MAX5986AETE+【PDF 16/23 页】IC POE SWITCH PSE 16TQFN

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MAX5986AMAX5986C/MAX5987A

IEEE 802.3af-Compliant, High-Efficiency, Class 1/

Class 2, PDs with Integrated DC-DC Converter

Maxim Integrated

Inductor Selection

Choose an inductor with the following equation:

where LIR is the ratio of the inductor ripple current to

full load current at the minimum duty cycle. Choose LIR

between 20% to 40% for best performance and stability.

Use an inductor with the lowest possible DC resistance

that fits in the allotted dimensions. Powdered iron ferrite

core types are often the best choice for performance.

With any core material, the core must be large enough

not to saturate at the current limit of the devices.

Input Capacitor Selection

The input capacitor reduces the current peaks drawn

from the input power supply and reduces switching noise

in the IC. The total input capacitance must be equal or

greater than the value given by the following equation

to keep the input-ripple voltage within specification and

minimize the high-frequency ripple current being fed

back to the input source:

where V

IN-RIPPLE

is the maximum allowed input ripple

voltage across the input capacitors and is recommended

to be less than 2% of the minimum input voltage. D is the

duty cycle (V

OUT

) and T

is the switching period (1/

The impedance of the input capacitor at the switching

frequency should be less than that of the input source so

high-frequency switching currents do not pass through

the input source, but are instead shunted through the

input capacitor. The input capacitor must meet the ripple

current requirement imposed by the switching currents.

The RMS input ripple current is given by:

where I

RIPPLE

is the input RMS ripple current.

Output Capacitor Selection

The key selection parameters for the output capacitor are

capacitance, ESR, ESL, and voltage-rating requirements.

These affect the overall stability, output ripple voltage,

and transient response of the DC-DC converter. The out-

put ripple occurs due to variations in the charge stored in

the output capacitor, the voltage drop due to the capaci-

tors ESR, and the voltage drop due to the capacitors

ESL. Estimate the output-voltage ripple due to the output

capacitance, ESR, and ESL:

RIPPLE

= V

RIPPLE(C)

+ V

RIPPLE(ESR)

RIPPLE(ESL)

where the output ripple due to output capacitance, ESR,

and ESL is:

or whichever is larger. The peak-to-peak inductor current

P-P

)

Use these equations for initial output capacitor selec-

tion. Determine final values by testing a prototype or an

evaluation circuit. A smaller ripple current results in less

output-voltage ripple. Since the inductor ripple current is

a factor of the inductor value, the output-voltage ripple

decreases with larger inductance. Use ceramic capaci-

tors for low ESR and low ESL at the switching frequency

of the converter. The ripple voltage due to ESL is negli-

gible when using ceramic capacitors.

Load-transient response depends on the selected output

capacitance. During a load transient, the output instantly

changes by ESR x I

LOAD

. Before the controller can

respond, the output deviates further, depending on the

inductor and output capacitor values. After a short time,

the controller responds by regulating the output voltage

back to its predetermined value. The controller response

time depends on the closed-loop bandwidth. A higher

bandwidth yields a faster response time, preventing the

output from deviating further from its regulating value.

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