LM5071HEEVAL National Semiconductor, LM5071HEEVAL Datasheet - Page 7

• Manufacturer Part Number: LM5071HEEVAL
• Manufacturer: National Semiconductor

Specifications of LM5071HEEVAL

• Lead Free Status / Rohs Status: Not Compliant

Performance Characteristics
Horizontal Resolution: 5 ms/Div.
Trace 1: RTN pin, elevated until UVLO release. 5V/Div.
Trace 2: SS pin, starts when VCC achieves minimum regu-
lation. 5V/Div.
Trace 3: VCC, starts when RTN
winding. 5V/Div.
Horizontal Resolution: 2 ms/Div.
Trace 1: RTN pin, elevated until UVLO release. 5V/Div.
Trace 2: Vout, cross-regulate VCC after output regulation is
established. 1V/Div.
Trace 3: VCC, starts when RTN
winding. 2V/Div.
Figure 6 shows a normal 3.3V line startup, along with the
softstart pin for reference.
(Continued)
FIGURE 4. Normal Startup Sequence
FIGURE 5. VCC Startup Detail
<
<
1.5V, elevated by auxiliary
1.5V, elevated by auxiliary
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Horizontal Resolution: 1 ms/Div.
Trace 1: +3.3V output voltage, 1V/Div.
Trace 2: Softstart pin, 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 (hiccups). Applying a dead
short to the +3.3V line causes a number of protection
mechanisms to occur sequentially. They are:
1. Feedback raises duty cycle in an attempt to maintain the
2. The average current in the internal PD interface MOS-
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
The re-try mode due to a shorted output condition can be
observed in Figure 7. The soft-start pin is observed to rise
quickly as the LM5071 reacts to the fault. This is because it
references RTN, while all scope measurements reference
VEE.
FIGURE 6. Regulator Output (+3.3V) Startup Detail
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.
FET rises until it is current limited around 390 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 MOSFET into the saturation region, the drain voltage
(RTN pin) rises. When it reaches 2.5V with respect to
VEE, the internal power good signal is de-asserted.
the soft-start 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 soft-start sequence.
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