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CMOS to TTL monostable multivibrator

P

PPP

Jan 1, 1970
0
Hola!

I am looking at Texas Instrument's CD74HC538 monostable multivibrator.
Going thru the datasheet, I noticed that it does not have anything for
Voh (high level output voltage for TTL loads) at VCC ranges lower than
4.5V.

Does that mean that I won't be able to drive a TTL switch if my voltage
supply range is around 3V?

I found an analog switch also on Texas Instruments that work at 3V, but
it is labeled as having a TTL input. I wanted to interface this with
multivibrator.

Any hints?

Gracias!
 
A

Andy

Jan 1, 1970
0
Hello
Hola!

I am looking at Texas Instrument's CD74HC538 monostable multivibrator.
Going thru the datasheet, I noticed that it does not have anything for
Voh (high level output voltage for TTL loads) at VCC ranges lower than
4.5V.


The standard TTL (SN54) is not specified with the Vcc below 4.5V -
that's probably the reason why the CD74HC4538 is not specified neither
with TTL loads with the Vcc below 4.5V.
Does that mean that I won't be able to drive a TTL switch if my voltage
supply range is around 3V?

It could work, but is not specified.
I found an analog switch also on Texas Instruments that work at 3V, but
it is labeled as having a TTL input. I wanted to interface this with
multivibrator.

What is the part number ?
Any hints?

Gracias!
-- Andy
 
P

PPP

Jan 1, 1970
0
Hello Andy!

The switch part number is TS5A3159. It does not say that it has a TTL
Control Input feature on the datasheet, but it states this in the TI
Analog Switch Guide.

I'm new to this CMOS to TTL interfacing. After giving it much thought,
the TI selection guide probably meant that TTL devices can also drive
the switch (?). I figured since CMOS devices normally have voltage high
outputs close to VCC and voltage low outputs close to VSS (GND), the
voltage input high and voltage input low specifications for the
TS5A3159 will be within the CD74HC538 voltage high and low output
range, no?

Thanks!
 
A

Andy

Jan 1, 1970
0
PPP said:
Hello Andy!

The switch part number is TS5A3159. It does not say that it has a TTL
Control Input feature on the datasheet, but it states this in the TI
Analog Switch Guide.

I'm new to this CMOS to TTL interfacing. After giving it much thought,
the TI selection guide probably meant that TTL devices can also drive
the switch (?).

Most certainly, I have not looked through the guide, but the input logic
thresholds of the TS5A3159 at 5V suppy (4.5 to 5.5v) match the standard
TTL output specifications.

I figured since CMOS devices normally have voltage high
outputs close to VCC and voltage low outputs close to VSS (GND), the
voltage input high and voltage input low specifications for the
TS5A3159 will be within the CD74HC538 voltage high and low output
range, no?

Look at the input logic voltage specifications of the TS5A3159, for the
3.3V supply. For the Vcc range from 3.0 to 3.6V, the input logic high is
=2V, the input logic low is <=0.6V. While the CD74HC4538 is not
specified at 3Vcc, from the specs at 2Vcc and 4.5Vcc (CMOS loads!), one
can safely derive that at 3 to 3.6Vcc its outputs will finely drive the
TS5A3159 digital inputs (powered at the same Vcc).

When interfacing TTL & CMOS, one also has to pay attention to the
input/output current specifications, and ensuring the required switching
speed is not quite simple.
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
who cares what the voltage is providesd is surpasses the voltage threshold. 3v hi is good believe it or not TTL things to watch for is fan-in fan-out capability. basicaly is specified as unit loads. confused want to know more?
 
J

John Fields

Jan 1, 1970
0
Hola!

I am looking at Texas Instrument's CD74HC538 monostable multivibrator.
Going thru the datasheet, I noticed that it does not have anything for
Voh (high level output voltage for TTL loads) at VCC ranges lower than
4.5V.

Does that mean that I won't be able to drive a TTL switch if my voltage
supply range is around 3V?

I found an analog switch also on Texas Instruments that work at 3V, but
it is labeled as having a TTL input. I wanted to interface this with
multivibrator.

Any hints?
 
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