Instructions

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9.5.3 Constant Conductance and Resistance Modes
In both these modes, the analogue multiplier-divider is used to derive the current required from
the sensed voltage. In Conductance mode the current required is calculated by multiplying the
sensed voltage by the specified conductance; in Resistance mode the current required is
calculated by dividing the difference between the sensed voltage and the dropout voltage setting
by the specified resistance.
In both cases, the current rises as the applied voltage rises. At equivalent resistance and
conductance settings, the path from the voltage sense input through to the power stage is the
same, so the two modes will exhibit similar stability characteristics.
In transient operation, the two modes are very different. In Conductance mode, the current
required linearly follows the changing conductance value and the behaviour is fundamentally
similar to constant current mode. In Resistance mode, the required current is inversely
proportional to the linearly changing resistance value, so the resulting current waveform is very
non-linear, changing rapidly at the low resistance part of the cycle. This rapid change
accentuates the effect of inductance in the interconnecting leads and can easily lead to
bottoming and overshoots. Resistance mode is best used at higher voltages and modest
currents.
9.5.4 Dropout Voltage and Resistance Mode
The use of the Dropout voltage setting as an offset in Constant Resistance mode allows flexibility
in constructing load characteristics for particular circumstances. For example, setting a low value
of resistance and a significant value of dropout voltage yields a characteristic similar to a string of
LEDs or a Zener diode and provides an alternative to a Constant Voltage mode but without the
extreme stability problems of that mode.
9.6 Multiple Unit Operation
It is possible to operate two loads in parallel in Constant Current mode, which will double both the
current handling and power dissipation capability of a single unit. The connections to the source
should be matched as well as possible.
Note that additional stability issues may arise, because of phase response differences between
the units; the use of more than two units in parallel is not recommended. Multiple unit operation
should not be attempted in any operating mode other than Constant Current.