LM5015

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SNVS538C – NOVEMBER 2007 – REVISED APRIL 2013

FUNCTIONAL DESCRIPTION

The LM5015 high voltage switching regulator features all the functions necessary to implement an efficient power

converter using a Two-Switch Forward or Two-Switch Flyback topology. The voltage across the MOSFETs

employed in the two-switch topology is clamped to the input voltage, allowing the input voltage range to approach

the rating of the MOSFETs. The regulator control method is based upon current mode control providing cycle-by-

cycle current limit, inherent input voltage feed-forward and simple feedback loop compensation.

Referring to Figure 13, the operating principle of the LM5015 regulator is as follows:

At the beginning of each switching cycle, the oscillator sets the driver logic and turns on both the high and low

side power MOSFETs to conduct current through the inductor or power transformer. The peak current in the

MOSFET is controlled by the voltage at the COMP pin. The COMP voltage, which is determined by the feedback

circuit, is compared with the sensed current signal of the internal low side power MOSFET. When the current

signal exceeds the COMP voltage, the PWM comparator resets the driver logic, turning off both power

MOSFETs. At the end of the switching cycle the driver logic is set again by the oscillator to initiate the next

switching period.

The LM5015 also contains dedicated circuitry to protect the IC from abnormal operating conditions. Cycle-by-

cycle current limiting prevents the power MOSFET current from exceeding 1 Amp. 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 approximately 25°C. The EN pin can be used as an input voltage under-voltage

lockout (UVLO) during start-up to prevent operation with less than the minimum desired input voltage.

Bias Input (VIN) and Power Input (PVIN)

The LM5015 provides two separate input power pins, VIN and PVIN, allowing for flexible decoupling options. The

VIN pin provides power to the low drop-out VCC bias regulator which powers all internal control blocks. The

PVIN connects directly to the high side MOSFET drain.

If used with a single input source, the recommended configuration is shown in Figure 2a. Separate input pins

allow the bias input (VIN) to be de-coupled from the main power input as shown. It is possible to directly connect

the VIN and PVIN pins for applications with localized de-coupling capacitors.

For applications where a lower voltage auxiliary source is available, the configuration shown in Figure 2b can be

used. Powering the VIN pin with a relatively low voltage auxiliary source reduces the IC power dissipation and

increases the conversion efficiency, especially when the main power source for PVIN is relatively large. The VIN

and PVIN pins are independent and can be separately biased at any voltages within the 4.25V to 75V

recommended operating range.

In high voltage applications extra care should be taken to ensure that the VIN and PVIN pins do not exceed the

Absolute Maximum Voltage Ratings of 76V. Voltage ringing on the input line during line transients that exceeds

the Absolute Maximum Ratings can damage the IC. Both careful PC board layout and the use of quality bypass

capacitors located close to the VIN and AGND pins, and to the PVIN to PGND pins, are essential.

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