Voltage level shifting question

Thread Starter

rougie

Joined Dec 11, 2006
410
Hello,

I have a question concerning how voltage level shifting works. I am using a light sensing device TSOP4838 from Vishay. This device is connected to a 5VDC power source and its output is an 38 KHZ alternating signal aprroximately 5VDC (4.7VDC). At default, when the sensor is not detecting anything, it outputs a constant 5VDC.

I need to feed the output signal of the TSOP4838 (5VDC)to a CPLD which only accepts 3.3 VDC input.

I am using the MAXII, EPM240 CPLD from Altera and they show a sample on how to interface a 5VDC signal or voltage level to a 3.3VDC input of a CPLD.

As you can view my attached schematic AAC_SCHEMATIC.doc, which shows my circuit, I was just wondering mathematically how and why it works.

You can view Altera's sample which starts on p. 8-7 of the AAC_max2_mii51009[1].pdf attachment. On p. 8-8 of this document, they speak of IOH. The IOH specification can be found on p.6 of 32 of the AAC_max2_mii51005[1].pdf attachment.

When the TSOP4838 is outputing the 5VDC, the internal transistor of TSOP4838 not forward biased (See TSOP4838 spec), and the current is actually going from the 5VDC supply (Pin 3 of the TSOP4838) and through an internal
30K resistor and through R2 and then through the clamping diode. How is it that at this instance I read 0.13ma????

Using ohms law, I don't see how this current causes the necessary voltage drop of 1.7VDC?

Any help as to why my circuit actually works is very appreciated.

With regards
Robert
 

Attachments

Thread Starter

rougie

Joined Dec 11, 2006
410
Thankyou papabravo for your response,

Maybe I am wrong, but are you sure this part will convert a 5VDC to a 3.3 VDC?

As stated in its spec:

"The input protection circuitry on this device allows overvoltage tolerance on the input, allowing the device to be used as a logic−level translator from 3 V CMOS logic to 5 V CMOS Logic or from 1.8 V CMOS logic to 3 V CMOS Logic while operating at the high−voltage power supply."

Please do get back!

Regards
Robert
 

Papabravo

Joined Feb 24, 2006
13,931
The device can use a Vcc of 3.3 volts and has 5V tolerant inputs.

In the reverse direction it can ues a 5V Vcc but the TTL thresholds of 2.0V and 0.8 Volts allow it to be used with a 3.3V CMOS output.
 

Thread Starter

rougie

Joined Dec 11, 2006
410
Hello papabravo,

So for my case I would just plug VCC to 3.3 volts and input the 5VDC at IN-A and expect a 3.3VDC at OUT-Y, right?. And when IN-A is low I should expect a low at OUT-Y, right?

Get back!

With regards
Robbert
 

neverlog

Joined Dec 4, 2008
9
Hi papabravo,

Please let me ask an additional question:

Can I use the same parts as a voltage translator?

I would like to use on a bluetooth module(Vdd 3.3V) and my RFID device(Vdd 5V), RX and Tx pin.

Because the Rx and Tx pin from bluetooth module has lower voltage level compare to Rx and Tx pin from RFID module.

Please let me know if you need more info.

Attached is the electric characteristics of the bluetooth and RfiD module:

I would appreciate if you could help me. I am a newbie to electronics.

Regards,

Lam
 

Attachments

Papabravo

Joined Feb 24, 2006
13,931
That is what the chip is made for. One chip will have a VCC of +5V, and the other will have a VCC of +3.3V. Vcc goes according to where the output is connected. The inputs will take care of themselves. The key is the presentce of "TTL thresholds" of 0.8 V and 2.0V on those inputs.
 

neverlog

Joined Dec 4, 2008
9
Thanks Papabravo,

Which means I need 2 unit of this part, right? Because I have Tx and Rx pin on both Bluetooth and RFID module.

Hope to hear from you soon.

Thank you.
 

Papabravo

Joined Feb 24, 2006
13,931
Yes, each unit has one gate. The two units WILL have different VCC's so having two separate parts is a definite advantage.
 

neverlog

Joined Dec 4, 2008
9
Hi Papabravo,

By using the MC74VHC1GT50, I have successfully connected/activated my device, but it could retrieve any data/signal. Do I need to add a capacitor at the output?

Please get back.

Thanks!

Merry Christmas & Happy New Year!

rgds,
Lam
 
Top