MAX26239/MAX26240

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

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Equation 9:

������������������������ =

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

(△ ������������������������ × ������������������ × ������ − ������������������������ × ������������������������ × ������������������) × ������������������������ × ������������������

When the input voltage reaches the minimum value, and the output voltage reaches the maximum value, the output

voltage is the largest.

Meanwhile the output capacitance is selected to satisfy the load-transient requirements. During a load step, the output

current changes almost instantaneously, whereas the inductor is slow to react. During this transition time, the load-charge

requirements are supplied by the output capacitors, which causes an undershoot in the output voltage. Select a capacitor

based on the maximum allowable undershoot on the output voltage. Typically, the worst-case response from a load

transient is in boost mode. The output capacitance for the allowable undershoot is calculated under the load transient in

boost mode, as in Equation 10:

Equation 10:

������������������������ =

△ ������������������������

2 × ������ ×△ ������������������������

������������ × ������������

where:

fC = Crossover frequency

∆IOUT = Transient load step

∆VOUTUS = Maximum allowable undershoot

Select the larger output capacitance of Equation 9 and Equation 10 as the final value of the output capacitance COUT

that can handle the given RMS current at the operating frequency. Ceramic capacitors are recommended for their low

ESR, ESL, small size, and high current ripple capability to bypass the pulsing ripple current, which helps reduce the peak-

to-peak voltage ripple at the output voltage and electromagnetic interference (EMI). The RMS ripple current of the output

capacitors is calculated in Equation 11:

Equation 11:

������������������������������

������������������ = ������������������������ × √

������������������������ − ������������������

������������������

Output Voltage Setting

Connect FB to VCC to enable the fixed output voltage set by a preset internal resistive voltage-divider connected between

the feedback pin (FB) and AGND. To externally adjust the output voltage between 3V and 20V, connect a resistive voltage-

divider from the output (OUT) to FB to AGND, as shown in Figure 1. Calculate RFB1 and RFB2 using Equation 12.

Equation 12:

������FB1 = ������FB2[(

������OUT

������FB

) − 1]

where:

VFB = 0.8V, typ

RFB2 < 50kΩ, can be typically set to 10kΩ