I am not sure what device you are using but what i believe you want to do is a retriggerable finite pulse train. So, everytime you see an external trigger you want to generate one pulse with a specified pulse width (5% of the trigger pulse train). The NI Example finder has an example that does this already. All you need to do is adjust the parameters to meet your needs. You can adjust the frequency, duty cycle, trigger delay, and trigger source terminal (one of the PFI lines). The example I am referencing can be found by going to Help>>Find Examples:
Hardware Input and Output>>DAQmx>>Generating Digital Pulses>>Gen Dig Pulse Train-Finite-Retriggerable.vi
Hope this helps!