Simple and high efficiency low voltage cutoff for MC34063A regulator needed

[Telek]

Hello everyone - this looks like a fantastic forum. I know that this is a topic that comes up somewhat frequently however I have searched and couldn't find anything specific to my needs. I'm a hobbiest with no formal training but have fooled and built more than my fair share of circuits.

I have a design right now that uses 2xCR123A rechargeable batteries in series to power a big array of LEDs. I've figured out so far that what appears to be my best/cheapest bet is to use the MC34063A regulator in step-up configuration to power 4-led-series in parallel (for example a grid of 4x12 LEDs). The output voltage is 12.8V and Vin is up to 8.3V.

What I'd like to do is cutoff the power from the cells once they hit about 6.4V. I know that I could go lower but it's a tradeoff and besides from my testing and research there's only about 5% total power left between 3.2V and 3.0V on these cells - but the power supply design is easier if I cut off higher.

I was looking at several different circuit types to accomplish this, preferably without impacting the efficiency of the system at all. The basic idea seems to be to use some sort of voltage supervisor with a transistor. I could use a TL431 and a mosfet (from here) but that incurs a loss through the mosfet. Small but I don't think necessary.

The MC34063A regulator (datasheet here) is a cheap and efficient step-up converter that I would like to use.

Here's a sample circuit:

**broken link removed**

Now here's where my ignorance kicks in.

What I'm wondering is if I could use a similar circuit to TL431/mosfet but instead use a simple NPN transistor on either pin 4 (which is the ground connection for the reference regulator) or between R1 and ground. Alternately could I actually replace the 1N5819 with a mosfet instead?

The MC34063A was chosen because it's cheap and efficient - and this is for something that will be sold at small profit margins. However if there is something else equally as inexpensive that would also work I am open to suggestions!

Thanks everyone!

 

[Boncuk]

I suggest you refer to TI (Texas Instruments) step-up regulators of the TPS61XXX family.

They all have an ENABLE pin for shutoff.

Attached is the datasheet of the TPS61081.

TI's free sample service is quickly and uncomplicated.

Boncuk

 

[Telek]

I suggest you refer to TI (Texas Instruments) step-up regulators of the TPS61XXX family.

They all have an ENABLE pin for shutoff.

TI's free sample service is quickly and uncomplicated.

Thanks for the suggestion - however I've got an eye on keeping the price down. The MC34063 can be obtained for $0.25 in small quantities where the TPS61x are > $3 for the current required.

 

[MikeMl]

Here is the simplest low voltage detector circuit I know

 

[Telek]

Here is the simplest low voltage detector circuit I know

Thanks Mike! I had noticed that one already - I assume you're referring to the one lower down which uses the TL431? In between R1/Q3 (base of Q1) essentially acts as the active-low trigger with R5/D2 being the load, correct?

However how would I adapt that to turning off the MC34063A regulator?

 

[MikeMl]

Starve Q2 by breaking the connection to pin8?
Break the connection from pin1 to ground?
Trick it into shutting off by dragging pin 5 high?

Read the data sheet?

Page 31 of this App Note.

 

[Telek]

Starve Q2 by breaking the connection to pin8?
Break the connection from pin1 to ground?
Trick it into shutting off by dragging pin 5 high?

Read the data sheet?

Thanks again Mike,

I am realizing now that best case scenario with any method like those just disables the regulator - but since it's operating in a step-up configuration won't the battery voltage simply be present through the sense resistor, inductor and diode straight to Vout? I.e. if the regulator is disabled then Vout=Vin (less any losses in those components).

Could I instead replace the diode with a transistor, and trigger that with the voltage supervisor circuit?

 

[mneary]

You could arrange the LEDs so that they won't light when Vout=Vin. 4 white LEDs in series is likely to meet this requirement.

Alternatively you could put a (cheap?) NMOS switch in the return (0V) line of the LED array. It doesn't have to be in the regulator circuit itself, does it?

 

[Telek]

You could arrange the LEDs so that they won't light when Vout=Vin. 4 white LEDs in series is likely to meet this requirement.

Alternatively you could put a (cheap?) NMOS switch in the return (0V) line of the LED array. It doesn't have to be in the regulator circuit itself, does it?

That's true - 6.4V will not be enough to power 4 LEDs in series. However I wanted it as a part of the control portion of the regulator in order to avoid any loss as being a part of the circuit itself. I.e. the NMOS switch in the return line of the LED array would still have some conduction loss of it's own. No, it's not much, but it'd still be something. I will look into that anyway.

Since the diode is necessary in the circuit anyway wouldn't I be able to replace that with a transistor triggered by the voltage supervisor circuit? Since the voltage supervisor is based on the input voltage (not the output) shouldn't that work?

 

[mneary]

An NMOS switch has a body diode.

 

[Telek]

An NMOS switch has a body diode.

Ahh ok - sorry I'm not all that familiar with mosfets. Do you have a model in particular that you think would work well for this application?

Thanks!

 

[Wade Hassler]

Don't know if this helps, but.....
ON Semi makes a version of the 34063 with a high-true enable on pin 8.
NCP3064 appears to cost the same as NCCP34063, and runs at 150KHz versus 100KHz
Wade Hassler

ON Semiconductor NCP3064: 1.5 A Switching Regulator with ENABLE
ON Semiconductor MC34063A: 1.5 A, Step-Up/Down/Inverting Switching Regulator

 

[Telek]

Don't know if this helps, but.....
ON Semi makes a version of the 34063 with a high-true enable on pin 8.
NCP3064 appears to cost the same as NCCP34063, and runs at 150KHz versus 100KHz
Wade Hassler

ON Semiconductor NCP3064: 1.5 A Switching Regulator with ENABLE
ON Semiconductor MC34063A: 1.5 A, Step-Up/Down/Inverting Switching Regulator

Thanks for the suggestion!

According to DigiKey the NCP3064 is $1.33 whereas the MC34063A is $0.78 - however via Newark I can get the MC34063A for $0.30 and Allied for $0.43.

If I can't find a simpler solution then the 3064 looks like a good bet.

 

[Telek]

An NMOS switch has a body diode.

Ok so if I wanted to replace the diode then I would need a n-channel mosfet depletion mode which would be brought to Vout to conduct (giving Vgs=0) and grounded (giving Vgs=-12.8V) to turn it off?

However, as I'm just realizing if I replace the diode with a mosfet then since the sense circuitry is on the off-side the regulator is going to go nuts trying to raise the voltage = bad. So I'd need to couple that with some way of disabling the regulator and now we're just overly complicating things. Plus that and the Vbd of the diode in those mosfets are around 0.8V - higher than the 0.5V of the normal diode.

So, basically, I either need to put a transistor of some kind on the LEDs and incur the loss or upgrade the chip to something with an enable pin.