Several methods to improve efficiency at light load and reduce IC consumption current

One of the biggest goals that is expected with DC/DC converters is the efficient conversion of energy. The way to increase efficiency is to reduce losses. As shown in FIG. 3, as a problem of occurrence of loss, there are significant causes such as consumption current of the IC, heat loss caused by the on-resistance of the driver transistor, loss caused by the series connection of the coils, and the like.

Figure 3 The main power loss part of a step-down DC/DC converter

Among them, in order to improve the efficiency at light load, it is very important to reduce the consumption current of the IC, and the control mode of the IC is extremely critical.

PFM control work and PWM control work

Here, the PFM control operation corresponding to the number of load current control switches is very effective. The PFM operation is to reduce the switching time per unit time when the load current is small, and to reduce the inefficient current such as the consumption current and the through current to improve the efficiency. This IC is operated by the PFM to reduce the current consumption at no load to 15 μA or less. As the load current increases, the consumption current of the IC becomes relatively small, and a PWM operation state with a smaller ripple voltage is required at this time.

PWM/PFM automatic switching work

The XC9236 has the ability to automatically switch between PFM operation and PWM operation in response to load changes. This function can simultaneously improve efficiency at light loads and maintain low ripple at heavy loads, which helps improve efficiency over the entire load range. (Refer to Figure 4)

Figure 4 Energy conversion efficiency

From Fig. 5, the load transient response characteristics from PFM operation to PWM operation and reverse transition can be confirmed.

Figure 5 PFM control and PWM control automatic conversion waveform (load current: 1mA⇔100mA)

From the load transient response characteristics when the load changes steeply, it can be recognized that the voltage recovery only starts at the 20mV voltage difference, and no overshoot or ringing occurs at the time of recovery, which makes the phase system have sufficient margin.

Manually switch working state

Based on the automatic PWM/PFM conversion, the XC9237 adds the function of forcing the transition from the external signal to the fixed PWM operation. Corresponding to the use of the machine's working state, it can make it work in the lower ripple PWM working state, can control the noise reduction at any time (refer to Figure 6)

Figure 6 Circuit diagram of the XC9237 manual switching PWM/PFM operation