Consideration of Current Mode

In succession to the voltage mode discussed last time, we consider the second type of slope transfer functions, which are current mode transfer functions.

Current-mode slope waveform = fixed ramp wave + current sense gain (R_s)×coil current (I_L)

The current sense gain is a constant that indicates by how many times the error amplifier output has changed compared with a change in the coil current.

As shown in Fig. 3, generally a method is employed in which the coil current is detected when the high-side switch (transistor) is turned on and the information is used. Fig. 4 shows the slope waveform when using this technique.

Fig. 4

Similarly to Fig. 2 in the section on “Voltage mode”, when V_c > V_slope, control to turn on the high-side switch is continued. An offset corresponding to the product of the DC component of the coil current I_L and R_s occurs with the timing of turning-on the high-side switch, and from then until the switch is turned off, a slope is generated by the ripple current in I_L. In other words, the current-mode slope waveform changes within the range indicated by equation 3-4.

In the current mode, the slope waveform shown in Fig. 4 and described by equation 3-4 is used to control the duty cycle, and so the duty cycle is determined by the output voltage of the error amplifier (V_c), minus the coil current feedback (R_s・I_L).

Next time, a current-mode transfer function will be derived drawing on what has been discussed up to this point, and including a comparison with the voltage mode.