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[SOLVED] Protection circuit for a Fiber Optic driver

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flote21

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Hello folks!

I am using the next schematic to drive a fiber optic tx:

pic.jpg

Note that Vcc= 5V/3A

I am wondering because the driver SN55452BJG is broken very often due to the high current. I would like to find a solution in order to limit the current only in the collector of the driver trt, because I need that such high current on the fiber optic side in order to achieve an enough luminosity power....

On the other hand after reading the datasheet od the FO Tx HFBR-1412Z, I could see that they use a diode on the collector og the trt (see test circuit). This diode could be used as a safety protection of the driver? See next pic:

pic.jpg



Any idea?

Thanks.
 
Last edited:

Hi,

according your schematic the current is limited to 5V/50Ohms = 100mA.
The driver is rated for 400mA continous current, therefore I don´t think the problem is "high current".

Wrong PCB layout, long wiring, stray inductance, dirty supply voltage, dirty signal voltage....comes into my mind.
Show us the PCB layout.

You use the "Test circuit" from the datasheet. But there is also a "recommended driver circuitry". It is not that lossy than your circuit.
What´s your data rate and fiber length?

Klaus
 

Hi KlausST!

Thanks for your quick answer!

The data rate is around 3MBd and the fiber length around 1 Km. I have been mesuring the noise on power supply and signal and it is around 15-20mV. So it does not look a noise problem.

Futhermore the schematic I used is the same schematic of the demo board:

**broken link removed**

And the layout, is pretty easy, here you can see it:

**broken link removed**

Greetings

- - - Updated - - -

Finally I have been also thinkinb to change the schematic to this:

pic.jpg

On this way, if the trt is broken, the default status of the Fiber Optic LED is OFF. Do you think that this solution is going to work?

Greetings,
 

Hi,

I have been mesuring the noise on power supply and signal and it is around 15-20mV
Click to expand...
* on the power supply is not of interest. "At the driver" is of interest.
15-20mV means what? RMS, Vpp? --> use VPP measurement with a scope and high bandwidth.

And the layout, is pretty easy, here you can see it:
Click to expand...
but the important things for clean signals is not visible: The capacitors for VCC. With that fast signals you need a fast ceramics capacitor (100nF) very close at the diode as well as at the driver.
Additionally (because of the relatively high current) i recommend a 100uF bulk capacitor.
--> Important: GND wiring, VCC wiring, VCC capacitors.
Please confirm
* that the resistors are not wire wound resistors.
* the wiring from microcontroller to driver is short, and refers to the same GND plane than the driver.

Klaus
 

Hi!

I have measured at the driver and I got: 15-20 mVpp with the Oscilloscope adjustments you say.

The caps are close to LED and the driver as well and I am using SMD resistors. The distance between the uC and the driver is 50mm and they share the same ground...

I think that the problem is on the schemetic selected.....With this schematic, the driver should be cooler and therefore it would not be broken:

pic.jpg
 

Hi,

I don´t think this works. For low frequency, yes. But not for your frequency.

There is the LED capacitance. When you switch OFF the current tehn the LED will be alight for a short time caused by the energy storen in the capacitance.
Therefore they use the resistors in the inital circuit to quickly discharge the capaciatance.
Your (somehow exotic) driver circuit is a open collector output. In your latest circuit it drives the LED and charges the capacitance. But it can not actively dischrge this capacitance.

Why don´t you simply use the recommended driver circuit of the datasheet? (like already mentioned in my post#2)
It is a push-pull driver and doesn´t suffer from the problems of a open collector driver.

Klaus
 

Hi KalusST,

pic.jpg

The recommended circuit, you mean the above one right??

because there is the below one, which is a little bit different....dont think so?

pic.jpg

Greetings
 

Hi,

basically both are push-pull drivers.

But I wonder where you have the second one from? Again a very exotic IC ... is it available in your country?
From the circuit and the driving method very similar to the above, but one is high side driving, the other low side driving.

Klaus
 
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