SN74LS06 CMOS?

I think I found an error in a TI datasheet, and I want to ask whether others see it as an error as well. I need five digital inputs to my circuit. The voltage levels it will see are 0V, 5V and open. I don't want to use pull-up or pull-down resistors because simply I don't have the space. I don't even have 0.1" for a SIP package. So, I want to use the following open-collector inverter and I will use pull-ups in the microcontroller. The microcontroller is uses 3.3V. I thought the SN74LS06 was a BJT based device, but Note 4 on page 3 of the datasheet says that all unused inputs need a pull-up or pull-down resistor. The schematic on page 2 shows BJTs (actually Schottky transistors). What should I believe?

http://www.ti.com/lit/ds/symlink/sn74ls06.pdf

Why not use a voltage divider and connect directly to the uC.

If you read the datasheet for 74LSXX you need pullups on all not used inputs for the whole temperature range. TI MUST print that. In real life, it works for 99.9 % of all cases but in some cases the leakages are to big and the input goes low.

SMD?

Pelle

Pelleplutt: Why not use a voltage divider and connect directly to the uC.

If you read the datasheet for 74LSXX you need pullups on all not used inputs for the whole temperature range. TI MUST print that. In real life, it works for 99.9 % of all cases but in some cases the leakages are to big and the input goes low.

SMD?

Pelle

Yes, it is a SMD design. I completed the layout then I realized that the inputs may float. The customer requirement is not clear about this detail so I'm assuming the worst. I only have the space of a 20-SOIC package left. I don't think I have enough space for five voltage dividers. Where is the leakage keeping from if the inputs are left open.

This is basic design technique with TTL ICs. NO inputs should EVER float.

http://www.fairchildsemi.com/an/AN/AN-363.pdf

TYING ALL UNUSED INPUTS TO A SOLID LOGIC LEVEL

Unused inputs on TTL devices float at threshold, anywhere from 1.1V to 1.5V, depending upon the device and its family. While this usually simulates a “high”, many application problems can be traced to open inputs. Inputs floating at threshold are very susceptible to induced noise (transmitted from other lines) and can easily switch the state of the device. A good design rule is to tie unused inputs to a solid logic level. Inputs are usually tied to VCC through a 1 k? to 5 k? resistor, since tying them to ground means supplying the IIL current instead of the IIH current. IIL is several orders of magnitude greater than IIH. The resistor is recommended to protect the input against VCC voltage surges and to protect the system against the possibility of the input shorting directly to ground. A single 1k resistor can handle up to 10 inputs.

All LS devices are TTL and not CMOS.

How can you not have room for pull-up or pull-down resistors? If 0603 is too big, use 0402 or 0201. ]:D

0201

Otherwise known as pepper.

Thanks!

I’m switching to a smaller IC. With that IC, I should be able to use 0603 resistors. 0603 is small enough.

Why are you using TTL at all? Why not 74HC06?

TTL has the problem that high inputs cannot be tied to the 5V rail safely (as there is no safety margin, the input transistors will fail with any spikes on the rail). Thus all the 1k or 10k resistors (TTL and LS respectively).

CMOS families like HC can have unused inputs safely tied to either rail.

MarkT: Why are you using TTL at all? Why not 74HC06?

I was going to say something similar, but using the word "obsolete".

The whole question is confused; I gave up on it; there surely is a level converter chip somewhere which does the precise job with rail-to-rail drive and no additional components - I just don't happen to know what is is.