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MC34063 with small inductors

J

Joerg

Hello Folks,

Have to correct a design with a 34063 in there. Buck, VIN 16-30V, VOUT
slightly under 12V, 400mA max output. And no, I don't like it much :)

One of the issues is a saturating inductor. Can't change the layout and
to be able to cram one in there I probably have to drop down to 47uH,
maybe 68uH if lucky.

SPICE says it's ok but none of the datasheets or app notes recommends
going that low. Any reason why not?
 
T

tm

Just stick a CD4060 in there. That oughta fix it ;-)

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon athttp://www.analog-innovations.com| 1962 |

The only thing bipartisan in this country is hypocrisy
Click to expand...

Won't a 555 do the trick?


I think it needs an ideal diode. :)
 
S

Spehro Pefhany

Hello Folks,

Have to correct a design with a 34063 in there. Buck, VIN 16-30V, VOUT
slightly under 12V, 400mA max output. And no, I don't like it much :)

One of the issues is a saturating inductor. Can't change the layout and
to be able to cram one in there I probably have to drop down to 47uH,
maybe 68uH if lucky.

SPICE says it's ok but none of the datasheets or app notes recommends
going that low. Any reason why not?
Click to expand...

Too slow. IIRC there's somewhat faster (and more expensive) parts that
might be pin-compatible at OnSemi.


Best regards,
Spehro Pefhany
 
G

Grant

Hello Folks,

Have to correct a design with a 34063 in there. Buck, VIN 16-30V, VOUT
slightly under 12V, 400mA max output. And no, I don't like it much :)

One of the issues is a saturating inductor. Can't change the layout and
to be able to cram one in there I probably have to drop down to 47uH,
maybe 68uH if lucky.

SPICE says it's ok but none of the datasheets or app notes recommends
going that low. Any reason why not?
Click to expand...

Try the NCP3063 from On Semi, it's a faster pin compatible for 34063

Grant.
 
J

Joerg

John said:
What's the frequency? It's spec'd for 100K max.
Click to expand...

During ramp-up it runs around 130kHz, then drops to around 30kHz
depending on load. There ain't much flexibility because of the way the
34063 comparator works. It can issue a cycle but it can never stop one.
That's why it behaves like one of those old aircraft rotary engines
during landing, rat-tat-tattatta-tat ... poof ... rat-tat-tat ...

It's a pretty crude PWM chip but what can ya do?

Those old things are slow. With 24 in, 12 out, on time is ~5 usec at
100 KHz. Ripple current into 47 uH is 1.25 A p-p. Not good. It will go
seriously discontinuous. Does Spice really approve?
Click to expand...

These things run discontinuous nearly all the time. That would be ok.

If you can't change the layout, push the frequency above spec and go
with 68 uH maybe. Best of luck.

Or kluge in one of the new National packaged switchers, the dpak ones
with inductors and all.
Click to expand...

Can't do a relayout on this one, else I'd already be home :-(
 
J

Joerg

Grant said:
Try the NCP3063 from On Semi, it's a faster pin compatible for 34063
Click to expand...

Thanks! Technically I can't change it but if needed it's needed. It's
only 50% faster but maybe that helps here.
 
J

Joerg

With 30v in, 12v out, that's 18v across 47uH, producing 380mA/uS thru
the inductor.

So, for a 400mA load, you'll need a 1uS on-time, multiplied by 1/(duty
factor) to yield 400mA average.

At 100KHz you could charge the inductor 0-to-1.2A in roughly 3uS, let
it discharge 1.2A-to-zero in 4.5uS, stand 2.5uS at zero current, and
get an average of 400mA. The lighter the load, the shorter the on-
time needed, and the faster the switcher will have to be.

That might barely work. Sure is ugly.
Click to expand...

The 34063 doesn't quite work that way, it is not a fixed frequency
device. When the output is above threshold it just stops for a while.
When it falls the comparator issue another cycle. Since the oscillor
itself does run at fixed frequency (unless the current limit comes on)
it starts the conduction somewhere, sometimes in the middle of a cycle.
That results in rather ugly looking ripple.

Inductance goes with turns^2 while saturation goes with ampere-turns,
so higher inductance works much to your favor. If you can keep the
thing continuous, that lowers peak currents in the inductor, and
saturation concerns with it.
Click to expand...

If I could just find one that fits into the cramped space :-(

Or just get a faster part.
Click to expand...

The NCP3063 that others have suggested might work but it's not
breathtakingly faster, just by 50%.
Because it's marginal timing-wise, places high stress on the inductor
and switch, and is likely inefficient.
Click to expand...

Inductor stress would be ok, just has to stomach the current. I do get
1.5A peaks which pushes the 34063 a bit but dissipation is "only" around
500mW.
 
J

Joerg

Jim said:
Just stick a CD4060 in there. That oughta fix it ;-)
Click to expand...

