MAX42408/MAX42410

36V, 8A/10A Fully Integrated Buck Converter

with 10μA Quiescent Current and Dual-Phase

Capability

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Applications Information

Setting the Output Voltage

To externally program the output voltage between 0.8V and 10V for 400kHz switching frequency, and between 0.8V and

6V for 1.5MHz switching frequency, connect a resistor-divider from the buck converter output to FB, and then to GND.

Select RFB2 between FB and GND in Typical Application Circuits less than 20kΩ. Calculate RFB1 between buck output

and FB with the following equation:

RFB1 = RFB2 × (

VOUT

VFB

− 1)

Where, VFB = 0.8V, and RFB2 is less than 20 kΩ.

Table 2 provides components selection recommendation for each output range (CFF is recommended based on RFB2 =

10 kΩ).

Table 2. Recommended Component Selection

SWITCHING FREQUENCY

(kHz)

VOUT (V)

INDUCTOR

(µH)

OUTPUT CAPACITOR (µF)

CFF (pF)

400

0.8 to 1.8

0.68

500

N/A

400

1.8 to 3.3

1

220

N/A

400

3.3 to 5

2.2

120

82pF

400

5 to 7

2.2

88

56pF

400

7 to 10

3.3

66

47pF

1500

0.8 to 1.8

0.38

88

N/A

1500

1.8 to 3.3

0.47

88

N/A

1500

3.3 to 5

0.68

66

47pF

1500

5 to 6

0.68

44

15pF

Input Capacitor

The input capacitors reduce peak current drawn from the power source, and improve noise and voltage ripple on the SUP

nodes caused by the buck converter switching cycles. Use two ceramic input capacitors with 0.1μF and 4.7μF capacitance

in parallel at each side of the IC for proper buck operation.

Place a 0.1μF ceramic capacitor with 0402 or 0603 size next to SUP and PGND at each side of the IC to reduce input

noise and improve EMI performance. A 4.7μF ceramic capacitor after 0.1μF capacitor is required on each input side to

reduce input voltage ripple. Additional buck capacitor might be required if high impedance exists in the input supply or

traces.

The input capacitor RMS current requirement (IRMS) is defined by the following equation:

IRMS = ILOAD(MAX) × (

√VOUT × (VSUP − VOUT )

VSUP

)

IRMS has a maximum value when the input voltage equals twice the output voltage:

VSUP = 2 × VOUT

Therefore:

IRMS =

ILOAD(MAX)

2

Choose an input capacitor that exhibits less than +10°C self-heating temperature rise at the RMS input current for optimal

long-term reliability. The input-voltage ripple comprises ΔVQ (caused by the capacitor discharge) and ΔVESR (caused by

the ESR of the capacitor). Assume the contribution from the ESR and capacitor discharge equal to 50%. Calculate the

input capacitance and ESR required for a specified input voltage ripple using the following equations: