MAX5971A Maxim Integrated Products, MAX5971A Datasheet - Page 19

• Manufacturer Part Number: MAX5971A
• Description: Single-Port 40W IEEE 802.3af/at PSE Controller with Integrated MOSFET
• Manufacturer: Maxim Integrated Products

The MAX5971A also contains an internal square wave,
PWM signal generator. The PWM runs at a typical fre-
quency of 25kHz with an approximate typical duty cycle
of 6.25%. PWMEN is used to enable or disable the PWM.
PWMEN is internally pulled up to the digital supply, and
can be left unconnected to enable the internal PWM.
When enabled, the LED pulses are driven by the PWM
to reduce the power dissipation and increase the system
efficiency. Force PWMEN low to disable the internal
PWM; the LED is then driven directly.
If the MAX5971A die temperature reaches 150NC, an
overtemperature fault is generated and the device shuts
down. The die temperature must cool down below 130NC
to remove the overtemperature fault condition. After a
thermal shutdown condition clears, the device is reset.
Careful PCB layout is critical to achieve high efficiency
and low EMI. Follow these layout guidelines for optimal
performance.
Figure 6. Typical Operating Circuit 1 (DC Load Removal Detection, Internal PWM Enabled for LED Indication, and Class 5
Detection Enabled)
Single-Port, 40W, IEEE 802.3af/802.3at PSE
Applications Information
______________________________________________________________________________________
-54V
47µF
100V
Controller with Integrated MOSFET
Thermal Shutdown
Layout Procedure
1N4448
0.1µF
100V
LED
10mH
5.1kI
1nF
LED
EN
V
V
LEGACY
MIDSPAN
OSC
EE
EE_DIG
MAX5971A
AGND
1) Place the input bypass capacitance and the output
2) Use large SMT component pads for power dissipat-
3) Use short, wide traces whenever possible for high-
4) Use the MAX5971 Evaluation Kit as a design and
5) The exposed pad (EP) must be soldered evenly to the
PWMEN
bypass capacitor (0.1µF ceramic capacitor from
AGND to OUTP) as close to the MAX5971A as pos-
sible.
ing devices such as the MAX5971A and the external
diodes in the high-power path.
power paths.
layout reference.
PCB ground plane for proper operation and power
dissipation. Use multiple vias beneath the exposed
paddle for maximum heat dissipation. A 1.0mm to
1.2mm pitch is the recommended spacing for these
vias and they should be plated (1oz copper) with a
small barrel diameter (0.30mm to 0.33mm).
OUTP
SMJ58A
ILIM1
ILIM2
OUT
DET
1kI
1kI
1N4448
0.1µF
100V
2.2MI
-54V
PSE OUTPUT
19