Driving 74LS from 3.3V uP

bitrex · 2016-07-12T17:52:29+00:00

Prototyping a little toy for the kiddos out of junk box parts... I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts is boosted to around 5 to power the 74LS and Numitron segments via an eBay special micro boost converter with shutdown pin. The test code right now simply outputs a BCD to the '47 inputs and pulls the shutdown pin on the boost high to turn it on. The problem is the uP starts up fine and all the output voltages on the 4 BCD pins looks good in isolation, but when the circuit is powered up with the Numitron and '47 connected up the bench supply immediately falls over into overcurrent protection at about 80 mA, way more than say two incandescent segments to form the numeral 1 should be drawing at 5 volts, I believe. The boost has plenty enough oomph to power all the Numitron digits from either the LiPo or the bench supply, I tested that in isolation. Tested two different '47s with the same result. Not sure exactly what's going on here...could there be some ESD/clamp diode pathway that's being driven that I don't know about? Or a startup sequencing issue?

Jul 12, 2016 48 Replies

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Jim Thompson 10 years ago

That's a good idea... turn on "warm" then full. ...Jim Thompson

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bitrex 10 years ago

Haha, around here you need a state license for that level of passive-aggressiveness.

I do appreciate the advice, though. ; )

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Phil Hobbs 10 years ago

Assuming the switcher has the usual 1.2V reference, then a single open-drain MCU pin and a resistor can switch it between high and low output voltage.

To simulate open drain, you write 0 to the bit in the register and toggle the output enable.

Vout

Output off -> resistor R3 not connected -> low output voltage Output on -> resistor R3 connected -> high output voltage.

The feedback pin doesn't go above 1.2V in normal operation. It'll jump up when R3 is disconnected, but should never go over 3.3V unless the voltage ratio is more than 2.5:1. If it is, a Schottky diode to the supply will fix it.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net

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krw 10 years ago

But would he get invited to the Whitehouse to show it off?

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bitrex 10 years ago

OK, so everything is working good now! I found that the boost's enable pin responds pretty quickly, so just ramping the ~5V supply voltage up via a PWM output to that pin, for about a half second, allows everything to start up well without any extra supply filtering.

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tabbypurr 10 years ago

OK. Now what happens if your display goes from . to 8888?

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bitrex 10 years ago

It can't, there's only one tube. It counts up from 0 to 9 and back down again in 1000 mS, 900, 800, etc. intervals well...

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David Eather 9 years ago

meausre the R of the cold incandescent segments and you will your answer

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bitrex 9 years ago

"soft starting" the "enable" pin of the converter with PWM to bring up the voltage on the filament slowly got it sorted

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