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PDF MAX5942A Data sheet ( Hoja de datos )
| Número de pieza | MAX5942A | |
| Descripción | IEEE 802.3af Power-Over-Ethernet Interface/PWM Controller for Power Devices | |
| Fabricantes | Maxim Integrated | |
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19-3024; Rev 0; 10/03
IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
General Description
The MAX5942A/MAX5942B integrate a complete power
IC for powered devices (PD) in a power-over-ethernet
(PoE) system. The MAX5942A/MAX5942B provide a PD
interface and a compact DC-DC PWM controller suitable
for flyback and forward converters in either isolated or
nonisolated designs.
The MAX5942A/MAX5942B PD interface complies with
the IEEE 802.3af standard, providing the PD with a detec-
tion signature, a classification signature, and an integrat-
ed isolation switch with programmable inrush current
control. These devices also feature power-mode under-
voltage lockout (UVLO) with wide hysteresis and power-
good status outputs.
The MAX5942A/MAX5942B also integrate all the build-
ing blocks necessary for implementing DC-DC fixed-
frequency isolated power supplies. This device is a
current-mode controller with an integrated high startup
circuit suitable for isolated telecom/industrial voltage
range power supplies. A high-voltage startup circuit
allows the PWM controller to draw power directly from the
18V to 67V input supply during startup. The switching
frequency is internally trimmed to 275kHz ±10%, thus
reducing magnetics and filter components. The
MAX59412A allows an 85% operating duty cycle and can
be used to implement flyback converters. The MAX5942B
limits the operating duty cycle to less than 50% and can
be used in single-ended forward converters. The
MAX5942A/MAX5942B are designed to work with or with-
out an external diode bridge in front of the PD.
The MAX5942A/MAX5942B are available in 16-pin SO
packages.
Applications
IP Phones
Wireless Access Nodes
Internet Appliances
Computer Telephony
Security Cameras
Power Devices in Power-Over-Ethernet/
Power-Over-MDI
Typical Operating Circuit appears at end of data sheet.
Features
o Powered Device Interface
Fully Integrated IEEE 802.3af-Compliant PD
Interface
PD Detection and Programmable Classification
Signatures
Less than 10µA Leakage Current Offset During
Detection
Integrated MOSFET for Isolation and Inrush
Current Limiting
Gate Output Allows External Control of the
Internal Isolation FET
Programmable Inrush Current Control/ULVO
PGOOD/PGOOD Outputs Enable PWM
Controller
o PWM Controller
Wide Input Range: 18V to 67V
Isolated (Without Optocoupler) or Nonisolated
Power Supply
Current-Mode Control
Leading-Edge Blanking
Internally Trimmed 275kHz ±10% Oscillator
Soft-Start
Ordering Information
PART
TEMP RANGE PIN-
MAX DUTY
PACKAGE CYCLE (%)
MAX5942AESE* -40°C to +85°C 16 SO
85
MAX5942ACSE 0°C to +70°C 16 SO
85
MAX5942BESE* -40°C to +85°C 16 SO
MAX5942BCSE 0°C to +70°C 16 SO
50
50
*Future product—contact factory for availability.
Pin Configuration
TOP VIEW
V+ 1
VDD 2
FB 3
SS_SHDN 4
ULVO 5
RCL 6
GATE 7
VEE 8
MAX5942A
MAX5942B
SO
16 VCC
15 NDRV
14 V-
13 CS
12 GND
11 PGOOD
10 PGOOD
9 OUT
________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1 page  
IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
ELECTRICAL CHARACTERISTICS (PWM Controller) (continued)
(All voltages referenced to V-. VDD = 13V, a 10µF capacitor connects VCC to V-, VCS = V-, V+ = 48V, 0.1µF capacitor connected to
SS_SHDN, NDRV = open circuit, VFB = 3V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX UNITS
SOFT-START
SS Source Current
ISSO
VSS_SHDN = V-
2.0 4.5 6.5
µA
SS Sink Current
1 mA
Peak Soft-Start Voltage Clamp
No external load
2.331 2.420 2.500
V
Shutdown Threshold
VSS_SHDN falling (Note 11)
VSS_SHDN rising (Note 11)
0.25
0.53
0.37
0.59
0.41
0.65
V
Note 1: All min/max limits for the PD interface are production tested at +85°C (extended grade)/+70°C (commercial grade). Limits
at +25°C and -40°C are guaranteed by design. All PWM controller min/max limits are 100% production tested at +25°C
and +85°C (extended grade)/+70°C (commercial grade). Limits at -40°C are guaranteed by design, unless otherwise
noted.
Note 2: The input offset current is illustrated in Figure 1.
Note 3: Effective differential input resistance is defined as the differential resistance between GND and VEE without any external
resistance.
Note 4: Classification current is turned off whenever the IC is in power mode.
Note 5: See Table 2 in the PD Classification Mode section. RDISC and RCL must be 100ppm or better.
Note 6: See Thermal Dissipation section for details.
Note 7: When UVLO is connected to the midpoint of an external resistor-divider with a series resistance of 25.5kΩ (±1%), the turn-
on threshold set point for the power mode is defined by the external resistor-divider. Make sure the voltage on the UVLO
pin does not exceed its maximum rating of 8V when VIN is at the maximum voltage.
