How to design cost-sensitive battery-monitoring circuits

In portable electronics designs, typical battery-monitoring systems measure battery voltage and battery current to detect when the battery needs charging or replacement. In this post, I’ll demonstrate battery-voltage and current-monitoring circuitry for cost-optimized systems using operational amplifiers (op amps).

Op amps used in battery-monitoring circuitry must meet the required accuracy levels while consuming minimal power-supply quiescent current, i_Q­, to conserve battery life. Table 1 lists the key specifications for two new op amps, the TLVx369 and TLVx379 families, which are designed for low-power, cost-sensitive applications.

Table 1: Key specifications for TLV369 and TLV379 op amps

In figure 1, you will see an example battery-voltage measurement circuit using the TLV379 configured as a unity-gain buffer.  To prevent violations of the amplifier’s common-mode input voltage range or output voltage swing, the battery is divided down using R1 and R2. In this case, a 1.8V-5.5V battery voltage will create a 0.393V-1.2V output voltage, which fits within the common 0V-1.2V range for analog-to-digital converters (ADCs) on many low-power microcontrollers.

Equation 1 shows the transfer function for the circuit in Figure 1. Be sure to use high-value resistors for the divider to minimize current consumption. You can place a low-pass filter at the output of the circuit to limit the signal bandwidth and output noise. However, like most low-power op amps, the TLV379 does not perform well while driving capacitive loads, so check the stability of the circuit when designing output filters with capacitances to GND.

Figure 1: Battery-voltage measurement circuit using the TLV379

The main error contributors in the circuit shown in Figure 1 are the tolerance of the resistors in the divider and the offset voltage of the op amp. Other error sources come from the op-amp’s CMRR and the input bias current flowing through the voltage-divider resistors.

Table 2 uses the typical specifications for the TLV379 listed in Table 1 to calculate expected circuit performance.  The resistor divider tolerance, R_TOL, is set to 0.1%.

Table 2: Error calculations for the TLV379 voltage-measurement circuit shown in Figure 1

Measuring the voltage drop across a low-side current-shunt resistor is often the simplest method to determine battery/load current. Figure 2 shows an example low-side current-sensing circuit using the TLV379. The circuit in Figure 2 was designed to create a 0V-1.2V output voltage for a 0A-1A load current, i_LOAD. Equations 2 and 3 calculate the input voltage, V_IN­, and output voltage, V_OUT, respectively; you can use these equations to adjust the circuit for other ranges.

In Figure 2, the shunt voltage, V_SHUNT, was limited to 100mV with the maximum 1A load current; you could use other V_SHUNT values depending on what the load can tolerate. Be sure to make a good Kelvin (or four-wire) connection across the shunt resistor, R_S, to reduce the effects of printed circuit board (PCB) impedances.

Figure 2: Low-side battery current-measurement circuit using the TLV379

Table 3 lists the error calculations for the circuit in Figure 2. Shunt-resistor values with <1Ω tolerances are typically 1%, which matches well with the error from the TLV379 V_OS.  The tolerance of the gain-setting resistors, R1 and R2 is set to 0.1%. 

Table 3: Error calculations for the TLV379 current-measurement circuit shown in Figure 2

The voltage drop across the shunt resistor in a low-side current-measurement circuit affects the voltage potential of the load GND, which is undesirable in some applications. Figure 3 shows a high-side battery-current-sensing circuit using the TLV369, which features a rail-to-rail input stage with >100dB of CMRR over the full supply voltage range. The circuit was designed to keep the output voltage below 1.2V for a 1A load current and 5.5V battery voltage.

Equations 4, 5 and 6 show simplified transfer functions for the circuit. The bias voltage, V_BIAS, created from the resistor divider pushes the output voltage away from the negative rail, allowing for current measurements down to 0A. You must combine this circuit with the battery-voltage measurement to obtain the value of V_BIAS. While I didn’t include them in this post, the error calculations for the circuit in Figure 3 are similar to those shown in Table 3.

Figure 3: High-side battery-current measurement circuit using the TLV369

Battery-powered electronics almost always include battery-voltage and current-monitoring circuitry for gauging and protection purposes. In this post, I provided circuit examples for a voltage-measurement circuit and both low- and high-side current-sensing circuits using some of our newest op amps, which provide an excellent price-to-performance ratio for cost-sensitive systems.

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Additional resources

• Read my colleague, Pete Semig’s blog, “Resistor Divider Drift:  5ppm + 5ppm = 5ppm” which I used to determine the gain error from the resistor divider in circuit 1.
• See TI’s portfolio of performance op amps for cost-conscious applications.
• Watch more than 40 on-demand precision amplifier training videos in our TI Precision Labs – Op Amps series.
• Find commonly used analog design formulas in the Analog Engineer’s Pocket Reference e-book.