SLAS986D November 2014 – February 2018 AMC7836
In applications that require current sensing of the power amplifier, an external high-side current sense amplifier can be added and configured to the unipolar ADC inputs. Figure 117 shows this design.
The LMP8480 device is a precision current sense device that amplifies the small differential voltage developed across a current-sense resistor in the presence of high input common-mode voltages. The LMP8480 device accepts input signals with a common-mode voltage range from 4 V to 76 V with a bandwidth of 270 kHz. The LMP8480 device offers different fixed gain settings. The optimal gain setting is dependent on the accuracy requirement of the application. To maintain precision over temperature, the output of the LMP8480 device should be directly connected to the AMC7836 unipolar ADC inputs. If the output range of the LMP8480 device is scaled by a voltage divider, as shown in Figure 117, an output amplifier may be required to drive the ADC unipolar input to ensure a low impedance source. If the series resistance, in this case R4, is low enough then the buffer may not be required because the LMP8480 device is capable of driving the input of the AMC7836 unipolar ADC channel.
The external resistors will cause some small error because of temperature drift and the input bias current of the operation amplifier.
Figure 117 also shows a simple method to ensure proper power sequencing of the power amplifier by adding a series PMOS transistor to the PA drain terminal. The activation of the PMOS transistor connects the PAVDD voltage supply to the drain pin of the power amplifier. The PMOS transistor is driven with a voltage divider that swings from the PAVDD voltage to PAVDD × (R2 / (R1 + R2)). The NMOS shown in Figure 117 is connected to a microcontroller output that controls the state of the PMOS transistor.