Aim TTi LD300 Fonctionnement page 22

Slew Rate
The SLEW RATE control and its associated three-position range switch set the slope of the
transitions between the two level settings. When the METERS switch is set to the SLEW &
DROPOUT position, the left hand meter indicates the slew rate: the measurement unit of the
presently selected mode is indicated above the display, with either the /ms (per millisecond) or /s
(per second) annunciator also lit.
The circuit provides a linear transition in the control value of the active mode, so, for example, in
Constant Power (CP) mode the slew rate is expressed in Watts per millisecond (or Watts per
second at very slow rates). The shape of the current transition is therefore not necessarily linear.
The slew rate setting is applied to all changes in level whether caused by manual adjustment, the
transient generator or external voltage control. It is also used to determine the rate of rise when
the slow start circuit is triggered.
The bandwidth of the power stages of the load is reduced whenever the slew rate range switch is
in the Lo position, even if the transient facilities are not being used. This changes the dynamic
behaviour of the unit and may improve stability with some difficult combinations of source and
load characteristics.
Slew Rate Error Conditions
There is an upper and a lower limit to the slew rate value that can be used in combination with
the settings of the other controls. In the top right hand corner of the display area there is an
indicator which lights, showing ERR, if either of the circumstances described below arise.
The lower limit is determined by the relationship of level settings, slew rate, frequency and duty
cycle. If the transition time (which is the difference in the level settings divided by the slew rate) is
longer than the oscillator period times the duty factor, then there is insufficient time for the
transition to complete before the oscillator initiates a return to the other level, so the desired level
will never be reached. In this case, the indicator flashes ERR.
The upper limit is determined by the minimum transition time of the power stages of the load
(which depends on the operating mode – see the specification). If a combination of a fast slew
rate and a small change in level imply a transition time shorter than this, then the settling time of
the power stage will dominate. In this case the ERR indicator is lit continuously. Note that this
indicator assumes an ideal source and optimum interconnections. Further limits on slew rate can
be imposed by the output impedance and dynamic characteristics of the source, and by
inductance in the connections.
In practice, the dynamic behaviour of a source and load combination at high slew rates depends
on many factors, particularly interconnection inductance and the damping factor of feedback
loops. In addition, the response of the power stages of the load is slower when operated at very
low or high currents, or at low voltages. In many circumstances, a lower slew rate setting will be
needed to avoid aberrations.
In Constant Resistance mode, where the current is inversely related to the resistance transition, it
is particularly difficult to predict the maximum useful slew rate setting.
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