Use A DSC For Digital Current-Mode Control

Calculating the PWM on and off times is similar to the process described earlier—by measuring the voltage across the inductor and the inductance and knowing the desired average current via the voltage error (see the figure). Then, calculate the PWM on and off times and load the PWM generator control registers with the duty-cycle and period values. It's also possible to measure the peak and valley currents instead of the midpoint average currents. Because software determines the PWM on and off times, it's easy to set the ADC sampling times to capture the peak and valley currents.

ADVANCED SENSORLESS DIGITAL CURRENT CONTROL
A more sophisticated method uses the voltage versus current equations for capacitors and inductors to predict the current through the inductor:

Given:
V(t) = L di/dt
I(t) = I(t0) + 1/L × ∫ V(t) dt
I(t) = C dv/dt
V(t) = V(t0) + 1/C × ∫ i(t) dt

If additional information is available (such as the ESR of the output capacitor) and the resistance of the inductor is known, more accurate current estimates can be made. Calculate the ripple current by knowing the voltage across the inductor and the time the voltage is applied.

By measuring the voltage drop across the inductor, you can calculate the dc component of the current by subtracting the inductive voltage drop from the measured voltage drop. To calculate current, divide the voltage difference by the inductor resistance. Use a similar process for the voltage ripple on the filter capacitor.

FEED-FORWARD TECHNIQUES
One of the benefits of current-mode control is improved response to input voltage variations. With digital control of SMPS systems, providing feed-forward compensation for input voltage variations becomes easy. Most SMPS topologies have relatively simple transfer equations that describe the output voltage in terms of input voltage, duty cycle, and transformer turns ratio. You can add a voltage feed-forward term to either to the current set point or in parallel with the current control loop. The transfer equation for a buck converter is:

V_OUT = V_IN Duty Cycle/Period

Usually, the desired result of any control calculation is to create a value to be loaded into the PWM duty-cycle register:

Duty Cycle = V_OUT (Period/V_IN)

The cost of calculating the feed-forward compensation for the input voltage is the time required to perform the division operation. The feed-forward compensation techniques are inherently stable and provide faster transient response.

Calculating the PWM on and off times is similar to the process described earlier—by measuring the voltage across the inductor and the inductance and knowing the desired average current via the voltage error (see the figure). Then, calculate the PWM on and off times and load the PWM generator control registers with the duty-cycle and period values. It's also possible to measure the peak and valley currents instead of the midpoint average currents. Because software determines the PWM on and off times, it's easy to set the ADC sampling times to capture the peak and valley currents.

ADVANCED SENSORLESS DIGITAL CURRENT CONTROL
A more sophisticated method uses the voltage versus current equations for capacitors and inductors to predict the current through the inductor:

Given:
V(t) = L di/dt
I(t) = I(t0) + 1/L × ∫ V(t) dt
I(t) = C dv/dt
V(t) = V(t0) + 1/C × ∫ i(t) dt

If additional information is available (such as the ESR of the output capacitor) and the resistance of the inductor is known, more accurate current estimates can be made. Calculate the ripple current by knowing the voltage across the inductor and the time the voltage is applied.

By measuring the voltage drop across the inductor, you can calculate the dc component of the current by subtracting the inductive voltage drop from the measured voltage drop. To calculate current, divide the voltage difference by the inductor resistance. Use a similar process for the voltage ripple on the filter capacitor.

FEED-FORWARD TECHNIQUES
One of the benefits of current-mode control is improved response to input voltage variations. With digital control of SMPS systems, providing feed-forward compensation for input voltage variations becomes easy. Most SMPS topologies have relatively simple transfer equations that describe the output voltage in terms of input voltage, duty cycle, and transformer turns ratio. You can add a voltage feed-forward term to either to the current set point or in parallel with the current control loop. The transfer equation for a buck converter is:

V_OUT = V_IN Duty Cycle/Period

Usually, the desired result of any control calculation is to create a value to be loaded into the PWM duty-cycle register:

Duty Cycle = V_OUT (Period/V_IN)

The cost of calculating the feed-forward compensation for the input voltage is the time required to perform the division operation. The feed-forward compensation techniques are inherently stable and provide faster transient response.