THS788 Datasheet, PDF (35/43 Pages) Texas Instruments – QUAD-CHANNEL TIME MEASUREMENT UNIT (TMU)

THS788
www.ti.com
SLOS616B – MARCH 2010 – REVISED JUNE 2011
An alarm output pin is provided that is an open-drain output. Connect this output through a pullup resistor to the
3.3-V power supply. The resistor must be at least 3.3 kΩ. This creates a slow-speed, low-voltage CMOS digital
output with a logical 1 being the normal operating state and a logical 0 being the overtemperature state.
Power Supply and Bypassing
All the high-speed time-measurement circuitry in the TMU is implemented in differential emitter-coupled logic
(ECL). Besides high speed, a characteristic of differential ECL is good rejection of power-supply noise and
variation. However, there is a great deal of CMOS logic, FIFO and output-serial interface circuitry that is an
excellent source of power-supply current noise. Therefore, to maintain the best accuracy, the TMU power supply
must be low-impedance. This is accomplished in the usual ways by careful layout, good ground and power
planes, short traces to the power and ground pins, and capacitive bypassing. Recommended is a quality,
low-inductance, high-frequency bypass capacitor close to each power pin of approximately 0.01 μF. The 0402
size works well. Additional bypass capacitors of larger value should be placed near the TMU, making
low-inductance connection with the power and ground planes. With a typical power-supply sensitivity of 30 ps/V,
a 1% power supply shift yields a 1-picosecond additional error, making power-supply regulation important for the
best accuracy.
Thermal Considerations
The TMU package provides a thermally conductive heat slug at the top for connection to an additional heatsink.
The TMU can be placed into many different modes for optimization of performance vs power dissipation, and a
table has been provided to help determine the power required. The heat sink should be carefully considered in
order to keep the TMU temperature within required limits and to promote the best temperature stability. The TMU
time measurement drift with temperature is an excellent 0.1 ps/°C. A good heat sink design takes advantage of
the low temperature drift of the TMU.
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