LM5001ISOEVAL National Semiconductor, LM5001ISOEVAL Datasheet - Page 8

• 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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Block Diagram
Functional Description
The LM5001 high voltage switching regulator features all the
functions necessary to implement an efficient boost, flyback,
SEPIC or forward current mode power converter. The oper-
ation can be best understood by referring to the block dia-
gram. At the start of each cycle, the oscillator sets the driver
logic and turns on the power MOSFET to conduct current
through the inductor or transformer. The peak current in the
MOSFET is controlled by the voltage at the COMP pin. The
COMP voltage will increase with larger loads and decrease
with smaller loads. This voltage is compared with the sum of
a voltage proportional to the power MOSFET current and an
internally generated Slope Compensation ramp. Slope Com-
pensation is used in current mode PWM architectures to
eliminate sub-harmonic current oscillation that occurs with
static duty cycles greater than 50%. When the summed signal
exceeds the COMP voltage, the PWM comparator resets the
driver logic, turning off the power MOSFET. The driver logic
is then set by the oscillator at the end of the switching cycle
to initiate the next power period.
The LM5001 has dedicated protection circuitry to protect the
IC from abnormal operating conditions. Cycle-by-cycle cur-
rent limiting prevents the power MOSFET current from ex-
8
ceeding 1A. This feature can also be used to soft-start the
regulator. Thermal Shutdown circuitry holds the driver logic in
reset when the die temperature reaches 165°C, and returns
to normal operation when the die temperature drops by ap-
proximately 20°C. The EN pin can be used as an input voltage
under voltage lockout (UVLO) during start-up to prevent op-
eration with less than the minimum desired input voltage.
High Voltage VCC Regulator
The LM5001 VCC Low Drop Out (LDO) regulator allows the
LM5001 to operate at the lowest possible input voltage. The
VCC pin voltage is very nearly equal to the input voltage from
2.8V up to approximately 6.9V. As the input voltage continues
to increase, the VCC pin voltage is regulated at the 6.9V set-
point. The total input operating range of the VCC LDO regu-
lator is 3.1V to 75V.
The output of the VCC regulator is current limited to 20mA.
During power-up, the VCC regulator supplies current into the
required decoupling capacitor (0.47 µF or greater ceramic
capacitor) at the VCC pin. When the voltage at the VCC pin
exceeds the VCC UVLO threshold of 2.8V and the EN pin is
greater than 1.26V the PWM controller is enabled and switch-
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