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MAX618EEE

28V Internal Switch, Step-Up DC-DC Converter

Maxim Integrated 

MAX618

28V Internal Switch, Step-Up DC-DC Converter

Maximum Output Current

The MAX618’s 2.2A LX current limit determines the output

power that can be supplied for most applications. In some

cases, particularly when the input voltage is low, output

power is sometimes restricted by package dissipation

limits. The MAX618 is protected by a thermal shutdown

circuit that turns off the switch when the die temperature

exceeds +150°C. When the device cools by 10°C, the

switch is enabled again. Table 3 details output current

with a variety of input and output voltages. Each listing in

Table 3 is either the limit set by an LX current limit or by

package dissipation at +85°C ambient, whichever is lower.

The values in Table 3 assume a 40mΩ inductor resistance.

Capacitor Selection

Input Capacitors

The input bypass capacitor (CIND) reduces the input

ripple created by the boost configuration. High-impedance

sources require high CIND values. However, 68μF is

generally adequate for input currents up to 2A. Low-ESR

capacitors are recommended because they will decrease

the ripple created on the input and improve efficiency.

Capacitors with ESR below 0.3Ω are generally appropriate.

In addition to the input bypass capacitor, bypass IN with a

1μF ceramic capacitor placed as close to the IN and GND

pins as possible. Bypass VL with a 4.7μF ceramic capaci-

tor placed as close to the VL and GND pins as possible.

Output Capacitor

Use Table 4 to find the minimum output capacitance

necessary to ensure stable operation. In addition, choose

an output capacitor with low ESR to reduce the output

ripple. The dominant component of output ripple is the

product of the peak-to-peak inductor ripple current and

the ESR of the output capacitor. ESR below 50mΩ

generates acceptable levels of output ripple for most

applications.

Integrator Capacitor

The compensation capacitor (CCOMP) sets the dominant

pole in the MAX618’s transfer function. The proper com-

pensation capacitance depends upon output capacitance.

Table 5 shows the capacitance value needed for the

output capacitances specified in Table 4. However, if a

different output capacitor is used (e.g., a standard value),

then recalculate the value of capacitance needed for the

integrator capacitor with the following formula:

C COMP

=

CCOMP(Table 5) × COUT

COUT(Table 4)

Pole Compensation Capacitor

The pole capacitor (CP) cancels the unwanted zero intro-

duced by COUT’s ESR, and thereby ensures stability in

PWM operation. The exact value of the pole capacitor is

not critical, but it should be near the value calculated by

the following equation:

CP

=

R ESR

×

COUT (R1

R1 × R 2

+

R2)

where RESR is COUT’s ESR.

Layout Considerations

Proper PC board layout is essential due to high current

levels and fast switching waveforms that radiate noise.

Use the MAX618 evaluation kit or equivalent PC layout to

perform initial prototyping. Breadboards, wire-wrap, and

proto-boards are not recommended when prototyping

switching regulators.

It is important to connect the GND pin, the input bypass

capacitor ground lead, and the output filter capacitor

ground lead to a single point to minimize ground noise

and improve regulation. Also, minimize lead lengths to

reduce stray capacitance, trace resistance, and radiated

noise, with preference given to the feedback circuit, the

ground circuit, and LX. Place the feedback resistors as

close to the FB pin as possible. Place a 1μF input bypass

capacitor as close as possible to IN and GND.

Refer to the MAX618 evaluation kit for an example of

proper board layout.

Chip Information

PROCESS: BiCMOS

Package Information

For the latest package outline information and land patterns

(footprints), go to www.maximintegrated.com/packages. Note

that a “+”, “#”, or “-” in the package code indicates RoHS status

only. Package drawings may show a different suffix character, but

the drawing pertains to the package regardless of RoHS status.

PACKAGE TYPE PACKAGE CODE DOCUMENT NO.

16 QSOP

EF16+8F

21-0055

www.maximintegrated.com

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