LM5071HEEVAL National Semiconductor, LM5071HEEVAL Datasheet - Page 6

• Manufacturer Part Number: LM5071HEEVAL
• Manufacturer: National Semiconductor

Specifications of LM5071HEEVAL

• Lead Free Status / Rohs Status: Not Compliant

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The Factors Limiting the Minimum
Operating Input Voltage
The LM5071 is capable of operating with an AUX power
source of as low as 10.5V (after the AUX input OR-ing-diode
drop, the VIN pin sees 9.5V). However, the minimum oper-
ating AUX input voltage of the evaluation board at full load is
mainly determined by two factors; the flyback power trans-
former design and the values of the current sense resistors
R14 and R15.
The installed EP13 type power transformer (DA2257-AL or
DCT13EP-U12S005) is a low cost solution to operate with a
wide AUX input voltage range. However, the small cross-
sectional area of the EP13 magnetic core limits the maxi-
mum flux it can handle. To use such a small transformer from
14V to 60V under the full load condition, a compromise
between the minimum operating input voltage and maximum
inductance of the transformer must be made such that the
peak current at 14V input will not cause the peak flux density
to exceed 3000 Gauss. A drawback of this low cost solution
is that the rms currents flowing through the dc-dc converter
stage are increased and the efficiency of the dc-dc converter
is reduced by about 3%.
Replacing the originally installed transformer with the op-
tional power transformer DA2383-AL from Coilcraft improves
the efficiency, but the minimum operating input voltage will
be limited to 24V. To use this optional transformer for lower
input voltage, the load level should be scaled down accord-
ingly, as shown in Figure 3.
To optimize efficiency over the maximum input voltage range
of 9.5V to 60V, a larger magnetic core like the EFD20 should
be used. The EFD20 core has adequate cross-sectional
area to handle the peak currents at 9.5V input.
The effects of the current sense resistors R14 and R15 also
limit the minimum AUX input operating voltage. The
LM5071’s internal slope compensation stabilizes the feed-
back loop of the dc-dc converter when the duty cycle ex-
ceeds 50% for input voltages lower than 22V. However, the
relative magnitude of the slope compensation is inversely
proportional to the values of R14 and R15. The maximum
values of R14 and R15 are governed by the following rela-
tion:
FIGURE 3. Maximum Load Current vs. Minimum Input
Voltage as Limited by Different EP13 Type Power
Transformers
20175804
6
where
D
f
L
k
V
V
Selecting 0.30Ω for both R14 and R15 will allow a minimum
operating voltage of 14V. For lower AUX input voltage, Dmax
is greater and hence R14 and R15 must be reduced accord-
ingly. However, the smaller resistors increase the slope com-
pensation. Increasing the slope compensating makes the
feedback loop appear more like voltage mode than current
mode which requires the use of a low ESR capacitor for C16
rather than the low cost capacitor initially installed on the
evaluation board.
In summary, the 14V minimum operating AUX input voltage
of the evaluation board is limited by the low cost solution. In
order to use the evaluation board with a lower AUX source,
the power transformer T1, the output capacitor C16, R14
and R15 should be all modified in addition to the installation
of D2.
Performance Characteristics
POWER UP SEQUENCE
The high level of integration designed into the LM5071 al-
lows all power sequencing communications to occur within
the IC. Very little system management design is required by
the user. The power up sequence is as follows. Note that the
RTN pin (IC pin 8) is isolated from the +3.3V RTN output pin
of the board:
1. Before power up, all nodes in the non-isolated section of
2. Once the RTN pin of the IC drops below 1.5V (refer-
3. Once the V
4. As the switching regulator achieves regulation, the aux-
Figure 4 shows the voltages at RTN, VCC, and SS (Soft-
start) IC pins, all referenced to the VEE pin, during a normal
startup sequence. A more detailed scope plot of the V
regulator starting up is given in Figure 5. The auxiliary wind-
ing starts to supply a higher voltage as the switching regu-
lator output voltage rises.
sw
m
t
o
F
max
the transformer’s primary to secondary turns ratio
the output voltage, in volts
the forward drop of the output diode D5, in volts
the flyback transformer primary inductance, in µH
the switching frequency, in kHz
the power supply remain at high potential until UVLO is
released and the drain of the internal hot swap MOSFET
is pulled down to VEE (IC pin 7).
enced to VEE), the V
lowed to start. This signals the assertion of the internal
“Power Good” signal. The V
equal to its current limit, typically 20 mA, divided by the
V
about 7.6V referenced to RTN, the soft-start pin is re-
leased. The soft-start pin will rise at a rate equal to the
soft-start current source, typically 10µA, divided by the
soft-start pin capacitance, C26.
iliary winding will raise the V
thus shutting down the internal regulator and increasing
efficiency.
is the duty cycle at the minimum AUX input voltage
CC
load capacitance, C19.
CC
regulator is within minimum regulation,
CC
regulator is released and al-
CC
CC
regulator ramps at a rate
voltage to about 10V,
CC