unfamiliar symbol

[gearhead]

I had a couple of things I wanted to figure out about the SG3525 and
got to looking at the schematics for the oscillator and the error
amplifier at the bottom of page 6 of the datasheet. (I wanted to
figure out how to use the sync pin, since the datasheet doesn't
explain it. And how low the common mode of the error amplifier goes,
since the datasheet only states that it includes Vref, or 5 volts.)
http://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF
What does the symbol that looks like a transistor with two collectors
represent and if you can, help me figure out the answers to my two
questions.

 

[Bob Eld]

"gearhead" <nospam@billburg.com> wrote in message
news:52b1caa0-d5d2-43ce-906e-a38f46beb9f7@r13g2000vbp.googlegroups.com...

I had a couple of things I wanted to figure out about the SG3525 and
got to looking at the schematics for the oscillator and the error
amplifier at the bottom of page 6 of the datasheet. (I wanted to
figure out how to use the sync pin, since the datasheet doesn't
explain it. And how low the common mode of the error amplifier goes,
since the datasheet only states that it includes Vref, or 5 volts.)
http://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF
What does the symbol that looks like a transistor with two collectors
represent and if you can, help me figure out the answers to my two
questions.

The two collector transistors are current mirrors. The area of the two
collectors is in a precise ratio, usually one to one. Therefore the current
through the transistor divides per that ratio. If it's one to one, the
current divides 50%- 50%.

In a current mirror, one collector current is defined by the appropriate
voltage drops and the other collector follows or mirrors that current. This
kind of structure is available within integrated circuits but not available
in discrete devices. You can make discrete current mirrors but not with
divided collector areas.

Apparently the sync input must pulse high to restart the ramp. If you do not
use it, the device will self oscillate asynchronously.

The common mode range of the error amp is 1.5 volts to 5.2 volts to stay
within the specified common mode rejection values. It's in the spec. on page
4.

 

[Shaun]

Hello,

If you look on page 4 of the datasheet, the minimum voltage to operate the
sync line + 1.2 volts. That is because you have to turn on the transistor
(0.7 volts) called bias, and there is a voltage divider in front of it too.
This pin "sync" is for an external oscillator, if you don't want to use the
built in one plus Ct and Rt. You would have to supply it with a signal that
is greater than + 1.2 volts on the peak or add a positive offset voltage to
the signal.

The dual collector trasistor is common for transistors in an IC, don't ask
me why, I don't know what the advantage is.

Page 4 also gives information for the error amplifier.

Shaun

"gearhead" <nospam@billburg.com> wrote in message
news:52b1caa0-d5d2-43ce-906e-a38f46beb9f7@r13g2000vbp.googlegroups.com...

I had a couple of things I wanted to figure out about the SG3525 and
got to looking at the schematics for the oscillator and the error
amplifier at the bottom of page 6 of the datasheet. (I wanted to
figure out how to use the sync pin, since the datasheet doesn't
explain it. And how low the common mode of the error amplifier goes,
since the datasheet only states that it includes Vref, or 5 volts.)
http://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF
What does the symbol that looks like a transistor with two collectors
represent and if you can, help me figure out the answers to my two
questions.

 

[gearhead]

On Jan 31, 7:11 pm, "Bob Eld" <nsmontas...@yahoo.com> wrote:

"gearhead" <nos...@billburg.com> wrote in message

news:52b1caa0-d5d2-43ce-906e-a38f46beb9f7@r13g2000vbp.googlegroups.com...

I had a couple of things I wanted to figure out about the SG3525 and
got to looking at the schematics for the oscillator and the error
amplifier at the bottom of page 6 of the datasheet.  (I wanted to
figure out how to use the sync pin, since the datasheet doesn't
explain it. And how low the common mode of the error amplifier goes,
since the datasheet only states that it includes Vref, or 5 volts.)
http://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF
What does the symbol that looks like a transistor with two collectors
represent and if you can, help me figure out the answers to my two
questions.

The two collector transistors are current mirrors. The area of the two
collectors is in a precise ratio, usually one to one. Therefore the current
through the transistor divides per that ratio. If it's one to one, the
current divides 50%- 50%.

In a current mirror, one collector current is defined by the appropriate
voltage drops and the other collector follows or mirrors that current. This
kind of structure is available within integrated circuits but not available
in discrete devices. You can make discrete current mirrors but not with
divided collector areas.

Okay, I know about current mirrors like Q12 and Q13, now I know that
symbol means something similar.

Apparently the sync input must pulse high to restart the ramp. If you do not
use it, the device will self oscillate asynchronously.

Then a high signal on the sync would have to turn on Q14 in order to
discharge the timing capacitor;
but when I try following the current mirrors around the circuit it
looks like just the opposite happens.
I can settle this by sticking a chip in the breadboard.

The common mode range of the error amp is 1.5 volts to 5.2 volts to stay
within the specified common mode rejection values. It's in the spec. on page
4.

Thanks!

 

[Bob Eld]

"gearhead" <nospam@billburg.com> wrote in message
news:6ce75abd-0c54-45e2-86bc-1389cbb6878d@a39g2000prl.googlegroups.com...
On Jan 31, 7:11 pm, "Bob Eld" <nsmontas...@yahoo.com> wrote:

"gearhead" <nos...@billburg.com> wrote in message

news:52b1caa0-d5d2-43ce-906e-a38f46beb9f7@r13g2000vbp.googlegroups.com...

