MAX20021 datasheet(13/16 Pages) MAXIM

13 / 16 page

MAX20021/MAX20022

Automotive Quad, Low-Voltage

Step-Down DC-DC Converters

www.maximintegrated.com

Maxim Integrated │ 13

The external feedback resistive divider must be frequency

compensated for proper operation. Place a capacitor

across R1 in the resistive divider network. Use the follow-

ing equation to determine the value of the capacitor:

1, C1 C

else C1 C, where C 15pF



= 



Connect OUTS_ to VOUT_ for a fixed 1.0V output voltage.

Inductor Selection

The PMICs are optimized for use with a 1.5µH inductor

for 2.2MHz and 3.2MHz operation. Chip inductors can be

used for additional board-space savings.

Input Capacitor

The PMICs are designed to operate with a single 2.2µF

ceramic bypass capacitor on each PV_ input. Phase

interleaving of the four buck converters contributes to

a lower required input capacitance by canceling input

ripple currents. Place the bypass capacitors as close as

possible to their corresponding PV_ input to ensure the

best EMI and jitter performance.

Output Capacitor

All outputs of the PMICs are optimized for use with a 10FF

X7R ceramic capacitor. Additional output capacitance can

be used if better voltage ripple or load transient response

is required. Due to the soft-start sequence, the device is

unable to drive arbitrarily large output capacitors.

Thermal Considerations

How much power the package can dissipate strongly

depends on the mounting method of the IC to the PCB

and the copper area for cooling. Using the JEDEC test

standard, the maximum power dissipation allowed is

2285mW in the TQFN package. More power dissipation

can be handled by the package if great attention is given

during PCB layout. For example, using the top and bottom

copper as a heatsink and connecting the thermal vias to

one of the middle layers (GND) transfers the heat from the

package into the board more efficiently, resulting in lower

junction temperature at high power dissipation in some

PMIC applications. Furthermore, the solder mask around

the IC area on both top and bottom layers can be removed

to radiate the heat directly into the air. The maximum

allowable power dissipation in the IC is as follows:

(

)

J(MAX)

MAX

−

+ θ

where TJ(MAX) is the maximum junction temperature

(+150ºC), TA is the ambient air temperature, BJC (3ºC/W

for the 28-pin TQFN) is the thermal resistance from the

junctiontothecase,andθCA is the thermal resistance from

the case to the surrounding air through the PCB, copper

traces,andthepackagematerials.θCA is directly related

to system-level variables and can be modified to increase

the maximum power dissipation.

The TQFN package has an exposed thermal pad on its

underside. This pad provides a low thermal-resistance path

for heat transfer into the PCB. This low thermally resistive

path carries a majority of the heat away from the IC. The

PCB is effectively a heatsink for the IC. The exposed pad

should be connected to a large ground plane for proper

thermal and electrical performance. The minimum size of

the ground plane is dependent upon many system vari-

ables. To create an efficient path, the exposed pad should

be soldered to a thermal landing, which is connected to

the ground plane by thermal vias. The thermal landing

should be at least as large as the exposed pad and can be

made larger depending on the amount of free space from

the exposed pad to the other pin landings. A sample lay-

out is available on the MAX20022 evaluation kit to speed

designs.

PCB Layout Guidelines

Careful PCB layout is critical to achieve low switching

losses and clean, stable operation. Use a multilayer board

whenever possible for better noise immunity and power

dissipation. Follow these guidelines for good PCB layout:

1) Use a large contiguous copper plane under the PMIC

packages. Ensure that all heat-dissipating components

have adequate cooling.

2) Keep the high-current paths short, especially at the

ground terminals. This practice is essential for stable,

jitter–free operation. The high current path comprising

of input capacitor, inductor, and the output capacitor

should be as short as possible.

Figure 5. Adjustable Output-Voltage Configuration

MAX20022

VOUT_

OUTS_

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