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Power Boost Circuit with Current Sense and Kelvin Connection

Angel Caballero [angel.caballero@analog.com] and Greg DiSanto

In automatic test equipment (ATE), low-current adjustable voltage sources and high-current fixed voltage sources are available. An additional supply must be created when a high-current adjustable voltage source is required.

This article shows how an AD8397 high-current rail-to-rail op amp boosts the current of an adjustable voltage source to up to ±750 mA. The buffered voltage can be used as a power supply or a reference. A Kelvin connection eliminates resistive losses. This technique provides an accurate voltage and allows the current to be measured with a sense resistor.

Figure 1. The AD8397 as a Power Supply Boost Circuit

Figure 1 shows the circuit that generates the power supply for the device under test (DUT). The AD8397, configured for a closed-loop gain of one, buffers the supply voltage and delivers power to the DUT. Negative feedback and the open-loop gain of the amplifier force the inverting input to be at the same voltage as the non-inverting input. If the buffered voltage is much greater than the offset voltage of the op amp, any errors become negligible. The DUT load current is provided by the AD8397.

The current-sense resistor, R₂, converts the current to a voltage that can be easily measured with an instrumentation amplifier. This sensing technique allows one voltage to be buffered multiple times using separate boost circuits, with each current measured individually. The value of R₂ does not affect the amplifier’s dynamic behavior, but it can limit its headroom. Figure 2 shows how the output voltage changes as it drives more current. In this example, the supply voltage is 15 V, R₂ is 10 Ω, and the desired DUT supply voltages are 6 V and 9 V. The graph shows that the circuit saturates at around 650 mA for the 6-V case, and at around 500 mA for the 9-V case.

Figure 2: DUT Supply Voltage and AD8397 Output Voltage vs. Output Current

Keeping resistor R₂ small increases the amplifier’s headroom, but a larger resistance helps protect the circuit from accidental current overdrives that could damage the buffer. As the current increases, the amplifier’s output voltage goes up until the output saturates or the amplifier becomes damaged. The larger the resistor, the sooner the output saturates, keeping the power dissipation to a manageable level. The amplifier’s power dissipation during normal operation must also be considered, as the AD8397 can only drive 750 mA for short periods of time without being damaged.

The buffered DUT supply voltage is decoupled with capacitor C₂. In conjunction with resistor R₂, this capacitor forms a feedback pole that can lead to instability. To fix this problem, the closed-loop gain of the system could be increased, increasing the phase margin and stabilizing the loop, but the magnitude of the voltage that could be buffered would be limited by the output swing.

The network comprising R₁ and C₁ increases the closed-loop gain at high frequencies, while maintaining unity gain at low frequencies. The ratio of R₂ to R₁ defines the high frequency closed-loop gain of the system. The larger the gain, the more stable the system. Capacitor C₁ and resistor R₁ set the frequency where the transition from unity gain to non-unity gain occurs. This corner is set to at least one decade below the crossover frequency of the amplifier to maintain stability¹. With the values depicted in Figure 1, this boost circuit can drive up to 10 nF loads while maintaining unity-gain stability.

A Kelvin connection can be made by routing the inverting input (sense) and the AD8397 output (force) into the DUT separately, as shown in Figure 1. The output of the amplifier is forced to a voltage that compensates for the resistive loss due to the high current flow in the feedback path. Very little current flows through the sense line, so the inverting input tracks the non-inverting input. This technique keeps the voltage on the DUT supply pin at the desired value.

The AD8397 can source and sink current, so it can also be used to generate the negative power supply to a DUT.

Figure 3: The AD8397 as a Mid-Supply Reference

The AD8397 can also act as a mid-supply reference voltage for a single-supply DUT, as shown in Figure 3. Here, the AD8397 buffers half the supply voltage, as determined by the resistor divider. The amplifier can source and sink current while keeping the mid-supply voltage constant. It is essential to operate the AD8397 off of a single supply in order to obtain bidirectional current. To decouple the mid-supply voltage, the same compensation technique as previously discussed is necessary. A Kelvin connection and/or current-sense resistor can also be implemented.

Reference

¹Buxton, Joe, AN-257, “Careful Design Tames High Speed Op Amps.”

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