LM5001ISOEVAL National Semiconductor, LM5001ISOEVAL Datasheet - Page 14

• Manufacturer Part Number: LM5001ISOEVAL
• Description: BOARD EVALUATION LM5001ISO
• Manufacturer: National Semiconductor

Specifications of LM5001ISOEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Isolated
• Voltage - Output: 5V
• Current - Output: 1A
• Voltage - Input: 16 ~ 42V
• Regulator Topology: Flyback
• Frequency - Switching: 250kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5001
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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Printed Circuit Board Layout
The LM5001 Current Sense and PWM comparators are very
fast and may respond to short duration noise pulses. The
components at the SW, COMP, EN and the RT pins should
be as physically close as possible to the IC, thereby minimiz-
ing noise pickup on the PC board tracks.
The SW output pin of the LM5001 should have a short, wide
conductor to the power path inductors, transformers and ca-
pacitors in order to minimize parasitic inductance that reduces
efficiency and increases conducted and radiated noise. Ce-
ramic decoupling capacitors are recommended between the
VIN pin to the GND pin and between the VCC pin to the GND
pin. Use short, direct connections to avoid clock jitter due to
ground voltage differentials. Small package surface mount
X7R or X5R capacitors are preferred for high frequency per-
formance and limited variation over temperature and applied
voltage.
If an application using the LM5001 produces high junction
temperatures during normal operation, multiple vias from the
GND pin to a PC board ground plane will help conduct heat
away from the IC. Judicious positioning of the PC board within
the end product, along with use of any available air flow will
help reduce the junction temperatures. If using forced air
cooling, avoid placing the LM5001 in the airflow shadow of
large components, such as input capacitors, inductors or
transformers.
Application Circuit Examples
The following schematics present examples of a Non-Isolated
Flyback, Isolated Flyback, Boost, 24V SEPIC and a 12V Au-
tomotive range SEPIC converters utilizing the LM5001
switching regulator.
NON-ISOLATED FLYBACK
The Non-Isolated Flyback converter (Figure 6) utilizes the in-
ternal voltage reference for the regulation setpoint. The output
is +5V at 1A while the input voltage can vary from 16V to 42V.
The switching frequency is set to 250kHz. An auxiliary wind-
ing on transformer (T1) provides 7.5V to power the LM5001
when the output is in regulation. This disables the internal high
voltage VCC LDO regulator and improves efficiency. The in-
put under-voltage threshold is 13.9V. The converter can be
shut down by driving the EN input below 1.26V with an open-
collector or open-drain transistor. An external synchronizing
frequency can be applied to the SYNC input. An optional soft-
start circuit is connected to the COMP pin input. When power
is applied, the soft-start capacitor (C7) is discharged and lim-
its the voltage applied to the PWM comparator by the internal
error amplifier. The internal ~5 kΩ COMP pull-up resistor
charges the soft-start capacitor until regulation is achieved.
The VCC pull-up resistor (R7) continues to charge C7 so that
the soft-start circuit will not affect the compensation network
in normal operation. If the output capacitance is small, the
14
soft-start circuit can be adjusted to limit the power-on output
voltage overshoot. If the output capacitance is sufficiently
large, no soft-start circuit is needed because the LM5001 will
gradually charge the output capacitor by current limiting at
approximately 1A (I
ISOLATED FLYBACK
The Isolated Flyback converter (Figure 7) utilizes a 2.5V volt-
age reference (LM431) located on the isolated secondary
side for the regulation setpoint. The LM5001 internal error
amplifier is disabled by grounding the FB pin. The LM431
controls the current through the opto-coupler LED, which sets
the COMP pin voltage. The R4 and C3 network boosts the
phase response of the opto-coupler to increase the loop
bandwidth. The output is +5V at 1A and the input voltage
ranges from 16V to 42V. The switching frequency is set to
250kHz.
BOOST
The Boost converter (Figure 8) utilizes the internal voltage
reference for the regulation setpoint. The output is +48V at
150mA, while the input voltage can vary from 16V to 36V. The
switching frequency is set to 250kHz. The internal VCC reg-
ulator provides 6.9V bias power, since there isn’t a simple
method for creating an auxiliary voltage with the boost topol-
ogy. Note that the boost topology does not provide output
short-circuit protection because the power MOSFET cannot
interrupt the path between the input and the output.
24V SEPIC
The 24V SEPIC converter (Figure 9) utilizes the internal volt-
age reference for the regulation setpoint. The output is +24V
at 250mA while the input voltage can vary from 16V to 48V.
The switching frequency is set to 250kHz. The internal VCC
regulator provides 6.9V bias power for the LM5001. An aux-
iliary voltage can be created by adding a winding on L2 and
a diode into the VCC pin.
12V AUTOMOTIVE SEPIC
The 12V Automotive SEPIC converter (Figure 10) utilizes the
internal bandgap voltage reference for the regulation setpoint.
The output is +12V at 50mA while the input voltage can vary
from 3.1V to 60V. The output current rating can be increased
if the minimum VIN voltage requirement is increased. The
switching frequency is set to 750kHz. The internal VCC reg-
ulator provides 6.9V bias power for the LM5001. The output
voltage can be used as an auxiliary voltage if the nominal VIN
voltage is greater than 12V by adding a diode from the output
into the VCC pin. In this configuration, the minimum input
voltage must be greater than 12V to prevent the internal VCC
to VIN diode from conducting. If the applied VCC voltage ex-
ceeds the minimum VIN voltage, then an external blocking
diode is required between the VIN pin and the power source
to block current flow from VCC to the input supply.
LIM
) until regulation is achieved.