LA Series Linear Servo Amplifier Technical Reference Manual
7
1 Introduction
The LA series of Linear Amplifiers are the perfect choice for systems requiring low
radiated noise and zero distortion from the drive electronics. These high power current mode
linear amplifiers are well suited to drive low inductance/resistance loads such as brushless and
brush servo motors or voice coils. Commutation options include externally commutated 2-phase
sine input, trapezoidal commutation using motor mounted hall sensors, or single-phase control.
With their true class AB linear output stage, their design features pure analog control
from input to output. The on-board DSP only provides monitoring functions and does not get
involved in the current control of the output stage.
The LA amplifiers are both extremely quiet and provide the ultimate in zero cross-over
distortion for smooth output positioning. The design of these amplifiers includes an on board
high speed DSP which monitors all key system functions in real time and provides protection for
the outputs by only allowing output power within the “Safe Operating Area” of the output
transistors. An intelligent user interface allows setup and storage of all system parameters via
the serial interface. Non-volatile memory provides storage of the parameters during power off
conditions.
1.1
Safe Operating Area
The LA amplifiers include a sophisticated algorithm that protects the outputs from over
power conditions. This algorithm is matched to the power characteristics of the output
transistors in each amplifier model. With linear servo amplifiers (as opposed to PWM amplifiers),
it is very important to provide over-power protection (rather than simple over-current protection)
due to the linear nature of the output control. In the case of PWM amplifiers, only over-current
protection is required since the outputs are operating in saturation mode or “full on mode”. This
mode provides very little voltage drop across the output transistors, so simple current monitoring
is sufficient to provide protection of the outputs.
With linear servo amplifiers, the outputs are operating in their linear region, so the
voltage across the output transistors can be a substantial contribution to the total power
dissipated by the device. To properly protect the amplifier from damage, the amplifier must
provide protection by monitoring the power (voltage * current) in the output devices. To put this
in perspective, the outputs used in our LA-415 (5A continuous, 15A peak) can handle 60A under
the proper conditions! It’s the power that has to be kept under control.
The DSP in the LA series amplifiers monitors the power of each output device in real
time as the device is switched on by the control circuitry. This instantaneous power
measurement is compared with the transistor manufactures recommended “safe operating area”
curve (published in all transistor specifications) stored in the DSP memory. The amplifier is shut
down in the event the measured power exceeds the recommended ratings of the output
devices.
Our Safe Operating Area (SOA) algorithm has proven to be very effective in protecting
the amplifier from damage due to over power conditions. While the user may experience
“nuisance” tripping of the SOA protective function during system development and testing, be
aware that the conditions that caused the “nuisance” trip may have very well have destroyed an
amplifier without this SOA protection.
