PCF8574A overheating
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Posts: 46,084
I have what I think is a simple question. When I drive all 8 outputs of the
PCF8574A I/O expander high without a load (pins dangleing in the wind) the
chip gets so hot you can't touch it. now If all 8 outputs are driven high
and are connected through 680 ohm resistor and LED to +5V (ex, LED's not
on) the chip doesn't get hot. I'm confused. looks like either way the
outputs are not drawing any current. I thought you need a pull-up /
pull-down resistor on open-drain outputs to source/sink current. maybe I
don't fully understand open-drain outputs. any suggestions
oh yeah, just for grins and giggles, I drove all outputs high except for
the last 3 (ex, 3 LED's on) and notice the top of the chip a lot hotter
than the bottom. all of the outputs except for the last 3 were floating.
Daryl
PCF8574A I/O expander high without a load (pins dangleing in the wind) the
chip gets so hot you can't touch it. now If all 8 outputs are driven high
and are connected through 680 ohm resistor and LED to +5V (ex, LED's not
on) the chip doesn't get hot. I'm confused. looks like either way the
outputs are not drawing any current. I thought you need a pull-up /
pull-down resistor on open-drain outputs to source/sink current. maybe I
don't fully understand open-drain outputs. any suggestions
oh yeah, just for grins and giggles, I drove all outputs high except for
the last 3 (ex, 3 LED's on) and notice the top of the chip a lot hotter
than the bottom. all of the outputs except for the last 3 were floating.
Daryl
Comments
that part. If a "typical" LED has a forward voltage of 1.4 volts and
you're current limiter is 680, the current through the LED is about 5
mA, so eight active LEDs would be 40 mA, or 15 mA greater than the chip
specs (last time I looked, anyway).
I have given up on the PCF8574 and have switched to the MCP23016. It
has 16 IOs, can sink/source much more current, and doesn't use any of
the quasi-bidirectional silliness of the PCF8574.
-- Jon Williams
-- Applications Engineer, Parallax
-- Dallas Office
Original Message
From: Daryl Berryhill [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=ic2AaKHqoWinKHQMy5OxfVG35bBKmgwcqJ-90nSqsPHijYHKbCTk8o_BfoYvAytq7Cpeie702Zk]djberry@h...[/url
Sent: Saturday, June 12, 2004 6:17 PM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] PCF8574A overheating
I have what I think is a simple question. When I drive all 8 outputs of
the
PCF8574A I/O expander high without a load (pins dangleing in the wind)
the
chip gets so hot you can't touch it. now If all 8 outputs are driven
high
and are connected through 680 ohm resistor and LED to +5V (ex, LED's
not
on) the chip doesn't get hot. I'm confused. looks like either way the
outputs are not drawing any current. I thought you need a pull-up /
pull-down resistor on open-drain outputs to source/sink current. maybe I
don't fully understand open-drain outputs. any suggestions
oh yeah, just for grins and giggles, I drove all outputs high except for
the last 3 (ex, 3 LED's on) and notice the top of the chip a lot hotter
than the bottom. all of the outputs except for the last 3 were floating.
Daryl