As shown in the above block diagram, the ADC can also digitize the temperature information read by the internal sensor.

 

Current Sense Accuracy

It should be noted that at lower load currents, the accuracy of the current sense circuitry can reduce significantly for both analog and digital current sensing methods. For example, with the TPS1H100-Q1 from TI, the accuracy of current sensing for [I_{OUT} geq 1A] is ±3%; however, for an IOUT of about 5 mA, the reported accuracy is ±80%. 

The following figure shows the general trend of losing accuracy at lower output currents.

 

Current sense accuracy

Current sense accuracy. Image used courtesy of Texas Instruments

 

A similar trend is also observed with the digital current sensing mechanism employed in the recently announced ST products. The table below shows how the accuracy of the VN9D30Q100F is reduced at lower output currents. 

 

Digital current sense

Digital current sense. Image used courtesy of STMicroelectronics

 

Other Features

The VN9D30Q100F and VN9D5D20FN are designed for applications powered by a 12 V battery. Using ST’s latest-generation of VIPower* M0-9 technology, the new devices combine an efficient 40 V trench vertical MOSFET with 3.3 V digital logic and high-precision analog circuitry in a 6 mm x 6 mm QFN package.

The VN9D30Q100F has two 33 mΩ and four 90 mΩ channels. The VN9D5D20FN has two 7.6 mΩ and two 20 mΩ channels. The devices also incorporate a PWM generator with a resolution of 0.1 percent that facilitates handling functions such as lamp dimming. 



For lighting, electrical, signage, and technology solutions that allow you to do more call Sverige Energy today at +4(670) 4122522.

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