My systerm all are +5V supply,But now have a amp circuit need -5V+-10%---10mA supply,how can I use a simple circuit to converter +5V to -5V.the special charge pump chip all are expensive,I only need a simple circuit.
LM7905 is yet another linear regulator. You still need a negative voltage to start with. Also, note the voltage drop of these device is not insignificant. In general, use a +7V unregulated supply if you are going to make +5V regulated with 7805; -7V unregulated to make -5V regulated.
There are switchmode power supply ICs that handles the conversion. You can also try more common ICs like MAX232 (yes, the level converter for serial port, 0-5V in => +/- 15V out) or some 555 timer based homebrew switchmode.
Assuming your main circuit is digital, there might be a clock there somewhere. Run it to a spare buffer, and connect a voltage doubler circuit to its output, for a negative voltage (two diodes and two capacitors). Depending on the load, the voltage might come to somewhat above 5 volts.
The best solutio is to use a charge pump ICL 7660 as described below. This integrated circuit costs less than US 1 thne isn't so expensive. If you want to do it by yourself you can use a digital CMOS buffer as 74HC245 oscillator and capacitors. You can also use a buck converter using a low cost pwm regulator as LM3524 from national.
There are switchmode power supply ICs that handles the conversion. You can also try more common ICs like MAX232 (yes, the level converter for serial port, 0-5V in => +/- 15V out)