4.1 Relationship Between V_COMP and S_e

Since the PWM comparator is a high gain element, it is reasonable to consider the positive input equal to the negative input when the high-side FET is turned off. Figure 3 shows the steady-state. Equation 4 calculates the small signal relationship between V_COMP, i_L, and S_e.

Figure 3. PWM Comparator Input Signals

Equation 4.

where

• i_Lpp is the inductor ripple current. Equation 5 calculates i_Lpp.

Equation 5.

where

• R_i is the current sensing gain. Equation 1 and Equation 3 calculate R_i.
• T_ON is high-side FET ON time. Equation 6 calculates T_ON. You can also use an oscilloscope to measure the value.
• T_OFF is high-side FET OFF time

Equation 6.

where

• S_n is the inductor current rising slope
• S_f is inductor current falling slope

In addition, the other parameters including the V_COMP, switching frequency (F_SW), input voltage (V_IN), and output voltage (V_OUT) can be measured in the lab.