1 AMC7908 in PA Biasing

Figure 1-1 highlights the typical application of the AMC7908.

The AMC7908 has eight DAC outputs and four OUT outputs. The DAC are grouped in two blocks, each with a separate supply voltage. Each DAC group can operate in a range of 0V to 10V or–10V to 0V, enabling one device to power both LDMOS and GaN PA. The DAC buffers automatically detect the VCC and VSS voltages and set the DAC output range accordingly. At start-up, all DAC are clamped to VSS and the OUT switch is connected to VSS. Each DAC output has a selectable source/sink current maximums: 15mA, 30mA, 90mA, and 120mA

The OUT pins are low on-resistance switched outputs with three connection options: the neighboring even numbered DAC, the neighboring odd numbered DAC, and the VSS supply. While the OUT pins are intended for fast switching the PA gate using the neighboring DAC, this is not required. These switched outputs can also switch from a DAC voltage to VSS, allowing the unused DAC channels to be used for gate biasing that does not require fast switching. The switch between the OUT pin and the DAC pins have a typical resistance of 10Ω, and the switch between OUT and VSS have a typical resistance of 15Ω.

The integrated 16-bit ADC has six inputs: two high-voltage inputs (ADCHV) and two sets of high-side current shunt monitoring inputs (SENSE). The ADCHV channels support up to 85V to monitor the V_DRAIN of GaN and GaA PA. The SENSE pins are used to monitor V_DRAIN current and require an external shunt resistor. In addition, the AMC7908 has an on-board temperature sensor. The temperature sensor can be used to monitor the temperature of the PA if the AMC7908 is placed close to the PA. The high-voltage inputs, current sense inputs, and temperature sensor have configurable high and low alarm limits. If the ADC detects an alarm condition, the DAC and OUT outputs clamp to VSS to protect the PA.