lm5015mhx National Semiconductor Corporation, lm5015mhx Datasheet - Page 9

• Manufacturer Part Number: lm5015mhx
• Description: High Voltage Monolithic Two-switch Forward Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

Functional Description
The LM5015 high voltage switching regulator features all the
functions necessary to implement an efficient power convert-
er using a Two-Switch Forward or Two-Switch Flyback topol-
ogy. 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 compensa-
tion.
Referring to the functional block diagram shown in Figure1,
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 pow-
er MOSFETs. At the end of the switching cycle the driver logic
is set again by the oscillator to initiate the next switching pe-
riod.
The LM5015 also contains dedicated circuitry to protect the
IC from abnormal operating conditions. Cycle-by-cycle cur-
rent limiting prevents the power MOSFET current from ex-
ceeding 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 pre-
FIGURE 2. Analog and Power Input Ports
9
vent 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 config-
uration 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 inde-
pendent 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 en-
sure that the VIN and PVIN pins do not exceed the Absolute
Maximum Voltage Ratings of 76V. Voltage ringing on the in-
put line during line transients that exceeds the Absolute Max-
imum 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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