28V, PWM, Step-Up DC-DC Converter

10

______________________________________________________________________________________

Maximum Output Current

The MAX618’s 2.2A LX current limit determines the

output power that can be supplied for most applica-

tions. In some cases, particularly when the input volt-

age 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 40m

Ω inductor resistance.

Capacitor Selection

Input Capacitors

ripple created by the boost configuration. High-imped-

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 0.3

Ω are gener-

ally 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 capacitor 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 cur-

rent and the ESR of the output capacitor. ESR below

50m

Ω generates acceptable levels of output ripple for

most applications.

Integrator Capacitor

nant pole in the MAX618’s transfer function. The proper

compensation 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 capaci-

tance needed for the integrator capacitor with the fol-

lowing formula:

Pole Compensation Capacitor

ity 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:

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 proto-

typing 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 resis-

tance, 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 possi-

ble to IN and GND.

Refer to the MAX618 evaluation kit for an example of

proper board layout.

C

R

C

(R2 R2)

R1 R2

P

ESR

OUT

=

+

⋅

⋅

C

C

Table

C

C

Table

COMP

COMP

OUT

OUT

=

⋅

(

)

(

)

5

4