Hello,
I wish to migrate my legacy designs from 8051 to LPC 1768. I am not clear from
the datasheet of LPC1768 what total current can I sink on port pins individually
and collectively. Currently I can drive optocouplers like PC817 etc directly
with 8051 port sinking more than 5 ma each pin. Shall I need to interface
through some buffers like 74ACT245 or such to meet that requirement of current?
Request experienced design experts to clarify.
Mahesh Vyas
Current ratings of LPC 1768 Port Pins.
Started by ●October 16, 2011
Reply by ●October 16, 20112011-10-16
--- In l..., "Mahesh Vyas" wrote:
>
> Hello,
> I wish to migrate my legacy designs from 8051 to LPC 1768. I am not clear from the datasheet of LPC1768 what total current can I sink on port pins individually and collectively. Currently I can drive optocouplers like PC817 etc directly with 8051 port sinking more than 5 ma each pin. Shall I need to interface through some buffers like 74ACT245 or such to meet that requirement of current? Request experienced design experts to clarify.
> Mahesh Vyas
>
The graphs of pin current versus voltage (Figures 11 & 12 in section 10.3 of the datasheet) seem pretty specific.
Table 7 shows a recommended source and sink on each pin of 4 mA. It also lists the short circuit source and sink current at -45 and +50 mA as long as total package current isn't exceeded.
So, basically, you design for <= 4 mA per pin paying attention to the limiting values of Table 5 for total supply and ground current as well as total heat dissipation.
Richard
>
> Hello,
> I wish to migrate my legacy designs from 8051 to LPC 1768. I am not clear from the datasheet of LPC1768 what total current can I sink on port pins individually and collectively. Currently I can drive optocouplers like PC817 etc directly with 8051 port sinking more than 5 ma each pin. Shall I need to interface through some buffers like 74ACT245 or such to meet that requirement of current? Request experienced design experts to clarify.
> Mahesh Vyas
>
The graphs of pin current versus voltage (Figures 11 & 12 in section 10.3 of the datasheet) seem pretty specific.
Table 7 shows a recommended source and sink on each pin of 4 mA. It also lists the short circuit source and sink current at -45 and +50 mA as long as total package current isn't exceeded.
So, basically, you design for <= 4 mA per pin paying attention to the limiting values of Table 5 for total supply and ground current as well as total heat dissipation.
Richard
Reply by ●October 16, 20112011-10-16
On 10/16/2011 1:03 AM, Mahesh Vyas wrote:
> Hello, I wish to migrate my legacy designs from 8051 to LPC 1768. I
> am not clear from the datasheet of LPC1768 what total current can I
> sink on port pins individually and collectively. Currently I can
> drive optocouplers like PC817 etc directly with 8051 port sinking
> more than 5 ma each pin. Shall I need to interface through some
> buffers like 74ACT245 or such to meet that requirement of current?
> Request experienced design experts to clarify.
Richard has already answered the question of how much current drive is
available. I'll just add that I've never been comfortable using the
micro's pins has high current drive pins especially with the higher
integration parts.
I usually use small discrete FETs for their flexibility, current
capability and voltage withstand. My favourite at the moment is the
BSS123, better than 100mA drive and 100V breakdown. More robust but
also more expensive than a 245 if you need 8, cheaper for one. And you
are not limited to Vcc as the power rail (I am quit often switching 15V
to the actual output peripheral so it acts as a level changer as well.
Robert
--
http://www.aeolusdevelopment.com/
From the Divided by a Common Language File (Edited to protect the guilty)
ME - "I'd like to get Price and delivery for connector Part # XXXXX"
Dist./Rep - "$X.XX Lead time 37 days"
ME - "Anything we can do about lead time? 37 days seems a bit high."
Dist./Rep - "that is the lead time given because our stock is live....
we currently have stock."
> Hello, I wish to migrate my legacy designs from 8051 to LPC 1768. I
> am not clear from the datasheet of LPC1768 what total current can I
> sink on port pins individually and collectively. Currently I can
> drive optocouplers like PC817 etc directly with 8051 port sinking
> more than 5 ma each pin. Shall I need to interface through some
> buffers like 74ACT245 or such to meet that requirement of current?
> Request experienced design experts to clarify.
Richard has already answered the question of how much current drive is
available. I'll just add that I've never been comfortable using the
micro's pins has high current drive pins especially with the higher
integration parts.
I usually use small discrete FETs for their flexibility, current
capability and voltage withstand. My favourite at the moment is the
BSS123, better than 100mA drive and 100V breakdown. More robust but
also more expensive than a 245 if you need 8, cheaper for one. And you
are not limited to Vcc as the power rail (I am quit often switching 15V
to the actual output peripheral so it acts as a level changer as well.
Robert
--
http://www.aeolusdevelopment.com/
From the Divided by a Common Language File (Edited to protect the guilty)
ME - "I'd like to get Price and delivery for connector Part # XXXXX"
Dist./Rep - "$X.XX Lead time 37 days"
ME - "Anything we can do about lead time? 37 days seems a bit high."
