As technologies further advance, a variety of electronic devices, such as mobile phones, tablet PCs, digital cameras, MP3 players and/or the like, have become popular. Each electronic device requires direct current power at a substantially constant voltage which may be regulated within a specified tolerance even when the current drawn by the electronic device may vary over a wide range. In order to maintain the voltage within the specified tolerance, a power converter (e.g., a switching dc/dc converter) coupled to the electronic device provides very fast transient responses, while keeping a stable output voltage under various load transients.
Hysteretic-based power converter control schemes such as the constant on-time scheme or the constant off-time scheme can enable power converters to provide fast transient responses. A buck converter employing the constant on-time control scheme may only comprise a feedback comparator and an on-timer. In operation, the feedback circuit of the power converter (e.g., buck converter) directly compares a feedback signal with an internal reference. When the feedback signal falls below the internal reference, the high-side switch of the power converter is turned on and remains on for the on-timer duration. As a result of turning on the high side switch, the inductor current of the power converter rises. The high-side switch of the power converter turns off when the on-timer expires, and does not turn on until the feedback signal falls below the internal reference again. In summary, when the constant on-time control scheme is employed in a power converter, the on-time of the high-side switch of the power converter is terminated by the on-timer. The off-time of the high-side switch of the power converter is terminated by the feedback comparator. Similarly, a boost converter employing a constant off-time control scheme can achieve fast transient responses.
The power converters employing the constant on-time control scheme or the constant off-time control scheme are simple to design. However, the constant on-time control scheme and the constant off-time control scheme have an unwanted application issue. Under different operating conditions, the switching frequency of the constant on/off time controlled power converter varies in a wide range. Such a switching frequency variation is not preferable in many applications.
It would be desirable to provide an apparatus and/or a method for enabling the power converters employing the constant on-time control scheme or the constant off-time control to have a fixed switching frequency under a variety of operating conditions.