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

VIN

(36V TO 72V)

CDD

4.7µF

25.5kΩ

RCL

CCC

10µF

CSS

0.1µF

1N4148

6 NT

VDD

GND

V+

NDRV

RCL

14 NR

CMHD2003

CIN

3✕

0.47µF NP

14

M1

IRF640N

SBL204OCT

NS 20Ω

5

1nF

VCC MAX5942 CS

V-

UVLO

SS_SHDN

PGOOD

OUT

100Ω

RSENSE

100mΩ

GATE

VEE

GATE

FB

CFB

L1

4.7µH

(OPTIONAL)

VOUT

5V/10A

COUT

3✕

560µF

0.1µF

R1

2kΩ

R2

2kΩ

Figure 5. Forward Converter

Current-Sense Comparator

The current-sense (CS) comparator and its associated

logic limit the peak current through the MOSFET.

Current is sensed at CS as a voltage across a sense

resistor between the source of the MOSFET and GND.

To reduce switching noise, connect CS to the external

MOSFET source through a 100Ω resistor or an RC low-

pass filter (Figures 5, 6). Select the current-sense resis-

tor, RSENSE according to the following equation:

RSENSE = 0.465V /ILIMPrimary

where ILIMPrimary is the maximum peak primary-side

current.

When VCS > 465mV, the power MOSFET switches off.

The propagation delay from the time the switch current

reaches the trip level to the driver turn-off time is 180ns.

Internal Error Amplifier

The MAX5942A/MAX5942B include an internal error

amplifier that can be used to regulate the output volt-

age in the case of a nonisolated power supply (see

Figure 5). Calculate the output voltage using the follow-

ing equation:

VOUT =



1+

R1

R2





× VREF

where VREF = 2.4V.

Choose R1//R2 << RIN, where RIN ≅ 50kΩ is the input

resistance of FB. The gain of the error amplifier is inter-

nally configured for -20 (see Figure 4).

The error amplifier may also be used to regulate the out-

put of the tertiary winding for implementing a primary-

side regulated isolated power supply (see Figure 7).

Calculate the output voltage using the following equation:

VOUT

=

NS

NT



1+

R1

R2





× VREF

where NS is the number of secondary turns and NT is

the number of tertiary winding turns.

16 ______________________________________________________________________________________

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