Are TS555's really supposed to suck this much?
in [Design]
|
Prev: ZXSC400 LED driver problem
Next: calculate MTBF
From: Tim Williams on 20 Jan 2010 23:22 I was trying to blink LEDs from a TS555IN today, and discovered the awful truth that its output stage stinks. Somehow it's rated for 10mA source, 50mA sink (at 12V, 2V max. Vds(sat)). The datasheet happens to overlook the fact that internal voltages change when the output is loaded. Specifically, in astable mode, frequency drops and duty cycle drops. When heavily loaded, oscillation stops altogether. It seems like there's an insane amount of resistance in series with the power supply pin, and any current draw causes the rest of the circuit to poop out (i.e. dragging down the control voltage divider, comparator, etc.). So what's the deal with this? Are they really supposed to be so shitty? I know lots of CMOS is supposed to drive CMOS, but even CD4001 will drive a damn LED. Tim
From: dagmargoodboat on 20 Jan 2010 23:50 On Jan 20, 11:22 pm, Tim Williams <tmoran...(a)gmail.com> wrote: > I was trying to blink LEDs from a TS555IN today, and discovered the > awful truth that its output stage stinks. Somehow it's rated for 10mA > source, 50mA sink (at 12V, 2V max. Vds(sat)). The datasheet happens > to overlook the fact that internal voltages change when the output is > loaded. Specifically, in astable mode, frequency drops and duty cycle > drops. When heavily loaded, oscillation stops altogether. The output's buffered--that shouldn't happen. http://www.st.com/stonline/products/literature/ds/4077/ts555.pdf Are you taking feedback off the output pin? That would make it load sensitive. The original application circuits used the discharge pin instead. -- Cheers, James Arthur
From: Tim Williams on 21 Jan 2010 00:17 <dagmargoodboat(a)yahoo.com> wrote in message news:04b76c24-3e4e-42e8-8671-535f74cd6b7a(a)f12g2000yqn.googlegroups.com... > Are you taking feedback off the output pin? That would make it load > sensitive. The original application circuits used the discharge pin > instead. Good point, I am indeed! Well, that solves that... Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 21 Jan 2010 00:18 On Wed, 20 Jan 2010 20:22:49 -0800 (PST), Tim Williams <tmoranwms(a)gmail.com> wrote: >I was trying to blink LEDs from a TS555IN today, and discovered the >awful truth that its output stage stinks. Somehow it's rated for 10mA >source, 50mA sink (at 12V, 2V max. Vds(sat)). The datasheet happens >to overlook the fact that internal voltages change when the output is >loaded. Specifically, in astable mode, frequency drops and duty cycle >drops. When heavily loaded, oscillation stops altogether. > >It seems like there's an insane amount of resistance in series with >the power supply pin, and any current draw causes the rest of the >circuit to poop out (i.e. dragging down the control voltage divider, >comparator, etc.). > >So what's the deal with this? Are they really supposed to be so >shitty? I know lots of CMOS is supposed to drive CMOS, but even >CD4001 will drive a damn LED. > >Tim Well, duh, it's a 555. John
From: Son of a Sea Cook on 21 Jan 2010 00:43
On Wed, 20 Jan 2010 23:17:30 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: ><dagmargoodboat(a)yahoo.com> wrote in message >news:04b76c24-3e4e-42e8-8671-535f74cd6b7a(a)f12g2000yqn.googlegroups.com... >> Are you taking feedback off the output pin? That would make it load >> sensitive. The original application circuits used the discharge pin >> instead. > >Good point, I am indeed! Well, that solves that... > >Tim You are discharged! ;-) |