74 Application Programs
Application 2. Sequencing Multiple Modules to Power Down on Event
Overview Of Application
When testing devices, such as some GaAs and ECL devices that are sensitive to when bias voltages are removed, the order
of power-down of multiple power modules can be controlled. The power-down sequence can be initiated by an event, such
as a change in power module status, fault condition, detection of a TTL signal, etc.
For this example, there are three supplies + 5 V and ± 15 V. (See previous application for how to generate a power up
sequence.) Once the power has been applied to the DUT, the modules can be reprogrammed to perform the power down
sequence. The power down sequence is initiated when a fault in the DUT draws excessive current from the power module,
causing the module to change from CV to CC. To prevent damage to the DUT, it is necessary to remove the + 5 V first,
then the ± 15 V modules 15 ms later.
Once again, MPS triggering can solve the application. In this scenario, the CV-to-CC crossover event will be used as the
trigger source. The trigger will cause the modules, in the correct order, to change from their programmed voltages down to
0 V.
MPS Features Used
• Generate a trigger on a change in internal status.
• Change the voltage on trigger.
• Trigger in/out from MPS mainframe backplane TTL Trigger o Trigger delay.
• Active downprogramming.
Advantages/Benefits Of The MPS Solution
By using the modules' change in status to automatically generate a trigger, the computer is not devoted to polling the
modules to detect a change in state.
By letting each module monitor its status, the CC condition will generate a response faster than if the computer was polling
the module to detect a change in state.
The sequence is simpler to program (no timing loops).
By using trigger delay, the timing is accurate and repeatable because the computer does not provide timing for the sequence.
The active downprogrammers in the module output can quickly discharge the module's output capacitors and any
capacitance in the DUT.
Implementation Details
How The MPS Implements The Solution
All modules are set to listen for a backplane TTL Trigger.
When any module detects a change in status from CV to CC, it sends out a backplane TTL Trigger.
When the + 5 V module receives the trigger, it immediately goes to 0 V.
When the + 15 V and - 15 V modules receive the trigger, they wait the trigger delay time and then go to 0 V.
Note Any module can generate both the backplane TTL Trigger signal and be triggered by that same signal.