Power boost circuit with current sensing and Kelvin connection

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

Figure 1. AD8397 used as a power boost circuit.

Figure 1 shows the circuit for providing power to the device under test (DUT). The AD8397 is used to buffer the supply voltage and provide power to the DUT in a closed-loop gain configuration of 1. The negative feedback and open-loop gain of the amplifier make the voltage at the inverting input equal to the voltage at the noninverting input. If the buffered voltage is much higher than the offset voltage of the op amp, the error is negligible. The DUT load current is provided by the AD8397.

The current sense resistor R2 converts this current into a voltage that can be easily measured using an instrumentation amplifier. This sensing technique allows a voltage to be buffered multiple times using different boost circuits and each current to be measured individually. The value of R2 does not affect the dynamic characteristics of the amplifier, but it does limit its headroom. Figure 2 shows how the output voltage changes as the current driven by the circuit increases. In this example, the supply voltage is 15 V, R2 is 10 Ω, and the supply voltages required by the DUT are 6 V and 9 V. This figure shows that the circuit saturates at about 650 mA for the 6 V case and about 500 mA for the 9 V case.

Figure 2. DUT supply voltage and AD8397 output voltage vs. output current.

A smaller resistor, R2, increases the amplifier's headroom, but a larger resistor helps protect the circuit from accidental current overdrive that damages the buffer. As the current increases, the amplifier's output voltage rises until the output saturates or the amplifier is damaged. The larger the resistor, the faster the output saturates, keeping the power dissipation under control. The power dissipation during normal operation of the amplifier must also be considered; the AD8397 can only drive 750 mA for short periods of time without damage.

The buffered DUT supply voltage is decoupled using capacitor C2. This capacitor, along with resistor R2, forms a feedback pole, which can cause circuit instability. To address this issue, the closed-loop gain of the system can be increased, thereby increasing the phase margin and keeping the loop stable, but the voltage amplitude that can be buffered will be limited by the output swing.

The network formed by R1 and C1 increases the closed-loop gain at high frequencies while maintaining unity gain at low frequencies. The ratio of R2 to R1 determines the high-frequency closed-loop gain of the system. The greater the gain, the more stable the system. Capacitor C and resistor R1 set the frequency at which unity gain changes to non-unity gain. To maintain stability, this corner frequency should be at least 10 times lower than the amplifier's crossover frequency1. Using the values ​​shown in Figure 1, this boost circuit can drive loads up to 10 nF while maintaining unity-gain stability.

Connecting the inverting input (sense) and the AD8397 output (force) to the DUT separately forms a Kelvin connection, as shown in Figure 1. This forces the amplifier output to a certain voltage to compensate for the resistive losses caused by the high current in the feedback path. The current flowing in the sense line is very small, so the inverting input will follow the non-inverting input. This technique can keep the DUT power pins at the desired voltage value.

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

Figure 3. AD8397 used as reference at midsupply voltage.

For a DUT powered from a single supply, the AD8397 can also be used as a midsupply reference, as shown in Figure 3. In this case, the AD8397 buffers half the supply voltage as determined by the resistor divider. The amplifier can source and sink current while keeping the midsupply voltage constant. To achieve bidirectional current flow, the AD8397 must be powered from a single supply. To decouple the midsupply voltage, the compensation technique described above is required. Kelvin connections and/or current sense resistors can also be implemented.