LM2940ct 5.0 gets HOT quick.
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Hello,
In a circuit I made up to run a Amulet LCD I use the LM2940ct 5.0.
The only things being powered in the circuit is the stamp and the
LCD. The LM2940 gets hot after a few min which isn't nice. The burn
your finger kind of hot. I have a 22uf cap between the output and
ground as specified in the datasheet for stability.
Is ths just because of the load placed on it due to the lcd? Should I
use some other voltage regulator?
Thanks,
John
In a circuit I made up to run a Amulet LCD I use the LM2940ct 5.0.
The only things being powered in the circuit is the stamp and the
LCD. The LM2940 gets hot after a few min which isn't nice. The burn
your finger kind of hot. I have a 22uf cap between the output and
ground as specified in the datasheet for stability.
Is ths just because of the load placed on it due to the lcd? Should I
use some other voltage regulator?
Thanks,
John
Comments
What is the input voltage to the regulator -- can you measure it? The power
(in Watts) being dissipated within the regulator is the voltage drop across
the regulator times the current passing through it. In other words: (Vin -
Vout) Iout = W
If your Vin is high, you can develop significant heat even without a lot of
output current. ...Do you know about how much output current you have?
Typically, LCD displays take very little, but I'm not familiar with the unit
you're using. Does it have a backlight?
There is another possibility... Did you see the part in the LM2940CT data
sheet about the output capacitor ESR and potential oscillation? Did you
take steps to obtain caps with appropriate ESR? I haven't experienced it
personally, but you could have an issue there that might result in
overheating. Here's a snip from p. 10 of the National data sheet:
MINIMUM CAPACITANCE:
The minimum output capacitance required to maintain stability
is 22 µF (this value may be increased without limit).
Larger values of output capacitance will give improved transient
response.
ESR LIMITS:
The ESR of the output capacitor will cause loop instability if
it is too high or too low. The acceptable range of ESR plotted
versus load current is shown in the graph below. It is essential
that the output capacitor meet these requirements,
or oscillations can result.
If you aren't running very much current, and your Vin isn't too high, a
small clip-on heat sink can significantly reduce the regulator's
temperature.
Randy
www.glitchbuster.com
> In a circuit I made up to run a Amulet LCD I use the LM2940ct 5.0.
> The only things being powered in the circuit is the stamp and the
> LCD. The LM2940 gets hot after a few min which isn't nice. The burn
> your finger kind of hot. I have a 22uf cap between the output and
> ground as specified in the datasheet for stability.
>
> Is ths just because of the load placed on it due to the lcd? Should I
> use some other voltage regulator?
very hot. BTDT.
> What is the input voltage to the regulator -- can you measure it? The power
> (in Watts) being dissipated within the regulator is the voltage drop across
> the regulator times the current passing through it. In other words: (Vin -
> Vout) Iout = W
>
> If your Vin is high, you can develop significant heat even without a lot of
> output current. ...Do you know about how much output current you have?
> Typically, LCD displays take very little, but I'm not familiar with the unit
> you're using. Does it have a backlight?
>
> There is another possibility... Did you see the part in the LM2940CT data
> sheet about the output capacitor ESR and potential oscillation? Did you
> take steps to obtain caps with appropriate ESR? I haven't experienced it
> personally, but you could have an issue there that might result in
> overheating. Here's a snip from p. 10 of the National data sheet:
>
> MINIMUM CAPACITANCE:
>
> The minimum output capacitance required to maintain stability
>
> is 22 µF (this value may be increased without limit).
>
> Larger values of output capacitance will give improved transient
>
> response.
>
> ESR LIMITS:
>
> The ESR of the output capacitor will cause loop instability if
>
> it is too high or too low. The acceptable range of ESR plotted
>
> versus load current is shown in the graph below. It is essential
>
> that the output capacitor meet these requirements,
>
> or oscillations can result.
>
>
> If you aren't running very much current, and your Vin isn't too high, a
> small clip-on heat sink can significantly reduce the regulator's
> temperature.
>
> > In a circuit I made up to run a Amulet LCD I use the LM2940ct 5.0.
> > The only things being powered in the circuit is the stamp and the
> > LCD. The LM2940 gets hot after a few min which isn't nice. The burn
> > your finger kind of hot. I have a 22uf cap between the output and
> > ground as specified in the datasheet for stability.
> >
> > Is ths just because of the load placed on it due to the lcd? Should I
> > use some other voltage regulator?
All I have is a cap between the output and ground at 22uf. What are
the caps on the BS2P40 demo board? I can't find a schematic of it.
That would work alright. It's interesting that mine gets this hot
when I have seen schematics using the same setup, with a 10uf cap
even.
My input is 9V, I'm using the power supply that came with the P40
demo board.
John
and at minimum a 10 uF between the output and ground, both as close to
the regulator as possible.
Keep in mind the adapter you have may be putting out more than 9 volts,
or may not have a very clean output, both of which will make the
regulator get hot.
Also, remember the more voltage you have to drop and the more current
you want to pull, the hotter the device will run.
> All I have is a cap between the output and ground at 22uf. What are
> the caps on the BS2P40 demo board? I can't find a schematic of it.
> That would work alright. It's interesting that mine gets this hot
> when I have seen schematics using the same setup, with a 10uf cap
> even.
>
> My input is 9V, I'm using the power supply that came with the P40
> demo board.
> I believe the data sheets show a 0.1 uF between the input and ground,
> and at minimum a 10 uF between the output and ground, both as close to
> the regulator as possible.
>
> Keep in mind the adapter you have may be putting out more than 9 volts,
> or may not have a very clean output, both of which will make the
> regulator get hot.
