LPCXpresso IDEのトピックRe: Direct drive of LEDs?

Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:07 GMT

Content originally posted in LPCWare by wrighflyer on Wed Feb 24 09:33:34 MST 2010
I know the 1114/1343 boards have the single LED on the high capacity (20mA) pin but can you just confirm the current source/sink capability of the other I/Os. As far as I can determine they can sink 4mA - is that correct? I guess that with suitable resistors this should be enough to drive a handful of LEDs (one per pin) if only dimly?

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:08 GMT

Content originally posted in LPCWare by rkiryanov on Wed Feb 24 10:18:38 MST 2010

Quote: wrighflyer
I know the 1114/1343 boards have the single LED on the high capacity (20mA) pin

Quote:
High-current sink drivers (20 mA) on two I2C-bus pins in Fast-mode Plus.

LED is on the "PIO0_7/CTS", this is not "high capacity pin".

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:09 GMT

Content originally posted in LPCWare by rmteo on Wed Feb 24 11:48:16 MST 2010
According to the datasheet, High-drive output pin (PIO0_7) is capable of sourcing 20mA and sinking 4mA.

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:09 GMT

Content originally posted in LPCWare by rkiryanov on Wed Feb 24 11:56:03 MST 2010
Really. UM does not specify anything about PIO0_7 :(

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:10 GMT

Content originally posted in LPCWare by rmteo on Wed Feb 24 12:01:46 MST 2010
There is a reference on page 88 of the UM - don't see any electrical specs though, except in the datasheet.

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:10 GMT

Content originally posted in LPCWare by wrighflyer on Thu Feb 25 02:20:24 MST 2010
So no one else has ever wanted to add a few LEDs to their LPCXpresso? Well I guess I'll proceed cautiously and report results.

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:11 GMT

Content originally posted in LPCWare by ktownsend on Thu Feb 25 05:57:28 MST 2010
Particularly if you want to use more than one LED, you should be going through a MOSFET to toggle your LEDs. You don't want to source or sink 100mA with an LPC1100 or LPC1300. :)

http://www.microbuilder.eu/Tutorials/LPC2148/GPIO/GPIOMOSFET.aspx

Kevin.

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:12 GMT

Content originally posted in LPCWare by NXP_USA on Sun Feb 28 00:30:48 MST 2010

Quote: KTownsend
Particularly if you want to use more than one LED, you should be going through a MOSFET to toggle your LEDs. You don't want to source or sink 100mA with an LPC1100 or LPC1300. :)

http://www.microbuilder.eu/Tutorials/LPC2148/GPIO/GPIOMOSFET.aspx

Kevin.

The I/O pin source/sink specs give a current at one specific selected voltage drop. Different amounts of current could be supplied at other pin voltages (for example, sinking current at Vss + 1.5V) Although arbitrary specifications cannot be guaranteed, it may be more useful to review the IBIS models to determine the expected current capabilities at various I/O pin source or sink voltages. Please stay within the pin maximums and when driving multiple loads from the LPCs simultaneously, please make sure not to exceed the Vdd/Vss pin current ratings.

http://ics.nxp.com/support/models/lpc1000/

-NXP

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:12 GMT

Content originally posted in LPCWare by xpresso on Fri Mar 19 12:18:52 MST 2010
From the data sheet it quotes the sink/source for normal pins to be 4ma with at 0.4 volts. This implies a worst case internal FET resistance of 100 ohms. (0.4v/4ma = 100 ohms)

The short circuit current is specified as -45 and 50 ma. Again this Implies about a 100 ohm internal FET.

Note 14 on short circuit current:

[14]Allowed as long as the current limit does not exceed the maximum current allowed by the device.

From these specs one can see that it is possible to get say 10ma from several normal pins to direct drive an LED. The total number of will be limited by the 100ma limit on the supply and ground pins. The micro uses 17ma at 72 MHz. This leaves 83ma for current leaving other pins.

For example you could drive 8 LEDs a 10ma. For a 3.3 volt supply the external current limiting resistor will be about 47 ohms assuming a 1.8 volt drop on the LED.

Many high efficiency LEDs will look fine with less than 10ma, so more could be driven.

Another option to get higher current drive would be to parallel several ports. If you needed to drive 80ma into one LED you could use 8 ports, the defective output resistance would be about 12.5 ohms.

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:13 GMT

Content originally posted in LPCWare by wrighflyer on Tue Mar 23 04:29:23 MST 2010
Wow, thanks for that detailed answer - great info!

Re: Direct drive of LEDs?

lpcware — Thu, 16 Jun 2016 00:08:13 GMT

Content originally posted in LPCWare by bladerunner on Tue Mar 23 07:48:24 MST 2010

Quote: xpresso
From the data sheet it quotes the sink/source for normal pins to be 4ma with at 0.4 volts. This implies a worst case internal FET resistance of 100 ohms. (0.4v/4ma = 100 ohms)

The short circuit current is specified as -45 and 50 ma. Again this Implies about a 100 ohm internal FET.

Note 14 on short circuit current:

[14]Allowed as long as the current limit does not exceed the maximum current allowed by the device.

From these specs one can see that it is possible to get say 10ma from several normal pins to direct drive an LED. The total number of will be limited by the 100ma limit on the supply and ground pins. The micro uses 17ma at 72 MHz. This leaves 83ma for current leaving other pins.

For example you could drive 8 LEDs a 10ma. For a 3.3 volt supply the external current limiting resistor will be about 47 ohms assuming a 1.8 volt drop on the LED.

Many high efficiency LEDs will look fine with less than 10ma, so more could be driven.

Another option to get higher current drive would be to parallel several ports. If you needed to drive 80ma into one LED you could use 8 ports, the defective output resistance would be about 12.5 ohms.

Nicely put. Just want to add that many high-brightness, low-current LED's run just fine on 2mA-3mA. So very easy to drive even without a "strong" pin buffer. Regards :)