Smu Characterization Of Linear Voltage Regulators
Linear voltage regulators (LVRs) are essential in power management systems to supply the constant voltage needed by a DC electrical circuit
. When properly designed and selected, an LVR continuously maintains its specified output voltage within design limits for variations in load current and input voltage.
The two main types of LVRs, conventional and low dropout (LDO), function on the same principle. However, an LDO type requires a lower input voltage relative to output voltage, compared to a conventional type, and still maintains the specified output. This means that LDO LVRs consume less power than conventional types for the same operating conditions. As a result, low dropout regulators are better suited for battery-powered electronics and portable handheld communication devices.
LVR Test Instrumentation. To ensure you have the appropriate LVR for your application, it is best to do some testing that characterizes its line regulation, load regulation, dropout voltage and quiescent current. These parameters are applicable when qualifying either a conventional or LDO LVR.
LVR device characterization requires a variable power source for the input side and a variable load for the output side. Source Measurement Units (SMUs) are excellent instrumentation candidates for these applications because they can act as either a source or a load, and make precise voltage and current measurements needed on both sides of the regulator. One SMU can act as a DC power source on the input side; a second SMU on the output side can act as the load. A growing number of test equipment vendors have begun offering system-level SMUs that house multiple SMU channels in a single enclosure. For many applications a dual-channel SMU like Keithleys Model 2602A System SourceMeter instrument can serve as an economical substitute for two separate instruments.
Characterization System Setup. Used in the manner just described, the input side of the regulator is connected by test leads to the SOURCE terminals of SMU_1, which is configured to source voltage and measure current (SVMI). As described in more detail later, the voltages sourced will cover the appropriate range for line regulation testing. The compliance, or current limit, for SMU_1 is set to a value higher than the maximum output current of the voltage regulator in order to account for the LVRs current consumption.
SMU_2 is connected to the output side of the regulator, and is also configured for SVMI. Both the SOURCE and SENSE terminals are connected to the regulator output. All the test leads should be as short as possible. In addition, the SENSE test cable should have both low resistance and low inductance. Typically, this requires 10AWG size leads to avoid voltage drop errors in the load voltage measurements.
The voltage setting on SMU_2 is adjusted to a fixed value lower than the expected output voltage of the regulator. It will automatically switch to sinking, i.e., drawing current from the regulator, and thereby acts as a load. Its compliance current is set to the desired load current. Given that an SMU operates on the principles of ranging, its important that SMU_2s voltage range encompasses the expected output voltage of the regulator, which ensures the regulator output voltage is measured correctly.
LVR testing may require external capacitors to ensure stable operation of the regulator. These are usually bypass capacitors to ground, added across the input and output terminals of the regulator. On the other hand, large capacitance values at these terminals can cause the SMUs to be unstable at low currents. To avoid this problem, some SMUs offer advanced modes of operation to improve stability in cases of large capacitive loading. An alternative is to add a series resistor, or back-to-back parallel diodes, between the SOURCE HI terminals of the SMUs and the voltage regulator, which will also improve stability.
The use of heat sinking is another important consideration when characterizing LVRs. An LVRs parameters are temperature sensitive and overheating a packaged regulator will produce unintended damage. Therefore, heat must be removed from the device appropriately, taking into account power dissipation and the ambient temperature at the intended operating conditions.
Parameters Measured. SMUs are well suited for characterizing the LVR parameters of greatest interest to a design engineer. As mentioned earlier, these parameters include line regulation, load regulation, dropout voltage, and quiescent current.
Line regulation. This parameter is a measurement of an LVRs ability to maintain the specified output voltage as the input voltage changes under a constant load condition. Typically, the output voltage is expected to vary less than 100mV for specified line voltage variations. During testing, power dissipation should be carefully monitored to reduce the effect of temperature change on the regulator.
In a line regulation test, SMU_1 is configured for an output voltage sweep that stays within the specified input voltage range of the regulator. SMU_2 is configured to source a fixed voltage less than the expected output voltage of the regulator, allowing SMU_2 to act as a load (i.e., sink current). Its compliance current is set to the desired constant load condition. Depending on the size of the bypass capacitor on the input side of the regulator, a stepped sweep may be required, with sufficient time between steps to allow for capacitor charging.
Load regulation. This test characterizes the LVRs ability to maintain the specified output voltage as the load current varies under a constant input voltage. Again, the output voltage is typically expected to vary less than 100mV. As before, its important to remove heat from the device so voltage measurements are taken under a constant temperature condition.
In the load regulation test, SMU_1 is configured to a fixed input voltage. SMU_2 is configured to source a fixed voltage less than the expected output voltage of the regulator, allowing SMU_2 to operate in current sink mode. Its compliance current is varied to reflect the desired load current levels.
Dropout voltage. An LVRs dropout voltage indicates the minimum input voltage in excess of the output voltage necessary to achieve the specified output voltage. The input voltage must always be higher than the output voltage of an LVR. LDO regulators operate at a smaller input-to-output voltage differential than conventional LVRs. This characteristic is important in battery-powered applications, where current efficiency and low power consumption are critical.
In the dropout voltage test, SMU_1 is set up to sweep the input voltage of the regulator. SMU_2 is configured to source a fixed voltage less than the expected output voltage of the regulator in order to draw current. The compliance current is set to the desired constant load condition. Voltage measurements are collected on both the input and output sides of the regulator during the sweep to reveal its characteristic dropout voltage. When LDO and conventional regulators are tested under identical conditions, it will be found that the LDO regulators output voltage drops out at a significantly lower voltage than that of a conventional regulator.
Quiescent current (IDDQ). This is the difference between the input current and the output current of a voltage regulator. It is the current used to operate the regulator, but not delivered to the load. Quiescent current is normally specified at no-load or very low load conditions. Much as with dropout voltage, maintaining control over this parameter is critical in battery-powered applications.
The quiescent current test setup involves setting SMU_1 to sweep the input voltage of the regulator. SMU_2 is configured to source a fixed voltage less than the expected output voltage of the regulator to operate in sink mode. Its compliance current is set to the desired constant load condition. The differences in current measurements taken on the input and output sides at different load conditions will indicate the LVRs quiescent current characteristics. Typically, values of IDDQ are plotted against Vin values.
Conclusion. SMUs offer a wide variety of advantages for LVR device characterization, including their flexibility in sourcing either voltage or current while measuring both voltage and current. They can operate as a precision readback power supply or as a variable load with accurate measurement capabilities. They are good all-around workhorse additions to many researchers and engineers toolboxes.
by: Jennifer Cheney
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