Designing a high-side current monitor circuit is straightforward, provided that the amplifier electrical characteristics are carefully considered so that linear operation is maintained. Other additional characteristics, such as the input voltage range of the analog to digital converter (ADC) that follows the current monitor stage, must also be considered when configuring the system.

Figure 7-4 High Side Current Sense With Gain Stage

For example, consider the design of a TLVx888 high-side current monitor with an output voltage range set to be compatible with the input of an ADC with a full-scale input range of 0V to 5V. Although the TLVx888 is specified as a rail-to-rail output amplifier, the linear output operating range (like all amplifiers) does not quite extend all the way to the supply rails. This linear operating range must be considered.

In this design example, the TLVx888 is powered by ±15V; therefore, the device is easily capable of providing the 0V to 5V swing; or even more, if the ADC has a wider input range.

The best measure of an op-amp linear output voltage range comes from the open-loop voltage gain (A_OL) specification listed in the Electrical Characteristics table. The A_OL test conditions specify a linear swing range from 600mV from each supply rail (R_L = 10kΩ).

A nominal load current (I_L) of 100mA is used in this example. In most applications, however, the ability to monitor current levels far less than 100mA is useful.

Selection of current sense resistor R_S comes down to how much voltage drop can be tolerated at maximum current and the permissible power loss or dissipation. A good compromise for a 100mA sense application is an R_S of 100mΩ. That value results in a power dissipation of 1mW, and a 10mV drop at 100mA.

Next, determine the gain of the TLVx888 difference amplifier circuit. The maximum current of 100mA flowing through a 100mΩ sense resistor results in 100mV across the resistor. The gain of the difference amplifier is limited by the required input common-mode voltage. A gain of 1/2, for example, provides a 1/3 attenuation of the high side voltage seen by the circuit. The attenuation is enough to keep the input common-mode within the range of the TLVx888, (V+) − 1.7V.

The differential voltage that is applied across the TLVx888 difference amplifier circuit inputs is attenuated by the difference amplifier and a gain stage is needed for proper scaling. Conveniently, the second channel of the TLV2888 can used. The ultra-low offset and wide-bandwidth enable very high gain configurations. In this example, a gain of 1000V/V provides the necessary scaling for a 0V to 5V output.

The TLVx888 output voltage is intentionally limited to 5V. However, because of the ±15V supply, the output voltage can be much higher to allow for a higher voltage data converter with a wider dynamic range.

The TLVx888 output, as well as other CMOS output amplifiers, often swing closer to 0V (in single supply configurations) than the linear output parameters suggest. The voltage output swing, V_O (see the Electrical Characteristics table), is not an indication of the linear output range, but rather how close the output can move towards the supply rail. In that region, the amplifier output approaches saturation, and the amplifier ceases to operate linearly. Thus, in the current-monitor application, the current-measurement capability can continue to much less than the 600mV output level. However, keep in mind that the linearity errors are becoming large.

Lastly, some notes about maximizing the high-side current monitor performance:

1. All resistor values are critical for accurate gain results. Match resistor pairs of [R1 and R3] and [R2 and R4] as closely as possible to minimize common-mode mismatch error. Use a 0.1% tolerance, or better. Often, selecting two adjacent resistors on a reel provides close matching compared to random selection. The RES21A provides an even more elegant circuit design with better performance than discrete 0.1% resistors.
2. Keep the closed-loop gain, G, of the difference amplifier set to a reasonable value to reduce gain error and maximize bandwidth. The high bandwidth of TLVx888 enables very high gain configurations.
3. Although current monitoring is often used for monitoring dc supply currents, ac current can also be monitored. Special attention to the −3dB cutoff frequency of the circuit is warranted.

For more information about amplifier-based, high-side current monitors, see the TI Analog Engineer’s Circuit Cookbook: Amplifiers.