Dist./Rep - "that is the lead time given because our stock is live....
we currently have stock."
Reply by ●October 17, 20112011-10-17
On Monday 17 Oct 2011 01:37:57 Robert Adsett wrote:
> On 10/16/2011 1:03 AM, Mahesh Vyas wrote:
> > Hello, I wish to migrate my legacy designs from 8051 to LPC 1768. I
> > am not clear from the datasheet of LPC1768 what total current can I
> > sink on port pins individually and collectively. Currently I can
> > drive optocouplers like PC817 etc directly with 8051 port sinking
> > more than 5 ma each pin. Shall I need to interface through some
> > buffers like 74ACT245 or such to meet that requirement of current?
> > Request experienced design experts to clarify.
>
> Richard has already answered the question of how much current drive is
> available. I'll just add that I've never been comfortable using the
> micro's pins has high current drive pins especially with the higher
> integration parts.
>
> I usually use small discrete FETs for their flexibility, current
> capability and voltage withstand. My favourite at the moment is the
> BSS123, better than 100mA drive and 100V breakdown. More robust but
> also more expensive than a 245 if you need 8, cheaper for one. And you
> are not limited to Vcc as the power rail (I am quit often switching 15V
> to the actual output peripheral so it acts as a level changer as well.
With 74ls245 you are restricted to 5v. But it is ideal for multiple driver
situation. Parallel a few outputs for drive currents >25 ma per pin. I
recommend paralleling outputs if current drive is more than 20ma per pin.
> On 10/16/2011 1:03 AM, Mahesh Vyas wrote:
> > Hello, I wish to migrate my legacy designs from 8051 to LPC 1768. I
> > am not clear from the datasheet of LPC1768 what total current can I
> > sink on port pins individually and collectively. Currently I can
> > drive optocouplers like PC817 etc directly with 8051 port sinking
> > more than 5 ma each pin. Shall I need to interface through some
> > buffers like 74ACT245 or such to meet that requirement of current?
> > Request experienced design experts to clarify.
>
> Richard has already answered the question of how much current drive is
> available. I'll just add that I've never been comfortable using the
> micro's pins has high current drive pins especially with the higher
> integration parts.
>
> I usually use small discrete FETs for their flexibility, current
> capability and voltage withstand. My favourite at the moment is the
> BSS123, better than 100mA drive and 100V breakdown. More robust but
> also more expensive than a 245 if you need 8, cheaper for one. And you
> are not limited to Vcc as the power rail (I am quit often switching 15V
> to the actual output peripheral so it acts as a level changer as well.
With 74ls245 you are restricted to 5v. But it is ideal for multiple driver
situation. Parallel a few outputs for drive currents >25 ma per pin. I
recommend paralleling outputs if current drive is more than 20ma per pin.
Reply by ●October 17, 20112011-10-17
--- In l..., Robert Adsett wrote:
>>
> I usually use small discrete FETs for their flexibility, current
> capability and voltage withstand. My favourite at the moment is the
> BSS123, better than 100mA drive and 100V breakdown. More robust but
> also more expensive than a 245 if you need 8, cheaper for one. And you
> are not limited to Vcc as the power rail (I am quit often switching 15V
> to the actual output peripheral so it acts as a level changer as well.
>
> Robert
>
Robert,
I would like to use the small FET as well. How are you driving the gate from a 3.3V uC?
Richard
>>
> I usually use small discrete FETs for their flexibility, current
> capability and voltage withstand. My favourite at the moment is the
> BSS123, better than 100mA drive and 100V breakdown. More robust but
> also more expensive than a 245 if you need 8, cheaper for one. And you
> are not limited to Vcc as the power rail (I am quit often switching 15V
> to the actual output peripheral so it acts as a level changer as well.
>
> Robert
>
Robert,
I would like to use the small FET as well. How are you driving the gate from a 3.3V uC?
Richard
Reply by ●October 17, 20112011-10-17
On 10/17/2011 9:03 AM, rtstofer wrote:
> I would like to use the small FET as well. How are you driving the
> gate from a 3.3V uC?
That can be just about as simple as you want Richard.
The simplest is just to connect the gate directly to the micro's output
pin. With a BSS123 that typically will give somewhere around 7 or 8
ohms on resistance. The output impedance of the micro acts as a more
than adequate gate resistance.
From there you can get more sophisticated depending on your needs. Add
pull-ups or pull downs to enforce starting state if the micro pins start
as inputs.
Add gate resistors to either slow transitions or protect against either
failure or shoot through Miller capacitance.
Etc. etc. etc...
The simple direct connection to the micro is suitable for driving LEDs,
optos or power Fets (provided you don't need a lot of speed), with the
addition of flyback diode and either a resistor or preferably a zener
it'll drive an on board relay.
