LM5072EVAL National Semiconductor, LM5072EVAL Datasheet - Page 9

• Manufacturer Part Number: LM5072EVAL
• Description: BOARD EVALUATION LM5072
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM5072EVAL

• Main Purpose: Special Purpose DC/DC, Power Over Ethernet
• Outputs And Type: 1, Isolated
• Power - Output: 9.9W
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 38 ~ 60V
• Regulator Topology: Flyback
• Frequency - Switching: 250kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5072
• Lead Free Status / RoHS Status: Not applicable / Not applicable

Performance Characteristics
Horizontal Resolution: 1 ms/Div.
Trace 1: VOUT, 1V/Div.
Trace 2: SS pin voltage (referenced to RTN), 1V/Div
Trace 3: Input current (AC coupled), 200 mA/Div
OUTPUT DEAD SHORT FAULT RESPONSE
The evaluation board survives the output dead short condi-
tion by running into a re-try mode (hiccup). Applying a dead
short to the +3.3V line causes a number of protection
mechanisms to occur sequentially. They are:
1. The feedback signal increases the duty cycle in an
2. The current in the internal hot swap MOSFET rises until
3. Because linear current limit is accomplished by driving
4. The de-assertion of power good causes the discharge of
5. Once the switching stops, the current in the internal
Figure 9 shows the cycle-by-cycle peak current limit provid-
ing the over-current protection under output short-circuit con-
dition. The short-circuit condition causes a large over current
such that it intends to saturate the power transformer. Con-
sequently, a large peak current of about 4.33A is produced in
the primary circuit. This large peak current causes the
current-sense voltage at CS pin to exceed the peak limit
FIGURE 8. Output Voltage Vout (+3.3V) Soft-start Detail
(Continued)
attempt to maintain the output voltage. This initiates
cycle-by-cycle over-current limiting which turns off the
main switch when the current sense (CS) pin exceeds
the current limit threshold.
it is current limited around 440 mA. Some overshoot in
the current will be observed, as it takes time for the
current limit amplifier to react and change the operating
mode of the MOSFET.
the MOSET into the saturation region, the drain voltage
(RTN pin) rises. When it reaches 2.5V with respect to
VEE, power good is de-asserted and the nPGOOD pin
rises in voltage.
the softstart capacitor, which disables all switching ac-
tion in the dc-dc converter.
MOSFET will decrease and the drain voltage will fall
back below 1.5V with respect to VEE. When power good
is re-asserted, the dc-dc converter will automatically
restart with a new softstart sequence.
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threshold (
short, leading to a limited input current (the average current
of the current pulses) at about 0.34A.
Horizontal Resolution: 1 µs/Div.
Trace 1: Voltage at the CS pin, 200 mV/Div.
Trace 2: Input Current, 0.5A/Div.
Vin=48V. Iin=0.34A
Figure 10 shows the over-current protection by the hot swap
MOSFET’s dc current limit under the output short circuit
condition. The circuit operates in the FAUX power configu-
ration, and the FAUX input voltage is set to 24V. The input
current will exceed the 440 mA DC current limit of the hot
swap MOSFET, and causes the voltage at the RTN pin to
rise rapidly. It also discharges the soft start capacitor C26
connected to the SS pin, and the circuit enters the automatic
retry mode until the over-current condition is removed. The
voltage at the SS pin is observed to rise quickly as the
LM5072 reacts to the fault. This is because the internal
soft-start circuitry is referenced to RTN, while all scope
measurements are referenced to VEE.
Horizontal Resolution: 5 ms/Div.
Trace 1: RTN pin voltage (referenced to VEE), 2V/Div.
Trace 2: Softstart pin (referenced to VEE), 5V/Div.
Trace 3: Input current, 0.5A/Div.
FAUX input=24V
Protection Under Output Short-Circuit Condition
FIGURE 9. Cycle-by-Cycle Peak Current Limit
FIGURE 10. Shorted Output Fault Condition /
>
0.5V). Therefore the PWM duty cycle is cut
Automatic Re-try
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