Can someone explain how this circuit is functioning...when does the LED turn on?

 

Hello,

Did you read the datasheet?

This is at the introduction:

Undervoltage Sensing
Circuit
The MC34064 is an undervoltage sensing circuit specifically
designed for use as a reset controller in microprocessor−based
systems. It offers the designer an economical solution for low voltage
detection with a single external resistor. The MC34064 features a
trimmed−in−package bandgap reference, and a comparator with
precise thresholds and built-in hysteresis to prevent erratic reset
operation. The open collector reset output is capable of sinking in
excess of 10 mA, and operation is guaranteed down to 1.0 V input with
low standby current. The MC devices are packaged in 3−pin TO-92,
micro size TSOP−5, 8−pin SOIC−8 and Micro8t surface mount
packages. The NCV device is packaged in SOIC−8 and TO−92.
Applications include direct monitoring of the 5.0 V MPU/logic
power supply used in appliance, automotive, consumer and industrial
equipment.

Features

Trimmed−In−Package Temperature Compensated Reference
Comparator Threshold of 4.6 V at 25°C
Precise Comparator Thresholds Guaranteed Over Temperature
Comparator Hysteresis Prevents Erratic Reset
Reset Output Capable of Sinking in Excess of 10 mA
Internal Clamp Diode for Discharging Delay Capacitor
Guaranteed Reset Operation with 1.0 V Input
Low Standby Current

Bertus

 

Yes, the curcuit I posted was from the datasheet, but there was no example on using an external resistor. Am I to gather that the LED will turn on when the Supply voltage is 3.8V?
Is it designed for a 5V source only and can not be modified?

 

Hello,

The circuit is designed to reset micrprocessors at a to low voltage input.
There is no possibility to change the voltage with external resistors.

What voltage would you like to have a low voltage detection for?

Bertus

 

It offers the designer an economical solution for low voltage detection with a single external resistor. Apparently the resistor they are refering to is the current limiting resistor for the LED??
In the case of the circuit I provided, it has nothing to do with resetting anything and does not nessesarily have to do anything with a microprocessor.

 

Hello,

The single resistor is refering to the pull-up resistor at the output, also to be used in case of driving a led.

Bertus

 

I suppose a resistor in line with Vcc may work, but it could cause erratic operation due to the Vcc supplying the comparator as well.

If you are ordering many IC's, you can request a custom threshold voltage from Microchip.

 

Still no real answer...
Let's assume that the Power Supply is 5V. The LED will turn on when the Power Supply drops 1.2V to 3.8V, Yes? This is what is expected to be on Pin 2 for the LED to turn on. Actually probably a hair less than 3.8V. Are these assumptions correct?

Thanks

 

The trip voltage is clearly stated in the data sheet. It is fixed at 4.6V, with hysteresis on both sides.

 

Guess I got stuck on reading the text and didn't see too much there.

OK, so when the input source voltage is 5V the LED will come on when the source is ~4.6V and lower.

The real query here is this:

What would the trip voltage be when the input source is 9.0V?
What would the trip voltage be when the input source is 4.2V?
What would the trip voltage be when the input source is 3.0V?

Is the trip voltage fixed regardless of the source? I am assuming Yes... but am not sure.

Thanks

 

Trip voltage is 4.6V with a supply up to 10V.
It doesn't change depending on the input voltage.
If input is 3.0V the chip is in tripped mode.
If input is 4.2V the chip is in tripped mode.
If input is 9.0V the chip is in not tripped mode.

What are you trying to do? Seems like this is the wrong chip for whatever you are thinking about.

 

Hello,

Did you see this graph in the datasheet?

Bertus