DC-DC Isolator - Switching On and Off

Thread Starter

circuits9

Joined Feb 2, 2020
18
I am trying to provide a switching mechanism from an MCU to turn on/off a Trico DC-DC Isolator circuit that drives an LDO and 555 Timer. I am attempting to use a BC547 transistor to provide switching capability to the Trico DC-DC isolator. I also tried a 2n7000 Mosfet and have similar results. The Trico TBA1-0311 (Datasheet) takes a 3V input from the transistor and boosts it to 5V. The output is then sent to a LP2950 (Datasheet) LDO to get a clean 3.3V to send to a 555 timer.

The output voltage from the BC547 is measured at ~3.3V but as soon as the DC-DC isolator is added, the voltage drops significantly before the DC-DC isolator drops to 0.5V and the output from DC-DC isolator is about 1V.

Is there a better way to do this? I am very new to Isolators and can't quite figure out why I am not getting the output voltage I need. Attached is a partial schematic of the circuit. Any guidance would be much appreciated!

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AlbertHall

Joined Jun 4, 2014
12,344
With the MCU_ENABLE_PIN high, what is the voltage on the left end of R6?
The voltage on pins 1 and 2 of the isolator?
Voltages to be measured relative to GND.
 

AlbertHall

Joined Jun 4, 2014
12,344
And check that R6 is indeed 1k and R? is 10K.
Is the only load on the LP2950 the '555.
What circuitry is connected to the '555?
 

Thread Starter

circuits9

Joined Feb 2, 2020
18
Confirmed both resistors are correct. Also, I removed the 555 timer load. It is just the transistor, DC-DC-isolator and the LP2950.
 

KeithWalker

Joined Jul 10, 2017
3,063
You are not driving Q2 hard enough. Measure the collector to emitter voltage when the enable pin is high. It should be about 200mV. Decrease the value of R6 until you get that. Does the MCU you are using have an active sink output ? If it is open collector you will need to add a pullup resistor.
Regards,
Keith
 

Thread Starter

circuits9

Joined Feb 2, 2020
18
also, if I remove the transistor and power the isolator direct via 3.3. the DC-DC isolator outputs 5V and the LDO outputs a clean 3.3. Everything works fine until I add the transistor for switching.
 

Audioguru again

Joined Oct 21, 2019
6,672
I think the BC547 is saturating poorly because you connected its collector and emitter pins backwards because you are used to seeing the EBC pins of an American 2Nxxxx transistor and not the CBE pins of a European BCxxx transistor. Look at its pins in its datasheet.
 

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Audioguru again

Joined Oct 21, 2019
6,672
The datasheet for the LDO regulator recommends a larger output capacitor value of 2.2uF when the output voltage is only 3.3V. You have the 1uF that is used when the output is 5V.
 

Thread Starter

circuits9

Joined Feb 2, 2020
18
I see that now. Thanks for bringing that to my attention. One last question: if I add the 2.2uF capacitor after the LDO, would I still need to keep the 1uF and 0.1uF to use as a decoupling capacitor for the 555 timer?
 

Audioguru again

Joined Oct 21, 2019
6,672
I see that now. Thanks for bringing that to my attention. One last question: if I add the 2.2uF capacitor after the LDO, would I still need to keep the 1uF and 0.1uF to use as a decoupling capacitor for the 555 timer?
You do not have an antique very high current 555 that draws 400mA (!) each time the output switches, instead you have a Cmos low current C555 that works fine with a 0.1uF decoupling capacitor close to it. If the 1uF capacitor is close to the LDO regulator then replace it with a 2.2uF capacitor.
 
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