MAX5942ACSE(2003) データシートの表示（PDF） - Maxim Integrated

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MAX5942ACSE
(Rev.:2003)

IEEE 802.3af Power-Over-Ethernet Interface/PWM Controller for Power Devices

Maxim Integrated 

IEEE 802.3af Power-Over-Ethernet

Interface/PWM Controller for Power Devices

Table 3. Component Suppliers

COMPONENT

Power FETS

Current-Sense Resistors

Diodes

Capacitors

Magnetics

SUPPLIERS

International Rectifier

Fairchild

Vishay-Siliconix

Dale-Vishay

IRC

ON Semi

General Semiconductor

Central Semiconductor

Sanyo

Taiyo Yuden

AVX

Coiltronics

Coilcraft

Pulse Engineering

WEBSITE

www.irf.com

www.fairchildsemi.com

www.vishay.com/brands/siliconix/main.html

www.vishay.com/brands/dale/main.html

www.irctt.com/pages/index.cfm

www.onsemi.com

www.gensemi.com

www.centralsemi.com

www.sanyo.com

www.t-yuden.com

www.avxcorp.com

www.cooperet.com

www.coilcraft.com

www.pulseeng.com

6) Choose RSENSE according to the following equation:

RSENSE

≤

NS

NP

VILIM

×1.2 × IOUTMAX

where:

VILIM is the current-sense comparator trip threshold

voltage (0.465V).

NS/NP is the secondary-side turns ratio (5/14 in this

example).

IOUTMAX is the maximum DC output current (10A in

this example):

RSENSE ≤

6

0.465V

×1.2 ×10

=

90.4mΩ

14

7) Choose the inductor value so that the peak ripple

current (LIR) in the inductor is between 10% and

20% of the maximum output current:

L ≥ (VOUT + VD ) × (1-DMIN)

2 × LIR × 275kHz × IOUTMAX

where VD is the output Schottky diode forward-volt-

age drop (0.5V) and LIR is the ratio of inductor rip-

ple current to DC output current:

8) The size and ESR of the output filter capacitor deter-

mine the output ripple. Choose a capacitor with a

low ESR to yield the required ripple voltage.

Use the following equations to calculate the peak-to-

peak output ripple:

VRIPPLE = VR2IPPLE,ESR + VR2IPPLE,C

where:

VRIPPLE is the combined RMS output ripple due to

VRIPPLE,ESR, the ESR ripple, and VRIPPLE,C, the

capacitive ripple. Calculate the ESR ripple and

capacitive ripple as follows:

VRIPPLE,ESR = IRIPPLE x ESR

VRIPPLE,C = IRIPPLE/(2 x π x 275kHz x COUT)

Layout Recommendations

All connections carrying pulsed currents must be very

short, be as wide as possible, and have a ground plane

as a return path. The inductance of these connections

must be kept to a minimum due to the high di/dt of the

currents in high-frequency switching power converters.

Current loops must be analyzed in any layout pro-

posed, and the internal area kept to a minimum to

reduce radiated EMI. Ground planes must be kept as

intact as possible.

L ≥ (5.5) × (1- 0.198) = 4.01µH

0.4 × 275kHz ×10A

20 ______________________________________________________________________________________

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