negative supply converter

It is sometimes required in certain cir- cuits that are powered from just one battery to derive a negative supply voltage from the positive battery potential. As the loading of such negative lines is normally pretty minimal, it is possible to use a TL 497A IC to provide the negative voltage. This saves a transformer, rec- tifier, and a smoothing capacitor. The TL 497A is a switch-mode IC from Texas Instruments, that may be used as an upwards/downwards trans- former, but also as a negative supply converter. Inductor L makes it all possible, because when the on-chip transistor is switched off, a fairly large back- e.m.f. is generated across L, which causes a negative potential at the emitter of the transistor. The diode then conducts, and capacitor CF charges. The output voltage, U0, is determined by U„ =[—Ubt,/t0IV where U b is the supply voltage; ti is the time the transistor is switched on; tois the time the transistor is switched off. Period t is determined by the value of C...