negative voltage MC34063 pbm

[PRIYADHARSHINI PALANISAMY]

Hi all,

i have designed the circuit using MC34063AD
to produce -5V at the output for +5V input.
i designed the circuit as per the datasheet
I have calculated the resistor R2 and R1 as per the formula
Vout=1.25(1+R2/R1)
Hence i got that R2=8.2K and R1=2.7K
L=100uH,Ct=470pF,R sense=240 ohms
but i got only -0.2v at the output
can any one suggest me what was the wrong in my circuit?
Thanks in advance...

 

[betwixt]

If Rsense is 240 Ohms, the maximum current at the output will be around 250uA, is that what you want?

It is more normal to use a resistor of 10 Ohms or less.

Brian.

 

[PRIYADHARSHINI PALANISAMY]

If Rsense is 240 Ohms, the maximum current at the output will be around 250uA, is that what you want?

It is more normal to use a resistor of 10 Ohms or less.

Brian.

thanks for your reply
actually this -5v is used to supply the DG408L multiplexer
it s a negative power supply for the Multiplexer

 

[ud23]

**broken link removed**

few things your L inductor(not at filter) value should match in the test circuit and the diode 1N5819 part should be same and suggestion try to achieve the test circuit output -12v and then just replace the R1 and R2 value for -5v voltage.

 

[PRIYADHARSHINI PALANISAMY]

k ..i will try it..thanks for your reply ud23.

- - - Updated - - -

If Rsense is 240 Ohms, the maximum current at the output will be around 250uA, is that what you want?

It is more normal to use a resistor of 10 Ohms or less.

Brian.

I Got the output as -4.93v when R sense is 10 ohms
but is this current enough to drive the DG408L Multiplexer?

 

[betwixt]

You are confusing voltage with current but yes it is vastly more than adequate. The DG408 typically only draws about 1uA, your design fault is really that you used a relatively high current supply generator when a much smaller on would have been adequate. If I can use an example, it was like you used a jet engine to power a bicycle but starved it of fuel to make it less powerful.

Brian.

 

[PRIYADHARSHINI PALANISAMY]

k ....:grin:thanks for your kind reply

 

[PRIYADHARSHINI PALANISAMY]

Hi betwixt,

how do you calculate the R sense value as 10 ohms?actually it is used to limit the current.is it right?
thanks in advance..

 

[ud23]

yes it is the current limit resistor, Rsc it can be determined by using the current level of Ipk(switch).see the attached file. sorry you asked particularly to respected betwixt sir and I answered it.

 

[PRIYADHARSHINI PALANISAMY]

yes it is the current limit resistor, Rsc it can be determined by using the current level of Ipk(switch).see the attached file. sorry you asked particularly to respected betwixt sir and I answered it.

not like that ... u gave such a useful information..thanks for your reply.

 

[hemnath]

Use IC 7660 to convert +5V to -5V. You need only 2 external capacitors.

Best wishes

 

[PRIYADHARSHINI PALANISAMY]

Use IC 7660 to convert +5V to -5V. You need only 2 external capacitors.

Best wishes

nice..thanks for your useful information..i have one doubt...
if we use the opamp with voltage follwer configuration we get the -5v..
(i.e) +5v can be given as the negative input terminal we can get the -5v output
why can t we go for this...?will the opamp have the capability to drive the multiplexer.
or any other specific reasons are there?
mostly we are using the transistor to drive the circuits..why can t we use the opamp?

thanks in advance...

 

[betwixt]

That absolutely wont work! You are not understanding how op-amps work.

You are right that an inverting unity gain amp with +5 on it's inverting input would produce -5V at it's output but the -5V comes from the amps own negative supply. In other words, you have to supply at least -5V to the amp for it to give -5V out. You haven't produced -5V, just utilized one that had to be there already.

As sugested by Hemnath, the more appropriate device to use for a low current -5V supply would be the 7660 or one of it's equivalents.

Brian.

 

[PRIYADHARSHINI PALANISAMY]

HI
when i connected this -5v to multiplexer the 10 ohms resistor gets heat...i used 10 ohms with .125 W
may i use 1/4 W resistor...?

 

[ud23]

yes I think you need to use 1/4 W resistor

 

[PRIYADHARSHINI PALANISAMY]

sorry
i have already used 1/4 W resistor...not 1/8 resistor