Note 8: When the VUVLO is below VTH, G, UVLO, the MAX5942_ sets the turn-on voltage threshold internally (VUVLO,ON).
Note 9: An input voltage or VUVLO glitch below their respective thresholds shorter than or equal to tOFF_DLY will not cause the
MAX5942A/MAX5942B to exit power-on mode (as long as the input voltage remains above an operable voltage level of 12V).
Note 10: PGOOD references to OUT while PGOOD references to VEE.
Note 11: Guaranteed by design.
IIN
IINi +1
IINi
dRi
≅
(VINi
(IINi
+
+
1
1
-
-
VINi)
IINi)
=
(IINi
1V
+1
-
IINi)
IOFFSET
≅
IINi
-
VINi
dRi
dRi
IOFFSET
VINi 1V VINi +1
Figure 1. Effective Differential Input Resistance/Offset Current
_______________________________________________________________________________________ 5
5 Page 
IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
Table 1. PD Power Classification/RCL Selection
CLASS
0
1
2
3
4
USAGE
Default
Optional
Optional
Optional
Not allowed
RCL (Ω)
10k
732
392
255
178
MAXIMUM POWER USED BY PD (W)
0.44 to 12.95
0.44 to 3.84
3.84 to 6.49
6.49 to 12.95
Reserved*
*Class 4 reserved for future use.
Detailed Description
The MAX5942A/MAX5942B integrate a complete power
IC for powered devices (PD) in a power-over-ethernet
(PoE) system. The MAX5942A/MAX5942B provide PD
Interface and a compact DC-DC PWM controller suitable
for flyback and forward converters in either isolated or
nonisolated designs.
The MAX5942A/MAX5942B PD interface complies with
the IEEE 802.3af standard, providing the PD with a
detection signature, a classification signature, and an
integrated isolation switch with programmable inrush
current control. These devices also feature power-mode
undervoltage lockout (UVLO) with wide hysteresis, and
power-good status outputs.
An integrated MOSFET provides PD isolation during
detection and classification. The MAX5942A/MAX5942B
guarantee a leakage current offset of less than 10µA dur-
ing the detection phase. A programmable current limit
prevents high inrush current during power-on. The
devices feature power-mode UVLO with wide hysteresis
and long deglitch time to compensate for twisted-pair
cable resistive drop and to ensure glitch-free transition
between detection, classification, and power-on/off
phases. The MAX5942A/MAX5942B provide both active-
high (PGOOD) and active-low (PGOOD) outputs. Both
devices offer an adjustable UVLO threshold with a
default value compliant to the IEEE 802.3af standard.
The MAX5942A/MAX5942B are designed to work with or
without an external diode bridge in front of the PD.
Use the MAX5942A/MAX5942B PWM current-mode con-
trollers to design flyback- or forward-mode power sup-
plies. Current-mode operation simplifies control-loop
design while enhancing loop stability. An internal high-
voltage startup regulator allows the device to connect
directly to the input supply without an external startup
resistor. Current from the internal regulator starts the con-
troller. Once the tertiary winding voltage is established,
the internal regulator is switched off and bias current
for running the PWM controller is derived from the
tertiary winding. The internal oscillator is set to 275kHz
and trimmed to ±10%. This permits the use of small
magnetic components to minimize board space. Both the
MAX5942A and MAX5942B can be used in power sup-
plies providing multiple output voltages. A functional dia-
gram of the PWM controller is shown in Figure 4. Typical
application circuits for forward and flyback topologies are
shown in Figure 5 and Figure 6, respectively.
Powered Device Interface
Operating Modes
The PD front-end section of the MAX5942A/MAX5942B
operates in three different modes: PD detection signa-
ture, PD classification, and PD power, depending on its
input voltage (VIN = GND - VEE). All voltage thresholds
are designed to operate with or without the optional
diode bridge while still complying with the IEEE 802.3af
standard (see Application Circuit 1).
Detection Mode (1.4V ≤ VIN ≤ 10.1V)
In detection mode, the power source equipment (PSE)
applies two voltages on VIN in the range of 1.4V to
10.1V (1V step minimum), and then records the current
measurements at the two points. The PSE then com-
putes ∆V/∆I to ensure the presence of the 25.5kΩ sig-
nature resistor. In this mode, most of the
MAX5942A/MAX5942B internal circuitry is off and the
offset current is less than 10µA.
If the voltage applied to the PD is reversed, install pro-
tection diodes on the input terminal to prevent internal
damage to the MAX5942A/MAX5942B (see Figures 8
and 9). Since the PSE uses a slope technique (∆V/∆I) to
calculate the signature resistance, the DC offset due to
the protection diodes is subtracted and does not affect
the detection process.
Classification Mode (12.6V ≤ VIN ≤ 20V)
In the classification mode, the PSE classifies the PD
based on the power consumption required by the PD.
This allows the PSE to efficiently manage power distribu-
tion. The IEEE 802.3af standard defines five different
classes as shown in Table 1. An external resistor (RCL)
connected from RCL to VEE sets the classification current.
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet MAX5942A.PDF ] | |
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