I started with Infineon parts. I see NXP has a BSS123 as well but their
data sheet has no characteristic curves so I wouldn't spec it without
finding a more complete data sheet.
There are a fair number of small FETs that'll do the job.
Robert
--
http://www.aeolusdevelopment.com/
From the Divided by a Common Language File (Edited to protect the guilty)
ME - "I'd like to get Price and delivery for connector Part # XXXXX"
Dist./Rep - "$X.XX Lead time 37 days"
ME - "Anything we can do about lead time? 37 days seems a bit high."
Dist./Rep - "that is the lead time given because our stock is live....
we currently have stock."
> I would like to use the small FET as well. How are you driving the
> gate from a 3.3V uC?
That can be just about as simple as you want Richard.
The simplest is just to connect the gate directly to the micro's output
pin. With a BSS123 that typically will give somewhere around 7 or 8
ohms on resistance. The output impedance of the micro acts as a more
than adequate gate resistance.
From there you can get more sophisticated depending on your needs. Add
pull-ups or pull downs to enforce starting state if the micro pins start
as inputs.
Add gate resistors to either slow transitions or protect against either
failure or shoot through Miller capacitance.
Etc. etc. etc...
The simple direct connection to the micro is suitable for driving LEDs,
optos or power Fets (provided you don't need a lot of speed), with the
addition of flyback diode and either a resistor or preferably a zener
it'll drive an on board relay.
I started with Infineon parts. I see NXP has a BSS123 as well but their
data sheet has no characteristic curves so I wouldn't spec it without
finding a more complete data sheet.
There are a fair number of small FETs that'll do the job.
Robert
--
http://www.aeolusdevelopment.com/
From the Divided by a Common Language File (Edited to protect the guilty)
ME - "I'd like to get Price and delivery for connector Part # XXXXX"
Dist./Rep - "$X.XX Lead time 37 days"
ME - "Anything we can do about lead time? 37 days seems a bit high."
Dist./Rep - "that is the lead time given because our stock is live....
we currently have stock."
Reply by ●October 17, 20112011-10-17
--- In l..., Robert Adsett wrote:
>
> On 10/17/2011 9:03 AM, rtstofer wrote:
> > I would like to use the small FET as well. How are you driving the
> > gate from a 3.3V uC?
>
> That can be just about as simple as you want Richard.
>
> The simplest is just to connect the gate directly to the micro's output
> pin. With a BSS123 that typically will give somewhere around 7 or 8
> ohms on resistance. The output impedance of the micro acts as a more
> than adequate gate resistance.
>
> From there you can get more sophisticated depending on your needs. Add
> pull-ups or pull downs to enforce starting state if the micro pins start
> as inputs.
>
> Add gate resistors to either slow transitions or protect against either
> failure or shoot through Miller capacitance.
>
> Etc. etc. etc...
>
> The simple direct connection to the micro is suitable for driving LEDs,
> optos or power Fets (provided you don't need a lot of speed), with the
> addition of flyback diode and either a resistor or preferably a zener
> it'll drive an on board relay.
>
> I started with Infineon parts. I see NXP has a BSS123 as well but their
> data sheet has no characteristic curves so I wouldn't spec it without
> finding a more complete data sheet.
>
> There are a fair number of small FETs that'll do the job.
>
> Robert
>
Thanks!
It always bothers me that Vgs is usually in the range of 10V or so. Logic level devices are usually much lower.
As you point out, if Rds of 7 ohms or isn't a problem, direct connection may very well be the way to go. It clearly is not a problem for LEDs and such.
Richard
>
> On 10/17/2011 9:03 AM, rtstofer wrote:
> > I would like to use the small FET as well. How are you driving the
> > gate from a 3.3V uC?
>
> That can be just about as simple as you want Richard.
>
> The simplest is just to connect the gate directly to the micro's output
> pin. With a BSS123 that typically will give somewhere around 7 or 8
> ohms on resistance. The output impedance of the micro acts as a more
> than adequate gate resistance.
>
> From there you can get more sophisticated depending on your needs. Add
> pull-ups or pull downs to enforce starting state if the micro pins start
> as inputs.
>
> Add gate resistors to either slow transitions or protect against either
> failure or shoot through Miller capacitance.
>
> Etc. etc. etc...
>
> The simple direct connection to the micro is suitable for driving LEDs,
> optos or power Fets (provided you don't need a lot of speed), with the
> addition of flyback diode and either a resistor or preferably a zener
> it'll drive an on board relay.
>
> I started with Infineon parts. I see NXP has a BSS123 as well but their
> data sheet has no characteristic curves so I wouldn't spec it without
> finding a more complete data sheet.
>
> There are a fair number of small FETs that'll do the job.
>
> Robert
>
Thanks!
It always bothers me that Vgs is usually in the range of 10V or so. Logic level devices are usually much lower.
As you point out, if Rds of 7 ohms or isn't a problem, direct connection may very well be the way to go. It clearly is not a problem for LEDs and such.
Richard