Hold your coffee, but I did one with a CD40106. Still in mass
production, 15 years and counting. That was probably the lowest cost
switcher I ever did. Pretty darn efficient as well, nothing gets hot.

Oh, and this was one of the many cases where the client already had a
design but had a Maxim chip in there. Which, predictably, turned into
unobtanium. That called for the big knife.
 
J

Joerg

Tim said:
I think that one of the last things I'd trust SPICE for would be as an
indication that a chip really could run faster than its rated speed, at
least if the chip model came from the vendor.
Click to expand...

The model is from ON Semi and I am not running the chip above its rated
speed while in regulation. It's actually working (on the simulator) with
a 68uH inductance and well within normal timing limits. All I was
wondering is why the datasheets and app notes always prescribe higher
inductance values.

But you are right about models being suspicious. It does one thing it
ain't supposed to be doing and I still need to get to the ground of that
via the LTSpice group (this sim was with 200uH though):

http://www.analogconsultants.com/ng/images/MC34063_1.png

App notes and the internal schematic of the chip says that the feedback
comparator can never terminate the switch during a cycle (positive dark
blue ramp). And here it does just that ...
 
W

Winfield Hill

Jim Thompson wrote...
Is the model in text format or is it encrypted?
Click to expand...

The Intusoft version is in plain text. I think this
is it, along with an X1 call. It's not exactly what
I expected to see. E.g., who's Kehinde Omolayo? Hmm,
Magnetico, Inc. Hmm, 2003, "Standex International Corp.
has purchased substantially all of the assets..."; the
trail grows cold.

X1 7 4 5 0 8 6 7 7 MC34063AX2 { }
..SUBCKT MC34063AX2 1 2 3 4 5 6 7 8
*exempt 20030227 20395 -30736294
*BY KEHINDE OMOLAYO 2-20-03
*TERMINAL ID
*SWITCH COLLECTOR=1 SWITCH EMITTER=2 TIMING CAPACITOR=3 GND=4
*COMPARATOR INVERTING INPUT=5 VCC=6 IPK SENSE=7 DRIVER COLLECTOR=8
..MODEL DMC34063 D (CJO=2P N=0.05)
..MODEL QSWITCH NPN BF=75 CJC=2P IS=3E-9 RB=1 RC=0.45 RE=0
+ VJC=.75 VJE=.75 VJS=.75
E1 10 0 5 4 1
R1 4 5 10MEG
V1 20 0 PULSE 0 2
E2 11 0 3 4 1
B1 13 0 V=1M/(ABS((27.475-195M*V(12))+(36.002+244M*V(12))*V(9)-
+(302.302+651M*V(12))*V(9)^2)+1F)
E3 12 0 6 4 1
E4 9 0 6 7 1
R2 6 7 10MEG
B2 14 0 V=1M*((-10.765-151M*V(12))+(45.344+864M*V(12))*V(9)-
+(35.99+1.378*V(12))*V(9)^2+(8.341+839M*V(12))*V(9)^3)
B4 15 0 V=V(9)>0.32 ? V(14) : V(13)
C1 19 0 10P
B5 16 0 V= V(20)<1 ? 2 : V(24)>1 ? 2 : V(19)>1 ? 0 : 2
R3 16 17 150
C2 17 0 10P
B6 18 0 V=V(20)<1 ? 0 : V(24)>1 ? 0 : V(11)<(1.083-1.239*V(29)) ? 2 :
+ V(17)>1 ? 0 : 2
R4 18 19 150
D1 4 3 DMC34063
D2 3 6 DMC34063
C3 11 31 1N
B7 4 36 I=V(17)>1 ? -(224.4U+2.359U*V(12))*0.77 : V(15)*0.77
C5 23 0 10P
B9 21 0 V= V(20)<1 ? 2 : V(17)>1 ? 2 : V(26)>1 ? 0 : 2
R5 27 26 150
C6 26 0 10P
B10 27 0 V=V(20)<1 ? 0 : V(17)>1 ? 0 : V(10)<1.25 ? 2 : V(23)>1 ? 0 : 2
R6 21 23 150
B12 33 0 V=V(11)>(1.148+184.6M*V(29)) ? 2 : 0
R13 33 24 10K
C8 24 0 10P
Q1 8 30 25 QSWITCH
Q2 1 25 2 QSWITCH
R15 25 2 100
D5 2 30 DMC34063
G1 2 30 26 23 5M
R16 2 4 10MEG
R23 31 28 1M
V6 28 0
V7 36 3
B13 29 0 V=I(V6)/(I(V7)+866.8M*I(V6))
..ENDS
 
J

Joerg

Winfield said:
Jim Thompson wrote...

