MAX20004 datasheet(12/20 Pages) MAXIM

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Applications Information

Maximum Output Current

While there are device versions that supply up to 8A,

there are many factors that may limit the average output

current to less than the maximum. The devices can be

thermally limited based on the selected fSW, number of

PCB layers, PCB size, and the maximum ambient tem-

perature. See the Typical Operating Characteristics sec-

tion for guidance on the maximum average current. For a

more precise value, the θJA needs to be measured in the

application environment.

Setting the Output Voltage

Connect FB to BIAS for a fixed 5V or 3.3V output volt-

age. To set the output to other voltages between 1V and

10V, connect a resistive divider from output (OUT) to FB

(Figure 1). Select RFB2 (FB to GND resistor) less than or

equal to 100kΩ. Calculate RFB1 (OUT to FB resistor) with

the following equation:

OUT

FB1

FB2









−





















where VFB is the feedback regulation voltage. See the

Electrical Characteristics table.

Add a capacitor, CFB1, as shown to compensate the pole

formed by the divider resistance and FB pin capacitance

as follows:

FB2

FB1

10pf





× 







Note: Applications that use a resistor divider to set

output voltages below 4.5V should use IC versions

that are factory trimmed for 3.3V fixed output voltage

to ensure full output current capability.

Forced-PWM and Skip Modes

In forced-PWM (FPWM) mode, the devices switch at a

constant frequency with variable on-time. In skip mode,

the converter’s switching frequency is load-dependent.

At higher load current, the switching frequency becomes

fixed and operation is similar to PWM mode. Skip mode

helps improve efficiency in light-load applications by

allowing switching only when the output voltage falls

below a set threshold. Since the effective switching

frequency is lower in skip mode at light load, gate charge

and switching losses are lower and efficiency is increased.

Inductor Selection

Three key parameters must be considered when select-

ing an inductor: inductance value (L), inductor saturation

current (ISAT), and DC resistance (RDCR). The devises

are designed to operate with the ratio of inductor peak-

to-peak AC current to DC average current (LIR) between

15% and 30% (typ). The switching frequency, input volt-

age, and output voltage then determine the inductor value

as follows:

(

)

SUP

OUT

MIN1

SUP

MAX

30%

−

××

where VSUP and VOUT are typical values (so that effi-

ciency is optimum for typical conditions) and IMAX is 4A

for MAX20004, 6A for MAX20006, and 8A for MAX20008,

and fSW is the switching frequency set by RFOSC. Note

that IMAX is the maximum rated output current for the

device, not the maximum load current in the application.

The next equation ensures that the internal compensating

slope is greater than 50% of the inductor current down slope:

≥

where m is the internal compensating slope and m2 is the

sensed inductor current down-slope as follows:

OUT

where RCS is 0.38 for MAX20004, 0.28 for MAX20006,

and 0.21 for MAX20008.

1.35

2.2MHz

Solving for L and using a 1.3 multiplier to account for

tolerances in the system:

MIN2

OUT

1.3

Figure 1. Adjustable Output-Voltage Setting

VOUT

CFB1

RFB2

RFB1

MAX20004/MAX20006/

MAX20008

36V, 220kHz to 2.2MHz, 4A/6A/8A

Fully Integrated Automotive

Step-Down Converters

www.maximintegrated.com

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