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TTL vs LVTTL | Difference between TTL and LVTTL

This page compares TTL vs LVTTL and mentions difference between TTL and LVTTL with respect to voltage levels, advantages and disadvantages.

Introduction: First TTL IC 7400 was developed in 1965 and it was known as "Standard TTL". TTL family use transistor to perform basic logic operations. TTL devices are used as "glue logic" which connects more complex devices in a digital system.

The basic TTL logic circuit is NAND gate which uses BJTs internally.

TTL | Transistor Transistor Logic

• It is one of the most common I/O standard.
• TTL operates from +5V or 3.3V power supply.
• There are three different types of output configurations used by TTL viz. open collector output, Totem pole output and three state or tristate output.
• Standard TTL family ICs : 7404, 74S86 and 74ALS161
• Binary logic "1" : represented by voltage between 2.4 V and 5 V (i.e. VCC), must be more than 2V
• Binary logic "0" : represented by voltage between 0V and 0.8V, must be less than 0.8V

Following are the typical characteristics of TTL logic family.
• Basic gate used : NAND
• Fanout : 10
• Power per gate (mWatt) : 1-22
• Noise immunity : Very good
• Noise margin : 0.3 (High), 0.5 (Low)
• tPD (ns) : 1.5 to 33
• Output drive current : Asymmetric : High state : 0.4 to 2 mA, Low state : 8 to 20 mA

TTL and CMOS interfacing

The figure depicts interfacing between TTL and CMOS.

Advantages :
• Low power consumption, higher output swing, low implementation cost, low propagation delay, TTL circuits are fast etc.
• Totem pole TTL output configuration offers high fanout, low power dissipation and higher operating speed. Totem pole output does not require external pull up resistor.
• Refer more advantages of TTL logic family >>.

Disadvantages :
• Consumers more power than CMOS and hence not used for battery powered devices.
• Fanout (i.e. component density) is less than CMOS
• It has poor noise immunity.
• In Totem pole configuration, output of two gates can not be tied together.

TTL and LVTTL voltage levels

LVTTL | Low Voltage TTL

• It is a JEDEC standard which references input signal to the ground.
• The switching range at output is 0.4V to 2.4V for 3.3V LVTTL version.
• The switching range at output is 0.4V to 2.2V for 2.5V LVTTL version.

Following are the receiver input and driver output specifications of LVTTL.
LVTTL and TTL Driver output : At low logic level, maximum driver output voltage (VOL) is 0.4V for both LVTTL and TTL. The minimum output voltage is GND.
Driver output : At high logic level, minimum (VOH) is 2.4V for LVTTL and TTL and maximum is Vcc which is 3.3 V for LVTTL and 5V for TTL.

LVTTL and TTL Receiver Input : For low logic level, maximum input voltage (i.e. VIL) is 0.8V for LVTTL and TTL; minimum i/p voltage to receiver is GND
For high logic level, minimum input voltage (VIH) is 2 V for TTL/LVTTL and maximum receiver i/p voltage is Vcc i.e. 3.3 V for LVTTL and 5V for TTL

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