I had a couple of things I wanted to figure out about the SG3525 and
got to looking at the schematics for the oscillator and the error
amplifier at the bottom of page 6 of the datasheet. (I wanted to
figure out how to use the sync pin, since the datasheet doesn't
explain it. And how low the common mode of the error amplifier goes,
since the datasheet only states that it includes Vref, or 5 volts.)
http://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF
What does the symbol that looks like a transistor with two collectors
represent and if you can, help me figure out the answers to my two
questions.

The two collector transistors are current mirrors. The area of the two
collectors is in a precise ratio, usually one to one. Therefore the
current
through the transistor divides per that ratio. If it's one to one, the
current divides 50%- 50%.

In a current mirror, one collector current is defined by the appropriate
voltage drops and the other collector follows or mirrors that current.
This
kind of structure is available within integrated circuits but not
available
in discrete devices. You can make discrete current mirrors but not with
divided collector areas.

Okay, I know about current mirrors like Q12 and Q13, now I know that
symbol means something similar.

Apparently the sync input must pulse high to restart the ramp. If you do
not
use it, the device will self oscillate asynchronously.

Then a high signal on the sync would have to turn on Q14 in order to
discharge the timing capacitor;
but when I try following the current mirrors around the circuit it
looks like just the opposite happens.
I can settle this by sticking a chip in the breadboard.


Yes I agree it does look weird. I checked the Texas instruments version of
this part, UC3525 and they do NOT show a connection between the collector of
Q13 and the base of Q14. I have not studied it enough to figure out the
actual circuit or who is right. Obviously there is at least one error in at
least one of the schematics. So who knows?

Tell us how the breadboard reacts and then maybe we can make sense of the
schematics.

Keep in mind that the schematics are for reference and are abbreviated and
stylized for simplicity. They are NOT the actual circuitry used with
complete detail.

The common mode range of the error amp is 1.5 volts to 5.2 volts to stay
within the specified common mode rejection values. It's in the spec. on
page
4.

Thanks!

 

[gearhead]

On Feb 2, 12:01 pm, "Bob Eld" <nsmontas...@yahoo.com> wrote:

"gearhead" <nos...@billburg.com> wrote in message

news:6ce75abd-0c54-45e2-86bc-1389cbb6878d@a39g2000prl.googlegroups.com...
On Jan 31, 7:11 pm, "Bob Eld" <nsmontas...@yahoo.com> wrote:





"gearhead" <nos...@billburg.com> wrote in message

news:52b1caa0-d5d2-43ce-906e-a38f46beb9f7@r13g2000vbp.googlegroups.com....

I had a couple of things I wanted to figure out about the SG3525 and
got to looking at the schematics for the oscillator and the error
amplifier at the bottom of page 6 of the datasheet. (I wanted to
figure out how to use the sync pin, since the datasheet doesn't
explain it. And how low the common mode of the error amplifier goes,
since the datasheet only states that it includes Vref, or 5 volts.)
http://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF
What does the symbol that looks like a transistor with two collectors
represent and if you can, help me figure out the answers to my two
questions.

The two collector transistors are current mirrors. The area of the two
collectors is in a precise ratio, usually one to one. Therefore the
current
through the transistor divides per that ratio. If it's one to one, the
current divides 50%- 50%.

In a current mirror, one collector current is defined by the appropriate
voltage drops and the other collector follows or mirrors that current.
This
kind of structure is available within integrated circuits but not
available
in discrete devices. You can make discrete current mirrors but not with
divided collector areas.

Okay, I know about current mirrors like Q12 and Q13, now I know that
symbol means something similar.



Apparently the sync input must pulse high to restart the ramp. If you do
not
use it, the device will self oscillate asynchronously.

Then a high signal on the sync would have to turn on Q14 in order to
discharge the timing capacitor;
but when I try following the current mirrors around the circuit it
looks like just the opposite happens.
I can settle this by sticking a chip in the breadboard.

Yes I agree it does look weird. I checked the Texas instruments version of
this part, UC3525 and they do NOT show a connection between the collector of
Q13 and the base of Q14. I have not studied it enough to figure out the
actual circuit or who is right. Obviously there is at least one error in at
least one of the schematics. So who knows?

Tell us how the breadboard reacts and then maybe we can make sense of the
schematics.

Keep in mind that the schematics are for reference and are abbreviated and
stylized for simplicity. They are NOT the actual circuitry used with
complete detail.





The common mode range of the error amp is 1.5 volts to 5.2 volts to stay
within the specified common mode rejection values. It's in the spec. on
page
4.

Thanks!- Hide quoted text -

- Show quoted text -

The TI schematic is all wet.
Q14 is floating there in space, doing nothing... the base doesn't
connect to anything that can bias it on.
Likewise with the whole discharge circuit: it has no connection to
the components in the timing section.
So the TI diagram is missing connections at the base and emitter of
Q14 that the OnSemi circuit diagram shows.
The OnSemi diagram starts to make sense as I look at it now. I had
the mirrors turned around or something when I looked at it before.
Without going too deep into it--
The 7.4k and 14k resistors set a trip point for the timing ramp, not
far from one timing constant.
At that point the mirrors turn on Q14 (I had it backwards the first
time Iooked). That discharges the timing cap.
Same thing happens if you pull up the sync pin. You were right, Bob.