MAX26239/MAX26240

4.5V to 36V, 6A Buck-Boost Converters

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Applications Information

Inductor Selection

Design of the inductor is a compromise between the size, efficiency, control, bandwidth, and stability of the converter. For

a buck-boost application, selecting the right value of inductor becomes even more critical due to the presence of right-

half-plane (RHP) zero in boost and buck-boost mode. A larger inductance value would reduce RMS current loss in

MOSFETs, core and winding losses in the inductor. On the other hand, it slows the control loop and reduces the frequency

of the RHP zero, which can cause stability concerns.

Start the selection of the inductor based on the inductor peak-to-peak current ripple as a percentage of the maximum

inductor current in buck and boost modes of operation using Equations 1, 2, and 3. Typically, 40% ripple of the maximum

inductor current is a good compromise between speed and efficiency.

Equation 1:

������������������������������ =

(������������������_������������������ − ������������������������) × ������������������������

������������������ × ∆������������

������−������ × ������������������_������������������

where:

VIN_MAX = Maximum input voltage

VOUT = Output voltage

∆ILP-P = Peak-to-peak current ripple of the inductor

fSW = Switching frequency

Equation 2:

������������������������������������ =

(������������������������_������������������ − ������������������ ) × ������������������

������������������ × ∆������������

������−������ × ������������������������_������������������

where:

VIN = Input voltage

VOUT_MAX = Maximum output voltage

Select the larger of LBUCK and LBOOST as the final value of inductance L. Once the final value of inductance L is selected,

calculate the actual peak inductor current using Equation 3 and choose an inductor with saturation current ≈20% more

than the peak inductor current and the low DC resistance (DCR).

Equation 3:

������������

������������������������ =

������������������������ × ������������������������

������������������_������������������ × ������

+

������������������_������������������ × (1 −

������������������_������������������

������������������������

)

������ × ������������������ × 2

where:

VIN_MIN = Minimum input voltage

IOUT = Output current

η = Power conversion efficiency

L = Inductor value

Maximum Output Current

The MAX26239/MAX26240 sense the peak inductor current to limit the output current. The maximum output current is

determined by the operating conditions and component selection that impact the peak inductor current. At a heavy load

and high output voltage, thermal limitations impact the output current capability. Use θJA to estimate the junction

temperature at specific operating conditions to determine whether the device will trigger thermal shutdown.