8.2 Inductor Selection and Slope Compensation

To select the best inductor for the application, the following factors must be considered: inductance value, RMS rated current, saturation current and DC resistance (DCR).

The inductance value is critical for the correct operation of the MCP16367/8/9. For stable current loop operation where duty cycle is above 50%, slope compensation is internally added according to the output voltage and the input-output voltage ratio.

For the proper amount of slope compensation, it is recommended to keep the inductor down-slope current constant by varying the inductance with V_OUT.

Table 8-1. Recommended Inductor Values
V_OUT	L_STANDARD
2.0V	5.6 µH
3.3V	8.2 µH
5.0V	15 µH
12V	33 µH
15V	33 µH

The inductor RMS current represents the current at which the inductor’s temperature rises by +20°C to +40°C, depending on manufacturer. For inductor saturation current, it represents the peak current at which the inductor exhibits a loss of inductance value between 10% and 30%, depending on manufacturer. For proper operation, ensure that the nominal and peak currents of the application are well within the permissible current ratings of the inductor.

The peak inductor current can be calculated with Figure 1.

Equation 8-2. Peak Inductor Current

$I_{L P E A K} = (I_{O U T} + V_{O U T} \times \frac{1 - V_{O U T} / V_{I N}}{2 \times f_{s w} \times L})$

Where:


I_OUT	=	Nominal output current
V_OUT	=	Output voltage
V_IN	=	Input voltage
f_sw	=	Switching frequency
L	=	Inductance value

When choosing the inductor, a sufficient design margin must be taken into account so that the inductor does not enter deep saturation. Overcurrent situations must also be taken into account, especially at very high input voltages where the inductor current can increase very rapidly.