decoupling? Is this a capacitor?

[dazzlepecs]

I have this in a component documentation

The compass module requires a 5v power supply at a nominal 15mA, however the supply should be well decoupled near to the PCB as there is a high - very brief - current spike during coil switching, of up to 400mA

Is this a capacitor going from +Vcc to ground? Is it a electrolytic cap? ANy size will do if so?



thanks for any help

 

[Boncuk]

400mA is a pretty high switching current. So the decoupling capacitor should be like 10 to 47µF depending on frequency.

 

[audioguru]

On Intersil's datasheet for a Cmos 555 (their ICM7555) they say that a normal 555 produces a supply current spike of 400mA each time its output switches and therefore the 555 needs a decoupling capacitor close to the device.

 

[dazzlepecs]

400mA is a pretty high switching current. So the decoupling capacitor should be like 10 to 47µF depending on frequency.

so its just inbetween the +VCC and gnd lines? Is it a polarity-specific cap? thanks for the tip

On Intersil's datasheet for a Cmos 555 (their ICM7555) they say that a normal 555 produces a supply current spike of 400mA each time its output switches and therefore the 555 needs a decoupling capacitor close to the device.

So coupling may not be 100% necessary?

 

[audioguru]

so its just inbetween the +VCC and gnd lines? Is it a polarity-specific cap? thanks for the tip



So coupling may not be 100% necessary?

You mean a supply decoupling capacitor may not be necessary?
The 555 IC [/b]must[/b] have a supply decoupling capacitor. Its datasheet recommends two: a 0.1uf ceramic disc (for high frequencies) plus an electrolytic of at least 1uF (for lower frequencies).
Intersil says that the supply current spike caused by their ICM7555 Cmos IC is so low that a supply decoupling capacitor is not required.

Use a supply decoupling capacitor for ALL circuits.

 

[Boncuk]

Hi again,

without proper decoupling a complex circuit won't work properly. Imagine one IC just switches demanding high current for a split of a second. Drawing high current will cause the supply voltage to surge and mess up another IC which wants to switch simultaneously.

It is always good practice to use an electrolytic capacitor on a board just where the supply voltage is provided. From the electrolytic VDD and Ground continue to individual ICs.

The IC power pins should have a decoupling capacitor as close as possible to their supply pins. Normally that cap is placed close to the VDD-pin of the IC.

The reason for that is simple: When the layout is almost finished all free space is used to enforce the ground connections. (In some cases occupying the entire top layer.)

Look at the sample circuit for better understanding. The circuit is a null-function circuit to show how to decouple.

Boncuk

 

[dazzlepecs]

Hi again,

without proper decoupling a complex circuit won't work properly. Imagine one IC just switches demanding high current for a split of a second. Drawing high current will cause the supply voltage to surge and mess up another IC which wants to switch simultaneously.

It is always good practice to use an electrolytic capacitor on a board just where the supply voltage is provided. From the electrolytic VDD and Ground continue to individual ICs.

The IC power pins should have a decoupling capacitor as close as possible to their supply pins. Normally that cap is placed close to the VDD-pin of the IC.

The reason for that is simple: When the layout is almost finished all free space is used to enforce the ground connections. (In some cases occupying the entire top layer.)

Look at the sample circuit for better understanding. The circuit is a null-function circuit to show how to decouple.

Boncuk

thanks very much for this detailed explanation!! I wired up my compass as you suggested and it works fine with the other circuitry fed off the same power supply



you da man!!!