High-side current-sense measurement: Circuits and principles - AN746

This application note describes the use of current sense amplifiers, differential amplifiers and instrumentation amplifiers to measure battery charge and discharge currents in portable equipment, computer notebooks and USB accessories. It compares high side current sense amplifiers with low side differential amplifiers and recommends selection criteria for current sense resistors. A high voltage circuit breaker circuit is described to provide system over current protection due to faults and short circuits. Application circuits for a variable linear current source and a programmable 0-5A current source is included. Current measurement (monitoring current flow into and out of electronic circuits) is an essential skill in the designer's repertoire and necessary in a wide range of applications. Examples include overcurrent-protection and supervising devices, 4-20mA systems, programmable current sources, linear and switch-mode power supplies, battery chargers, and battery-operated circuits for which you must know the ratio of current flow into and out of a rechargeable battery (that is, the gauge function). As more applications become portable, the demand increases for dedicated current monitors that accomplish their task in a small package and with low quiescent current. The following discussion covers low-side and high-side current monitors and includes their architectures and applications. Highor low-side monitor? Most current-measurement applications employ either the low-side principle, in which the sense resistor connects in series with the ground path (Figure 1), or the high-side principle, in which it connects in series with the hot wire (Figure 2). These two approaches pose a trade-off in different areas: The low-side resistor adds undesirable extraneous resistance in the ground path, and circuitry associated with the high-side resistor must cope with relatively large common-mode signals. Moreover, if the op amp in Figure 1 has its GND pin referred to the positive side of RSENSE, then its common-mode input range must extend below zero, that is, to GND -(RSENSE x ILOAD). Figure 1. Principle of the low-side current monitor.