>
> Also, remember the more voltage you have to drop and the more current
> you want to pull, the hotter the device will run.
>
> > All I have is a cap between the output and ground at 22uf. What are
> > the caps on the BS2P40 demo board? I can't find a schematic of it.
> > That would work alright. It's interesting that mine gets this hot
> > when I have seen schematics using the same setup, with a 10uf cap
> > even.
> >
> > My input is 9V, I'm using the power supply that came with the P40
> > demo board.
backlights?
--- In basicstamps@yahoogroups.com, Bill Boyer <daweasel@s...> wrote:
> Another data sheet says 0.47 uF and 22 uF. Go figure.
>
> > I believe the data sheets show a 0.1 uF between the input and
ground,
> > and at minimum a 10 uF between the output and ground, both as
close to
> > the regulator as possible.
> >
> > Keep in mind the adapter you have may be putting out more than 9
volts,
> > or may not have a very clean output, both of which will make the
> > regulator get hot.
> >
> > Also, remember the more voltage you have to drop and the more
current
> > you want to pull, the hotter the device will run.
> >
> > > All I have is a cap between the output and ground at 22uf. What
are
> > > the caps on the BS2P40 demo board? I can't find a schematic of
it.
> > > That would work alright. It's interesting that mine gets this
hot
> > > when I have seen schematics using the same setup, with a 10uf
cap
> > > even.
> > >
> > > My input is 9V, I'm using the power supply that came with the
P40
> > > demo board.
i bet your 9v adaptor is pumping to much voltage in it
should be something like 6.5 to 7.5 going in
i would just replace it with a 7805 it will run cooler
off 9v good luck
--- Bill Boyer <daweasel@s...> wrote:
> Another data sheet says 0.47 uF and 22 uF. Go
> figure.
>
> > I believe the data sheets show a 0.1 uF between
> the input and ground,
> > and at minimum a 10 uF between the output and
> ground, both as close to
> > the regulator as possible.
> >
> > Keep in mind the adapter you have may be putting
> out more than 9 volts,
> > or may not have a very clean output, both of which
> will make the
> > regulator get hot.
> >
> > Also, remember the more voltage you have to drop
> and the more current
> > you want to pull, the hotter the device will run.
> >
> > > All I have is a cap between the output and
> ground at 22uf. What are
> > > the caps on the BS2P40 demo board? I can't find
> a schematic of it.
> > > That would work alright. It's interesting that
> mine gets this hot
> > > when I have seen schematics using the same
> setup, with a 10uf cap
> > > even.
> > >
> > > My input is 9V, I'm using the power supply that
> came with the P40
> > > demo board.
>
>
>
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and also a 7805 to compare performance.
Thanks,
John
--- In basicstamps@yahoogroups.com, kenneth magers
<kenneth_m_73149@y...> wrote:
> since the lm2940 is a low voltage drop out regulator
> i bet your 9v adaptor is pumping to much voltage in it
> should be something like 6.5 to 7.5 going in
> i would just replace it with a 7805 it will run cooler
> off 9v good luck
> --- Bill Boyer <daweasel@s...> wrote:
> > Another data sheet says 0.47 uF and 22 uF. Go
> > figure.
> >
> > > I believe the data sheets show a 0.1 uF between
> > the input and ground,
> > > and at minimum a 10 uF between the output and
> > ground, both as close to
> > > the regulator as possible.
> > >
> > > Keep in mind the adapter you have may be putting
> > out more than 9 volts,
> > > or may not have a very clean output, both of which
> > will make the
> > > regulator get hot.
> > >
> > > Also, remember the more voltage you have to drop
> > and the more current
> > > you want to pull, the hotter the device will run.
> > >
> > > > All I have is a cap between the output and
> > ground at 22uf. What are
> > > > the caps on the BS2P40 demo board? I can't find
> > a schematic of it.
> > > > That would work alright. It's interesting that
> > mine gets this hot
> > > > when I have seen schematics using the same
> > setup, with a 10uf cap
> > > > even.
> > > >
> > > > My input is 9V, I'm using the power supply that
> > came with the P40
> > > > demo board.
> >
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed.
> > Text in the Subject and Body of the message will be
> > ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
> > http://docs.yahoo.com/info/terms/
> >
> >
>
>
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> New Yahoo! Photos - easier uploading and sharing.
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minimize oscillation. Furthermore, hidden in the ap note (the ones that I
have seen) is an admonition to have a minimum current load. Otherwise the
output impedance is too high and it oscillates. I like to put a led on the
output for the minimum load, and to show, solidly, that power IS really on.
Original Message
From: Bill Boyer [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=L8V5XDx_MJO2IjlkfK3rdbtirqFNZYAcfQb3yFdnwhP6eSNLV8hYjH1ibYvkzlmJsAf90LgucOOgSyFEa90]daweasel@s...[/url
Sent: Friday, December 05, 2003 10:48 PM
To: Basic Stamp List
Subject: Re: [noparse][[/noparse]basicstamps] Re: LM2940ct 5.0 gets HOT quick.
I believe the data sheets show a 0.1 uF between the input and ground, and at
minimum a 10 uF between the output and ground, both as close to the
regulator as possible.
Keep in mind the adapter you have may be putting out more than 9 volts, or
may not have a very clean output, both of which will make the regulator get
hot.
Also, remember the more voltage you have to drop and the more current you
want to pull, the hotter the device will run.
> All I have is a cap between the output and ground at 22uf. What are
> the caps on the BS2P40 demo board? I can't find a schematic of it.
> That would work alright. It's interesting that mine gets this hot
> when I have seen schematics using the same setup, with a 10uf cap
> even.
>
> My input is 9V, I'm using the power supply that came with the P40
> demo board.
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
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