TL494 Lowest Duty Cycle

[sac1991]

Why TL494 from Texas is only giving least duty cycle of 11.66%. I have used it with a buck converter but at no load duty cycle is not decreasing below 11.66%. Am I using older version of TL494. In new datasheet it is mentioned that 3% duty cycle is achievable.

Another problem is,when Feedback Pin is high(4V), any change in DTC pin is not affecting the duty cycle, it remain 100%.

 

[treez]

are you isng tl494 in current or voltage mode.....at the end of the day, look at you pwm comparator......if the reference to it is not going low enough to give less than 11% duty then it wont.

 

[FvM]

I hear some confusion in your initial question. High feedback pin voltage will turn duty cycle down, low voltage up, as show in datasheet figure 1. Voltage at C1 and C2 is however inverted output, pulled low in active state.

Output duty cycle goes always down to zero with respective high feedback voltage, maximum duty cycle is however limited.

TL494 is designed for voltage mode only.

 

[sac1991]

Yes, datasheet has given the figures for base drive of output transistor. So if you will use it in common collector mode, then high voltage at comp pin will result in high duty cycle.

And I have tested the IC by giving variable voltage to Comp pin(Output of PWM comparator) directly. It turns out, the IC is actually some old version of TL494, it has minimum turn off time for gate drive is limited to 11% of the time period due to present of 1.2V before the DTC comparator.


Newer Texas TL494 IC has a 0.1V before DTC, and it has minimum gate turn off time of 3%.

 

[FvM]

So if you will use it in common collector mode, then high voltage at comp pin will result in high duty cycle.

You can do this and turn the DTC controlled maximum duty cycle in a minimum duty cyle, but it's not intended TL494 use. Minimum duty cycle in regular use is zero, as said.