TPS61200 Enable Pin

[MichaelKavanagh]

Greetings,
Another beginner question from me. Currently designing a circuit that will be powered by a single cell LiPo, and due to its popularity and pretty neat specs I decided on the TPS61200 converter (http://www.ti.com/lit/ds/symlink/tps61200.pdf).
Now, I somehow I need to build in an on/off switch for the circuit, and I was wondering whether I could use the enable pin to do this? A simple SPDT slide switch to either pull the EN pin up to VCC or down to GND? If I understand the datasheet correctly, it basically disconnects the load and everything internally from the supply, therefore not discharging the battery? (bottom of page 13)
Is this an acceptable thing to do? This way I'd also still be able to charge it when the thing is turned off.
Just a 17 year old student trying to learn as much as possible in between A-level work, 
Cheers,
Michael

[Alex]

Hi Michael,

Sure you can, that's what its for. It is a boost converter so the conventional design has a DC path in which case you wouldn't have been able to do this. Here however the output diode is made of FETs and synchronous, so it can be driven open-circuit. Just like the datasheet says.

Power consumption during shutdown should be very low, but worth checking in your application.

Also consider an RC circuit prior to the enable pin to suppress the bouncing of the switch.

Alex.

[MichaelKavanagh]

Thanks.
Also, is it necessary to have pull up/down resistors on PS and EN pins? Some people seem to use a 10k in their designs and others don't bother. The datasheet doesn't mention the use of them either.

[8086]

The datasheet doesn't mention the use of them either.

It does, in a roundabout way.

On page four, in reference to the EN pin, it says "Do not leave floating" - this means you must connect the pin to something.

So yes, it's advisable to use a pull-up. And this is good practice in general, so you have a known input at all times.

EDIT: I see you're using a double throw switch - no need for a resistor. But if you were using a button or a single throw switch, or similar, you would need one.

[Alex]

The pull ups/downs are required if you are driving the EN, PS pins with a circuit that has at least one high impedance output state. Such a state would leave the pin floating with undetermined behaviour. An example of this would be an open collector output from a transistor. There are also some special cases where it is required (over shorting to a power rail) due to the specific internal design of the IC. This is not the case here.

In this case the SPDT switch will switch between two low impedance states (+V and GND rails) so a pullup/down is not needed. But there will be bouncing when switching over. A 100nF capacitor from the pin to ground and a say 1k resistor will greatly suppress that giving the pin a clean state to detect.

Alex.