SN74LVTH16245A: Looking for a mil-spec (-55 - 125C) TTL (5V) to CMOS (3.3V) level shifter for a SiP design

Jonathan Phillippe

Part Number: SN74LVTH16245A
Other Parts Discussed in Thread: SN74AHC245-EP, SN74AHC244-EP, SN74LVC8T245-EP, SN74LVCH16T245-EP

Tool/software:

Here's my design problem. I have 13 inputs that need to support 5V TTL logic levels (ViL: 0 - 0.8V, ViH: 2V - 5V) and then level shift down to 3.3V. I'm working on a SiP design so package size is critical.

The SN74LVTH16245A-EP is ideal from a spec standpoint but the mil-spec package is 16mm x 10mm. That part comes in a VBGA package which would work but that package is only temp rated for -40 - 85C.

1. So I'm looking for 2 things - does anyone know why the temp ratings differ per package?

2. Does anyone have any alternative solution ideas that are smaller than 16mm x 10mm?

6 months ago

0 Clemens Ladisch 6 months ago

Guru 316510 points

1. Testing is expensive; I guess there have been no customers for this so far.

2. 2× SN74AHC244-EP or SN74AHC245-EP in the TSSOP package has 2 × 6.5 mm × 6.4 mm; 2× SN74LVC8T245-EP in the VQFN package has 2 × 5.5 mm × 3.5 mm; SN74LVCH16T245-EP in the TVSOP package has 9.7 mm × 6.4 mm.

0 Jonathan Phillippe 6 months ago in reply to Clemens Ladisch

Prodigy 20 points

Thanks for the suggestions but none of those parts have true TTL input levels of Vil: 0 - 0.8V and Vih: 2v - 5V.

0 Clemens Ladisch 6 months ago in reply to Jonathan Phillippe

Guru 316510 points

With a 3.3 V supply, the LVC devices have these switching thresholds.

0 Jonathan Phillippe 6 months ago in reply to Clemens Ladisch

Prodigy 20 points

Good catch! Ok - so if I drive the inputs to 5V with VCC = 3.3V will I just be forward biasing an ESD diode and therefore injecting current? I suppose I could use a resistor to limit the current.

Did some digging and found this note (pg 29) in TI's "Low-Voltage Logic (LVC) Designers Guide":

Case 1 Case 1 addresses a 5-V TTL device driving a 3.3-V TTL device. As shown in Figure 1–1, the switching levels for 5-V TTL and 3.3-V LVC are the same. Since 5-V tolerant devices can withstand a dc input of 6.5 V, interfacing these two devices does not require additional components or further design efforts. TI’s crossbar technology (CBT) switches can be used to translate from 5-V TTL to 3.3-V devices that are not 5-V tolerant. This is accomplished by using an external diode to create a 0.7-V drop (reducing 5 V to 4.3 V) with the CBT (of which the field effect transistor has a gate-to-source voltage drop of 1 V) that results in a net 3.3-V level. TI produces a CBTD device that incorporates the diode as part of the chip, thereby eliminating the need for an external diode.

0 Jonathan Phillippe 6 months ago in reply to Jonathan Phillippe

Prodigy 20 points

Also - do you know if the IBIS models correctly model this scenario? Vin = 5V, Vcc = 3.3V?

+1 Clemens Ladisch 6 months ago in reply to Jonathan Phillippe

Guru 316510 points

All these devices have overvoltage-tolerant inputs without a diode to V_CC; see [FAQ] Can the input voltage (Vi) to my logic device be higher than the supply voltage (Vcc)?

The IBIS models should model this correctly.

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SN74LVTH16245A: Looking for a mil-spec (-55 - 125C) TTL (5V) to CMOS (3.3V) level shifter for a SiP design