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Theory Of Operation 63
CV Amplifier
The CV amplifier controls the output of the power module when the module is operating in constant voltage mode. The CV
amplifier consists of two stages: an error amplifier stage and a voltage monitor stage. The calibrated CVPROG* signal from
the CV DAC pulls current out of the negative input of the error amplifier at a rate determined by the DAC’s programmed
value. The error amplifier in turn, controls the output of the module.
The output of the module is monitored by the voltage monitor stage. It reduces the module’s output voltage so that the 0 to
full-scale output voltage of the module is represented by a range of 0 to 2 volts at the output of the voltage monitor
amplifier. The voltage monitor amplifier’s output (VMON) is fed through a resistor to develop a current. This current is fed
into the negative input of the error amplifier stage at the same time that current is being pulled out of this input by the
CVPROG* signal from the CV DAC.
When the current pulled out of the error amplifier input by the CV DAC exceeds the current fed into the input by the
voltage monitor, the error amplifier turns on, which turns the output of the module on. When the current pulled out of the
error amplifier input by the CV DAC is less than the current fed into the input by the voltage monitor, the error amplifier
turns off, which turns off the output of the module. When the current pulled out of the error amplifier input by the CV DAC
is equal to the current fed into the input by the voltage monitor, the net current into the error amplifier is zero. At this point,
there is no change on the output voltage of the power module because the output voltage equals the programmed voltage
value.
CC Amplifier
The CC amplifier controls the output of the power module when the module is operating in constant current mode. The CC
amplifier also consists of two stages: an error amplifier stage, and a current monitor stage. The calibrated CCPROG* signal
from the CC DAC pulls current out of the negative input of the error amplifier at a rate determined by the DAC’s
programmed value. The error amplifier in turn, controls the output of the module.
The current monitor stage monitors the current at the output of the module by sensing the voltage drop across current
monitor resistor (RM). The current monitor amplifies this voltage (which is typically in the range of 0 to 50mV) so that the
0 to full-scale current output of the module is represented by a range of 0 to 2 volts at the output of the current monitor
amplifier. The current monitor amplifier’s output (IMON) is fed through a resistor to develop a current. This current is fed
into the negative input of the error amplifier stage at the same time that current is being pulled out of this input by the
CCPROG* signal from the CC DAC.
When the current pulled out of the error amplifier input by the CC DAC exceeds the current fed into the input by the current
monitor, the error amplifier turns on, which turns on the output of the module. When the current pulled out of the error
amplifier input by the CC DAC is less than the current fed into the input by the current monitor, the error amplifier turns off,
which turns off the output of the module. When the current pulled out of the error amplifier input by the CC DAC is equal to
the current fed into the input by the current monitor, the net current into the error amplifier is zero. At this point, there is no
change on the output current of the power module because the output current equals the programmed current value.
OR Gates
The outputs of the CV and CC amplifiers are "or"ed through two diodes, producing a single pulse-width modulator control
signal (VCNTRL). Whichever of the two signals (CC or CV) is the most negative, with respect to the output of the module,
controls the pulse-width modulator. The pulse-width modulator in turn controls the FETS. The more negative that the
controlling signal is with respect to the output, the harder it causes the pulse-width modulators to turn on the FETs.