Valley current mode control buck converter

Figure 3 shows a plot of the current-loop gain with different values of external ramp. With added external slope compensation, the shape of the gain and phase curves do not change, but the amplitude of the gain will decrease and phase margin will increase.

A new control-to-output transfer function is created when the current loop is closed. The domain pole is mainly determined by load resistor R L , C 1 , and C 2. The lower frequency pair of conjugate poles is determined by L 2 , C 1 , and C 2 , while the higher frequency pair of conjugate poles locate at half of the switching frequency.

Figure 4 shows a plot of the control-to-output loop gain with different values of external ramp. The additional resonant poles will give up to o additional phase delay.

Besides, Figure 4 clearly shows the transition from current-mode to voltage-mode control as the slope compensation is increased. This article presents a new hybrid feedback structure, as Figure 5 a shows. The idea of hybrid feedback is to stabilize the control loop by using an additional capacitor feedback from the primary LC filter. The outer voltage feedback from the output through resistor divider is defined as the remote voltage feedback and the inner voltage feedback though capacitor C F will be referred to as the local voltage feedback hereafter.

The remote feedback and local feedback carry different information on the frequency domain. Specifically, the remote feedback senses the low frequency signal to provide good dc regulation of the output, while the local feedback senses the high frequency signal to provide good ac stability for the system.

Figure 5 b shows the simplified small signal block diagram for Figure 5 a. The resulting equivalent transfer function see Equation 31 and Equation 32 in Appendix II of a hybrid feedback structure differs significantly from the transfer function of conventional resistor divider feedback.

The new hybrid feedback transfer function has more zeros than poles, and the additional zeros will lead to o phase ahead at the resonant frequency determined by L 2 and C 2.

Therefore, with the hybrid feedback method, the additional phase delay in control-to-output transfer function will be compensated for by the additional zeros in the feedback transfer function, which will facilitate the compensation design based on the complete control-to-feedback transfer function.

Apart from those parameters in the power stage, there are two more parameters in the feedback transfer function. The feedback transfer function has been simplified to a new form see Equation 33 in Appendix II. The rising current during the on-time is compared to a level proportional to the output of the error amplifier that corresponds to the required load current. When the switch current reaches that level, the on-time terminates. The same circuitry used to set the duty cycle can also detect the current limit by allowing an upper clamp of the control voltage.

There are some disadvantages to peak current limit. There may be considerable overshoot and ringing present. During this time, it is not possible to monitor the current accurately, so there is typically a minimum on-time or blanking time specified. During this time, the current is not monitored and may exceed the limit. You can see that valley current limit keeps the on-time constant while extending the off-time to reduce current below the limit, so it would be natural to think of using this type of current limit in converters using constant on-time control modes.

Constant on-time control sets the on-time to a fixed value and uses a hysteretic comparator to end the off-time when the fed-back portion off the output voltage falls below a preset level.

The valley current limit will override this control signal if the current is above the limit, increasing the off-time. Here is an advantage of valley current limit over peak current. Valley current limit is applied at the end of the off-time, before any switching transition. There is no need for any blanking time. Another consideration is the physical location of the sensing circuit.

For current limit, the actual current sometimes may not be sensed directly. Instead, a voltage proportional to the current may be monitored. Since the boost has continuous input current, a triangular waveform results and current is continuously monitored. Here, the peak switch current which is also the peak inductor current is monitored, resulting in a current waveform every half cycle.

A 4-switch buck-boost converter is shown below in Figure 6 with the sense resistor on the low side. The converter operates in buck mode when the input voltage is much higher than the output voltage, and in boost mode when the input voltage is much lower than the output voltage. In this circuit, the sense resistor is located at the bottom of the 4-switch H-bridge configuration.

The mode of the device buck mode or boost mode determines what current is being monitored. In buck mode switch D always on, switch C always off , the sense resistor monitors the bottom side switch B current and the supply operates as a valley current mode buck converter.

In this mode, since the valley inductor current is not monitored, it is difficult to detect the negative inductor current when the supply is in light load condition. MTD :. The PT series will operate off either. LAD : 1. It a dynamically adjustable output, ultra-fast transient response, high-DC accuracy, and high efficiency needed for leading-edge CPU core power supplies.

The boost converter runs at kHz fixed switching frequency to reduce output ripple, improve conversion efficiency, and allows for the use of small external.

These devices reduce the complexity and number of components required to monitor power-supply and battery functions.. Each LED output has its own 8-bit resolution steps fixed frequency individual PWM controller that operates at approximately To maximize efficiency, the converter operates in PWM mode with a nominal switching. UA : 3 Pin 1. These applications include on-card regulation for elimination of noise and distribution problems associated with single-point regulation.