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Applications Information—Quad-Phase Configurable Buck Converter

Inductor Selection

An inductor with a saturation current that is greater than or equal to the peak current limit setting (IPLIM) is recommended.

The load current requirement (per phase) of the system is also a consideration when choosing the RMS current rating

of the inductor. Inductors with lower saturation current and higher DCR ratings tend to be physically small. However,

higher values of DCR reduce the efficiency. To choose a suitable inductor for the given application, consider the trade-off

between the size of the inductor versus the DCR value. It is recommended to choose an inductance so that the ratio of

the inductor's ripple current to the average current is between 30% and 60%.

Consider the output-voltage range and switching frequency when choosing the inductance. In general, 0.47μH is suitable

for low-range outputs with 1.0MHz or 1.5MHz switching frequency. For mid-range and high-range outputs, 1.0μH–1.5μH

is recommended. Note that higher inductances slow down the maximum slew rate of the inductor current, and high-duty

cycles (VIN close to VOUT) coupled with large inductance can slow down the load transient response.

Table 19. Recommended Inductors

MANUFACTURER

PART NUMBER

INDUCTANCE (μH)

TYPICAL

DCR (mΩ)

TYPICAL

ISAT (A)

TYPICAL

ITEMP (A)

DIMENSION

(L x W x H) (mm)

DFE252012F-R47M

0.47 ±20%

6.7

4.9

2.5 x 2.0 x 1.2

DFE252012F-1R0M

1.0 ±20%

4.7

3.3

2.5 x 2.0 x 1.2

XEL4020-152ME

1.5 ±20%

21.45

7.4

5.2

4.0 x 4.0 x 3.25

For multiphase configurations, each phase on the same output needs its own inductor with the same inductance value

(do not short the LX nodes of different phases together on the PCB). See the Phase and Output Configuration section for

more information regarding different phase configurations.

Input-Capacitor Selection

The input capacitor (CIN) reduces the current peaks drawn from the battery or the input power source and reduces

switching noise in the device. The impedance of the CIN at the switching frequency should be kept very low. Ceramic

capacitors with X7R dielectric are highly recommended due to their small size, low ESR, and small temperature

coefficients. For most applications, a 10μF capacitor is sufficient.

Output-Capacitor Selection

The output-capacitor (COUT) is required to keep the output-voltage ripple small and to ensure regulation loop stability.

The COUT must have low impedance at the switching frequency. Ceramic capacitors with X7R dielectric are highly

recommended due to their small size, low ESR, and small temperature coefficients. The recommended minimum

effective output capacitance per phase is shown in Table 20.

Table 20. Recommended Minimum Effective Output Capacitance

VOUT RANGE

SWITCHING FREQUENCY

MINIMUM EFFECTIVE COUT*

Low (0.3V to 1.3V)

1MHz

42μF

Mid (0.6V to 2.6V)

1MHz

24μF

High (1.2V to 5.2V)

1MHz

16μF

*Required minimum COUT(EFF) is inversely proportional to the switching frequency setting. For example, a buck output

using Mx_RNG = 0x0 and 1MHz switching frequency requires 83μF of minimum effective output capacitance. Changing

the switching frequency to 1.5MHz decreases the effective output capacitance requirement to 56μF (= 83μF/1.5).

The effective COUT is the actual capacitance value seen by the buck output during operation. The nominal capacitance

(COUT) needs to be selected carefully by considering the capacitor's initial tolerance, variation with temperature, and

derating with DC bias. Refer to Tutorial 5527 for more information. Larger values of the COUT (above the required

minimum effective) improve load transient performance, but increase the input inrush currents during startup. The output

filter capacitor must have low enough ESR to meet output ripple and load transient requirements. The output capacitance

MAX77542

16VIN/16A, Quad-Phase High-Efficiency Buck

Converter

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