why in voltage regulator given fixed frequency internal oscillator for lm2576-5v 52Khz and lm2596-5v 150 khz but both also give same output voltage and current.my question is what does do internal oscillator for regulator.is it better to high range
It actually is written on the first page of the datasheet:
"The LM2596 series operates at a switching frequency 150 kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators"
So you could probably use a smaller inductor than in the LM2576.
There are drawbacks of using high frequency switchers also, as you need to concider EMI problems and switching losses. I wouldn't consider 150 kHz as a very high switching frequence though.
These are integrated SMPS buck voltage regulators and have an integrated PWM controller and switching transistor. In order to regulate the voltage the PWM controller adjusts the switching duty cycle of the transistor to maintain the voltage on the output for the given load. You need some base clock to generate a PWM signal. Linear Regulators like the LM7805 will not have a PWM or clock source.
Higher frequencies can give you more resolution/granularity for duty cycle and better regulation, but in general its not a real important performance characteristic since the integrated power transistor can't switch on that fast anyway. The datasheet claims that this higher frequency allows you to use smaller (cheaper) supporting components in the circuit.
Choosing or otherwise knowing the specific switching clock may be important in noise sensitive applications as the clock will inevitably bleed into the supply and the ground. If you know clock frequency you can filter it out or choose power supply components that are less likely to affect the signal you are trying to measure