What's the advantage of constant off time

[refugee]

I'd like to know what's the advantage of constant off time control method contrast to other control method ( constant on time and pwm method ).
And it's appreciated if any one could tell me the optimized relationship of the off time and working parameters (such as VIN, VOUT, ILoad...)

 

[cdh7]

Hi,

PWM is the basic standard control method. Constant on-time has the advantage over PWM that switching frequency is reduced under light loads, which improves efficiency because the switching loss is lessened. Constant off-time has the advantage that it helps keep the converter in continuous conduction mode, which reduces ripple amplitude and noise.

Regards,
Chris

 

[dick_freebird]

If you are trying for discontinuous conduction then you have
to ensure that the inductor current has enough time to quench
to zero. This sets a lower "off" time limit (w/ load).

Both constant-on and constant-off times become variable
fequency
and these are not preferred for any part where spurious
frequencies have to be well controlled. Hard to build a good
trap, or DSP out, a frequency that wanders around over a 10:1 range.