LM5001ISOEVAL National Semiconductor, LM5001ISOEVAL Datasheet - Page 11

• 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: -

Application Information
The following information is intended to provide guidelines for
the power supply designer using the LM5001.
VIN
The voltage applied to the VIN pin can vary within the range
of 3.1V to 75V. The current into the VIN pin depends primarily
on the gate charge of the power MOSFET, the switching fre-
quency, and any external load on the VCC pin. It is recom-
mended the filter shown in Figure 2 be used to suppress
transients which may occur at the input supply. This is par-
ticularly important when VIN is operated close to the maxi-
mum operating rating of the LM5001.
When power is applied and the VIN voltage exceeds 2.8V with
the EN pin voltage greater than 0.45V, the VCC regulator is
enabled, supplying current into the external capacitor con-
nected to the VCC pin. When the VIN voltage is between 2.8V
and 6.9V, the VCC voltage is approximately equal to the VIN
voltage. When the voltage on the VCC pin exceeds 6.9V, the
VCC pin voltage is regulated at 6.9V. In typical flyback appli-
cations, an auxiliary transformer winding is connected
through a diode to the VCC pin. This winding must raise the
VCC voltage above 6.9V to shut off the internal start-up reg-
ulator. The current requirements from this winding are rela-
tively small, typically less than 20 mA. If the VIN voltage is
much higher than the auxiliary voltage, the auxiliary winding
will significantly improve conversion efficiency. It also reduces
the power dissipation within the LM5001. The externally ap-
plied VCC voltage should never exceed 14V. Also the applied
VCC should never exceed the VIN voltage to avoid reverse
current through the internal VCC to VIN diode shown in the
LM5001 block diagram.
SW PIN
Attention must be given to the PC board layout for the SW pin
which connects to the power MOSFET drain. Energy can be
FIGURE 2. Input Transient Protection
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11
stored in parasitic inductance and capacitance which cause
switching spikes that negatively effect efficiency, and con-
ducted and radiated emissions. These connections should be
as short as possible to reduce inductance and as wide as
possible to reduce resistance. The loop area, defined by the
SW and GND pin connections, the transformer or inductor
terminals, and their respective return paths, should be mini-
mized.
EN / UVLO VOLTAGE DIVIDER SELECTION
Two dedicated comparators connected to the EN pin are used
to detect under-voltage and shutdown conditions. When the
EN pin voltage is below 0.45V, the controller is in a low current
shutdown mode where the VIN current is reduced to 95 µA.
For an EN pin voltage greater than 0.45V but less than 1.26V
the controller is in standby mode, with all internal circuits op-
erational, but the PWM gate driver signal is blocked. Once the
EN pin voltage is greater than 1.26V, the controller is fully
enabled. Two external resistors can be used to program the
minimum operational voltage for the power converter as
shown in Figure 3. When the EN pin voltage falls below the
1.26V threshold, an internal 100 mV threshold hysteresis pre-
vents noise from toggling the state, so the voltage must be
reduced to 1.16V to transition to standby. Resistance values
for R1 and R2 can be determined from the following equa-
tions:
where V
arbitrary current through R1 and R2.
For example, if the LM5001 is to be enabled when V
reaches 16V, I
set R1 to 29.4 kΩ and R2 to 2.49 kΩ. The voltage at the EN
pin should not exceed 10V unless the current into the 6V pro-
tection Zener diode is limited below 4 mA. The EN pin voltage
should not exceed 14V at any time. Be sure to check both the
power and voltage rating (some 0603 resistors are rated as
low as 50V) for the selected R1 resistor.
PWR
is the desired turn-on voltage and I
DIVIDER
could be chosen as 501 µA which would
DIVIDER
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is an
PWR