.
Power Monitoring LEDs
Each plane of the TLK3134 EVM has been equipped with a Voltage Monitoring circuit that will
monitor the voltage on the plane and light the LEDs when the voltage is within the min/max
datasheet limits for that power supply. A precision TI Voltage Reference chip is used along with
0.1% precision resistors setting min and max reference levels providing a detection circuit that is
accurate to approximately +/- 10mV. The LEDs should be used as a basic indication of the
status of power on the board being within the acceptable min/max limits given in the datasheet,
and not as a precise measurement tool as some LED circuits may turn off at slightly different
voltages when approaching the limits due to the manufacturing tolerances and available
component values.
The voltage monitor circuits can also be bypassed and the LEDs driven directly from the voltage
on the individual planes such as when performing voltage tolerance tests. Instead of being lit
only when the voltage on the plane is within the min/max range, the LED will be lit when the
voltage is greater than the voltage needed to turn on the LED drive circuit’s NPN transistor,
allowing current to flow, and the LED to be lit from the 5V source. In the Direct Connect mode,
the base resistors has been given extra margin to allow the LEDs to light when the voltage on
the plane is a little below the minimum limit of that supply in order to provide a LED indicator of
power on the plane during voltage tolerance tests near the lower supply limits.
Figure 5. TLK3134 EVM Voltage Monitor LED Enabled Example
Placing the jumper on the ENABLE side of the Voltage Monitor Enable/Disable header connects
the power plane to the input of the voltage monitoring circuit. This input is high impedance and
will not load down the power source providing the voltage to the plane.
Placing the header on the MONITOR side of the LED Monitor/Direct Connect selection header
connects the LED drive circuit to the output of the Voltage Monitor circuit causing the LED to be
lit only when the voltage is within the acceptable range.
TLK3134 XAUI Transceiver / 4 Channel Multi-Rate Transceiver Evaluation Module (EVM) Users’ Guide
9
SLLU104A - September 2007