The Intusoft version is in plain text. I think this
is it, along with an X1 call. It's not exactly what
I expected to see. E.g., who's Kehinde Omolayo? Hmm,
Magnetico, Inc. Hmm, 2003, "Standex International Corp.
has purchased substantially all of the assets..."; the
trail grows cold.

X1 7 4 5 0 8 6 7 7 MC34063AX2 { }
.SUBCKT MC34063AX2 1 2 3 4 5 6 7 8
*exempt 20030227 20395 -30736294
*BY KEHINDE OMOLAYO 2-20-03
*TERMINAL ID
*SWITCH COLLECTOR=1 SWITCH EMITTER=2 TIMING CAPACITOR=3 GND=4
*COMPARATOR INVERTING INPUT=5 VCC=6 IPK SENSE=7 DRIVER COLLECTOR=8
.MODEL DMC34063 D (CJO=2P N=0.05)
.MODEL QSWITCH NPN BF=75 CJC=2P IS=3E-9 RB=1 RC=0.45 RE=0
+ VJC=.75 VJE=.75 VJS=.75
E1 10 0 5 4 1
R1 4 5 10MEG
V1 20 0 PULSE 0 2
E2 11 0 3 4 1
B1 13 0 V=1M/(ABS((27.475-195M*V(12))+(36.002+244M*V(12))*V(9)-
+(302.302+651M*V(12))*V(9)^2)+1F)
E3 12 0 6 4 1
E4 9 0 6 7 1
R2 6 7 10MEG
B2 14 0 V=1M*((-10.765-151M*V(12))+(45.344+864M*V(12))*V(9)-
+(35.99+1.378*V(12))*V(9)^2+(8.341+839M*V(12))*V(9)^3)
B4 15 0 V=V(9)>0.32 ? V(14) : V(13)
C1 19 0 10P
B5 16 0 V= V(20)<1 ? 2 : V(24)>1 ? 2 : V(19)>1 ? 0 : 2
R3 16 17 150
C2 17 0 10P
B6 18 0 V=V(20)<1 ? 0 : V(24)>1 ? 0 : V(11)<(1.083-1.239*V(29)) ? 2 :
+ V(17)>1 ? 0 : 2
R4 18 19 150
D1 4 3 DMC34063
D2 3 6 DMC34063
C3 11 31 1N
B7 4 36 I=V(17)>1 ? -(224.4U+2.359U*V(12))*0.77 : V(15)*0.77
C5 23 0 10P
B9 21 0 V= V(20)<1 ? 2 : V(17)>1 ? 2 : V(26)>1 ? 0 : 2
R5 27 26 150
C6 26 0 10P
B10 27 0 V=V(20)<1 ? 0 : V(17)>1 ? 0 : V(10)<1.25 ? 2 : V(23)>1 ? 0 : 2
R6 21 23 150
B12 33 0 V=V(11)>(1.148+184.6M*V(29)) ? 2 : 0
R13 33 24 10K
C8 24 0 10P
Q1 8 30 25 QSWITCH
Q2 1 25 2 QSWITCH
R15 25 2 100
D5 2 30 DMC34063
G1 2 30 26 23 5M
R16 2 4 10MEG
R23 31 28 1M
V6 28 0
V7 36 3
B13 29 0 V=I(V6)/(I(V7)+866.8M*I(V6))
.ENDS
Click to expand...

There's actually three more models around but they essentially behave
similar. Thing is, the chip is not supposed to turn off the switch
during the positive ramp because the comparator does not tie into the
reset input of the register, only the set input.

<scratching_ head>
 
W

Winfield Hill

Joerg wrote...
[ snip ]
There's actually three more models around but they essentially
behave similar. Thing is, the chip is not supposed to turn off
the switch during the positive ramp because the comparator does
not tie into the reset input of the register, only the set input.

<scratching_ head>
Click to expand...

What, you analyzed it in two minutes? I have to
make a schematic drawing from the spice net list,
and analyze that. <scratching_ head>
 
J

Joerg

Winfield said:
Joerg wrote...
[ snip ]
There's actually three more models around but they essentially
behave similar. Thing is, the chip is not supposed to turn off
the switch during the positive ramp because the comparator does
not tie into the reset input of the register, only the set input.

<scratching_ head>
Click to expand...

What, you analyzed it in two minutes? I have to
make a schematic drawing from the spice net list,
and analyze that. <scratching_ head>
Click to expand...


No, I am scratching my head about this a bit longer than two minutes,
tried various models, etc. :)
 
J

Joerg

Yeah. I read TI's app note. So it can start an ON cycle any time
during ramp up, but can't ever stop until ramp-down. Crude. Burp,
sputter, and cough, in this application.
Click to expand...

Well, if it did that I'd be happy. But it does cut during ramp-down and
according to the internal wire-up it's not supposed to. Helmut
Sennewald, one of the LTSpice gurus, could not reproduce the error I
saw. Sent him my file, so we'll see.
 
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