8.2.2.2 Selecting the Inductor

It is suggested that the TPS61391 device works in the DCM operation; otherwise the output voltage would not be delivered for low input voltage to high output voltage.

With the device working in DCM operation, the maximum inductor could be calculated by equation Equation 1 and Equation 2:

Equation 1.

where

• V_IN is input voltage
• D is duty cycle
• f_SW is switching frequency
• I_LIM is current limit

For instance, if V_IN = 3.3 V, V_OUT = 60 V, f_SW = 600 kHz, I_LIM = 0.8 A, the L_MAX = 6.5 µH

However, there is minimum inductance is determined by the power delivered to the output side at given input condition.

Equation 2.

where

• V_OUT is output voltage
• I_OUT is output current
• eff is the efficiency
• f_SW is switching frequency
• I_LIM is current limit

For instance, if I_OUT = 8 mA, V_OUT = 60 V, f_SW = 600 kHz, I_LIM = 0.8 A, eff = 0.6, the L_MIN = 4.2 µH

With the calculation aforementioned, the operating inductor is recommended between the L_MIN and L_MAX.

The 4.7 µH inductance is optimum value for using the TPS61391 in application.