Datasheet

RT = 13.1 x 10
9
x
1
F
SW
- 83 ns
LM5002
SNVS496D –JANUARY 2007–REVISED MARCH 2013 www.ti.com
The LM5002 has dedicated protection circuitry to protect the IC from abnormal operating conditions. Cycle-by-
cycle current limiting prevents the power MOSFET current from exceeding 0.5A. 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 approximately 20°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.
High Voltage VCC Regulator
The LM5002 VCC Low Drop Out (LDO) regulator allows the LM5002 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 regulator 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 switching begins. The controller remains enabled until VCC falls below 2.7V or the EN pin falls
below 1.16V.
An auxiliary supply voltage can be applied to the VCC pin to reduce the IC power dissipation. If the auxiliary
voltage is greater than 6.9V, the internal regulator will essentially shut-off, and internal power dissipation will be
decreased by the VIN voltage times the operating current. The overall converter efficiency will also improve if the
VIN voltage is much higher than the auxiliary voltage. The externally applied VCC voltage should not exceed
14V. The VCC regulator series pass MOSFET includes a body diode (see the Block Diagram) between VCC and
VIN that should not be forward biased in normal operation. Therefore, the auxiliary VCC voltage should never
exceed the VIN voltage.
In high voltage applications extra care should be taken to ensure the VIN pin does not exceed the absolute
maximum voltage rating of 76V. Voltage ringing on the VIN line during line transients that exceeds the Absolute
Maximum Ratings will damage the IC. Both careful PC board layout and the use of quality bypass capacitors
located close to the VIN and GND pins are essential.
Oscillator
A single external resistor connected between RT and GND pins sets the LM5002 oscillator frequency. To set a
desired oscillator frequency (F
SW
), the necessary value for the RT resistor can be calculated from the following
equation:
(1)
The tolerance of the external resistor and the frequency tolerance indicated in the Electrical Characteristics must
be taken into account when determining the worst case frequency range.
External Synchronization
The LM5002 can be synchronized to the rising edge of an external clock. The external clock must have a higher
frequency than the free running oscillator frequency set by the RT resistor. The clock signal should be coupled
through a 100pF capacitor into the RT pin. A peak voltage level greater than 2.6V at the RT pin is required for
detection of the sync pulse. The DC voltage across the RT resistor is internally regulated at 1.5 volts. The
negative portion of the AC voltage of the synchronizing clock is clamped to this 1.5V by an amplifier inside the
LM5002 with ~100Ω output impedance. Therefore, the AC pulse superimposed on the RT resistor must have
positive pulse amplitude of 1.1V or greater to successfully synchronize the oscillator. The sync pulse width
measured at the RT pin should have a duration greater than 15ns and less than 5% of the switching period. The
sync pulse rising edge initiates the internal CLK signal rising edge, which turns off the power MOSFET. The RT
resistor is always required, whether the oscillator is free running or externally synchronized. The RT resistor
should be located very close to the device and connected directly to the RT and GND pins of the LM5002.
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