AN833 Datasheet, PDF (2/5 Pages) STMicroelectronics – FREQUENCY MODULATION ON L4981B

AN833
Figure 2. Measured V_iload vs. Input Current
Table 1. Current Sense Amplifier Circuit Supply Current vs. Load Current
Load Current
0
50 mA
Current Used by the Current Sense Amplifier
860 nA
48.1 µA
Amplifier input offset voltages need to be considered carefully in this circuit. The TS1001 op amp is specified with
±3 mV maximum input offset voltage at 25 °C, which corresponds to ±3 mA of error. Figure 2 shows an offset of
approximately 2.5 mA. Another consideration is that Q1 exhibits drain-source leakage current of a few 10s of
nanoamps at 25 °C and can approach 1µA over the commercial temperature range. Since the current through Q1
is effectively controlled by the TS1001’s loop, any drain-source leakage from Q1 appears as a current floor and
generates a corresponding minimum voltage output across R3 below which current cannot be measured until the
op amp “takes over” at higher measured currents. Therefore, normal methods of removing the current sense
amplifier input offset voltage by subtracting the zero-load voltage at V_iin will not work, since the offset voltage due
to the op amp’s Vos and the current floor from Q1’s drain-source leakage cannot be separated.
2
Rev. 1.1