Why Choose High-Precision Voltage Clamping for Low-Voltage Applications?
ALD recently announced a low-power, high-precision clamping solution that targets low-voltage electronic systems. Why do we turn to voltage clamping in such systems in the first place?
Voltage clamp circuits are used to protect electronic systems from unwanted voltage transients on power supply and signal lines. An example application where a high-precision voltage clamp is required is depicted below.
Diagram of a precision ADC design. Image used courtesy of Analog Devices
The analog signal provided by the ADC driver can exceed the ADC’s specified input range because the driver runs from ±15 V while the ADC is operated with a 5 V power supply. An overvoltage condition can cause permanent damage to the ADC or degrade its performance.
As depicted above, ADC inputs commonly have internal protection diodes that conduct when the input voltage exceeds the specified range. However, these diodes cannot carry a large current for an extended period of time. Therefore, some sort of external overvoltage protection is required.
Why is high-precision voltage clamping often the desired solution?
We need to match the analog input to the input range of the ADC. Without a high-precision clamp circuit, the analog signal should be restricted to a conservative level below the ADC reference voltage. This will be a waste of the ADC’s dynamic range and resolution.
Traditional Solutions
A pair of either Zener or Schottky diodes can be used at the driver output to protect the ADC from overvoltage conditions. These external diodes are capable of higher continuous current conduction compared to the ADC internal diodes.
Clamps providing input protection. Image used courtesy of Texas Instruments
Several aspects of the diode performance should be considered to have an efficient clamp circuit. The diodes should have low reverse leakage current not to increase the system power consumption when the clamp circuit is not activated.
Moreover, the diodes should add negligible parasitic capacitance to the signal path. Besides, the added parasitic capacitance should not vary significantly with the applied voltage level. This is important because the non-linear behavior of the parasitic capacitance can degrade the overall harmonic distortion of the system.
Another important parameter is the reverse recovery time of the diodes. With a fast reverse recovery, the diodes can immediately turn off as the analog signal returns to the ADC input range. This allows the system to rapidly restore its normal operation after an overvoltage condition.
There are many other types of high-speed voltage clamps, each one offering a different set of pros and cons. For more information, you can refer to op-amp based-solutions from Maxim Integrated and Analog Devices.
The Challenges With Low-Voltage, Low-Power Clamping
Zener diodes are not suited for low-voltage, low-power clamp circuits for several reasons.
First, Zener diodes draw a considerable amount of current. Precision Zener diodes can draw a leakage current as large as 50 µA (some even draw 20 mA). This cannot provide us with an acceptable protection solution for a low-power circuit that draws only a few hundreds of nanoamperes.
Second, even precision Zener diodes do not offer a precisely-defined threshold voltage. They have only about ±2% precision on their threshold voltage which doesn’t meet the needs of many sensitive circuits commonly found in wireless transmitters, battery management systems, supercapacitors, and energy harvesting applications.
Finally, Zener diodes cannot typically offer very low clamping voltages (for example, as low as 1.6 V).
Low-Voltage, Low-Power Voltage Clamps from Advanced Linear Devices
Advanced Linear Devices (ALD) has recently announced a low-power, high-precision clamping solution, SABMBOVP, that targets low-voltage electronic systems running from 5 V or below.
These modules are based on the company’s proprietary EPAD technology and employ very low-voltage precision enhancement-mode MOSFETs to implement low-power, low-voltage clamping solutions.
The schematic diagram of SABMBOVP2XX is shown below:
Schematic diagram of the SABMBOVP2XX. Image used courtesy of ALD
The module monitors the input voltage and turns on an output transistor to clamp the voltage at a predetermined value. This new solution is said to outperform traditional Zener-based clamp circuits in several different aspects of performance. It has a quiescent current of less than 100 nA and offers a more precise threshold voltage.
The response time is less than 100 ns and the module has a surge current handling capability of greater than 100 mA. The clamping voltages provided by the module are well below that of Zener-based solutions.
It is important to note that the module does not require any additional components such as a resistor divider, buffering circuits, or voltage regulators that are commonly needed when using Zener clamping circuits.
As a result, the new solution can reduce both complexity and power consumption.
