Discussion in 'General Electronics Chat' started by riyashini, Jan 3, 2015.

1. ### riyashini Thread Starter New Member

Sep 18, 2014
20
0
Hi all,

i have designed the circuit using MC34063AD
to produce -5V at the output for +5V input.This -5V used to give the supply to multiplexer
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=240ohms
but i got only -0.2v at the output
can any one suggest me what was the wrong in my circuit?

2. ### crutschow Expert

Mar 14, 2008
13,000
3,229
Hard to help without a schematic.

3. ### riyashini Thread Starter New Member

Sep 18, 2014
20
0
thanks for your reply..here i have attached the schematic..but the values mentioned above

File size:
144.3 KB
Views:
59
4. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,781
1,103
Perhaps you've lost a decimal point . If Rsense = 240 Ohms the circuit will shut down if you try to draw more than a few mA output current. In the datasheet circuit Rsense = 0.24 Ohms !

5. ### riyashini Thread Starter New Member

Sep 18, 2014
20
0
oh k ..but i got the -5v as the output if the R sense value as 10 ohms

6. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,781
1,103
Good. But the current will be limited to about 30mA. If you need more current, reduce the Rsense value.

7. ### riyashini Thread Starter New Member

Sep 18, 2014
20
0
k..thanks for your reply..actually the maximum output current will be given that 50 mA..
this 30mA is the current which has been reduced in the input side...is it right?
in my case i got that 43.5mA as the output current