Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

A start-up circuit for a voltage reference

Status
Not open for further replies.

vadim888

Member level 2
Joined
Feb 4, 2013
Messages
46
Helped
5
Reputation
10
Reaction score
5
Trophy points
1,288
Activity points
1,596
Dear All,

sorry, a new question...

I just need a small hit, because I can't fully understand how that (fig bellow) start-up circuit is working (three transistors on the left side):

vrvr.png

Best regards,
Vadim
 

erikl

Super Moderator
Staff member
Joined
Sep 9, 2008
Messages
8,112
Helped
2,690
Reputation
5,360
Reaction score
2,291
Trophy points
1,393
Location
Germany
Activity points
44,083
Vref.png
At start, Vref=0, so M3 is closed, long-length weak PFET M1 is open, and M2 is opened, too. The drain of M2 is at GND potential, opening the 3 PFET current sources. Then the Vref circuit goes starting to work, the OpAmp - with its low output impedance easily winning against the weaker M2 - reaches its correct operation point, so does Vref, which opens M3, which then wins against the weak M1 and draws M2's gate potential down to GND. M2 is open now, and the 3 PFETs' bias is fully determined by the opAmp.

No current through M2 any more now, but always cross current through M1-M3. That's why M1 should be rather weak (very long, i.e. W/L≪1).
 

vadim888

Member level 2
Joined
Feb 4, 2013
Messages
46
Helped
5
Reputation
10
Reaction score
5
Trophy points
1,288
Activity points
1,596
I should buy you a beer someday, thank you!
 

vadim888

Member level 2
Joined
Feb 4, 2013
Messages
46
Helped
5
Reputation
10
Reaction score
5
Trophy points
1,288
Activity points
1,596
So, I have an issue with that start-up circuit... it's oscillates. However, if I increase a width of the M1, hence and the current, it works good. But the current consumption is around 1mA...

Does anyone has a clue why I have oscillations in my start-up circuit?
 

erikl

Super Moderator
Staff member
Joined
Sep 9, 2008
Messages
8,112
Helped
2,690
Reputation
5,360
Reaction score
2,291
Trophy points
1,393
Location
Germany
Activity points
44,083
Instead of increasing the width of M1 (it should be a long transistor anyway, s. below), try reducing the width of M3.

Oscillation originates from not enough gain in the (positive) feedback loop during the switch-off period. Also try increasing the width of M2 - so increasing this gain. M2 is totally off after start, so no current through it, then. Only M1 - M3 leads current after startUp, which is determined by the W/L ratio of M1. That's why M1 usually is a very long transistor (W/L « 1).
 

dick_freebird

Advanced Member level 5
Joined
Mar 4, 2008
Messages
7,352
Helped
2,138
Reputation
4,281
Reaction score
2,003
Trophy points
1,393
Location
USA
Activity points
58,908
The trick with DC startup circuits is to get them to be always
effective, but also always "shut up" once the output is up at
its proper value. This depends to some extent on the loop amp's
gain at lower than normal output voltage (amp must "snap in"
nicely). By the time the output reaches designed setpoint, the
output voltage must be enough to make the startup circuit
quit pushing current at the P mirror rack altogether. If amp
gain or common mode range is inadequate then you may see
oscillation (if weak @ corner) or tempco issues (if strong @
corner) from startup current continuing to bleed into the
loop.
 

vadim888

Member level 2
Joined
Feb 4, 2013
Messages
46
Helped
5
Reputation
10
Reaction score
5
Trophy points
1,288
Activity points
1,596
That's why M1 usually is a very long transistor (W/L « 1).

but it will increase the static power consumption of the circuit. SO maybe I should try another start-up circuit?

- - - Updated - - -

If amp
gain or common mode range is inadequate then you may see
oscillation (if weak @ corner) or tempco issues (if strong @
corner) from startup current continuing to bleed into the
loop.

you were right, the opamp's gain was low. Thank you for your advise.
 

erikl

Super Moderator
Staff member
Joined
Sep 9, 2008
Messages
8,112
Helped
2,690
Reputation
5,360
Reaction score
2,291
Trophy points
1,393
Location
Germany
Activity points
44,083
That's why M1 usually is a very long transistor (W/L « 1).

but it will increase the static power consumption of the circuit. SO maybe I should try another start-up circuit?

In contrary: Long transistors (W/L « 1) provide low currents (low static power consumption).
 
Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top