Datasheet

LM5001
SNVS484E –JANUARY 2007–REVISED MARCH 2013 www.ti.com
Current Amplifier and Slope Compensation
The LM5001 employs peak current mode control which also provides a cycle-by-cycle over current protection
feature. An internal 50 mΩ current sense resistor measures the current in the power MOSFET source. The sense
resistor voltage is amplified 30 times to provide a 1.5V/A signal into the current limit comparator. Current limiting
is initiated if the internal current limit comparator input exceeds the 1.5V threshold, corresponding to 1A. When
the current limit comparator is triggered, the SW output pin immediately switches to a high impedance state.
The current sense signal is reduced to a scale factor of 1.05V/A for the PWM comparator signal. The signal is
then summed with a 450mV peak slope compensation ramp. The combined signal provides the PWM
comparator with a control signal that reaches 1.5V when the MOSFET current is 1A. For duty cycles greater than
50%, current mode control circuits are subject to sub-harmonic oscillation (alternating between short and long
PWM pulses every other cycle). Adding a fixed slope voltage ramp signal (slope compensation) to the current
sense signal prevents this oscillation. The 450mV ramp (zero volts when the power MOSFET turns on, and
450mV at the end of the PWM clock cycle) adds a fixed slope to the current sense ramp to prevent oscillation.
To prevent erratic operation at low duty cycle, a leading edge blanking circuit attenuates the current sense signal
when the power MOSFET is turned on. When the MOSFET is initially turned on, current spikes from the power
MOSFET drain-source and gate-source capacitances flow through the current sense resistor. These transient
currents normally cease within 50 ns with proper selection of rectifier diodes and proper PC board layout.
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the IC in the event the maximum junction temperature
is exceeded. When the 165°C junction temperature threshold is reached, the regulator is forced into a low power
standby state, disabling all functions except the VCC regulator. Thermal hysteresis allows the IC to cool down
before it is re-enabled. Note that since the VCC regulator remains functional during this period, the soft-start
circuit shown in Figure 20 should be augmented if soft-start from Thermal Shutdown state is required.
Power MOSFET
The LM5001 switching regulator includes an N-Channel MOSFET with 440 mΩ on-resistance. The on-resistance
of the LM5001 MOSFET varies with temperature as shown in the Typical Performance Characteristics graph.
The typical total gate charge for the MOSFET is 4.5 nC which is supplied from the VCC pin when the MOSFET is
turned on.
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 frequency, and any external load on
the VCC pin. It is recommended the filter shown in Figure 17 be used to suppress transients which may occur at
the input supply. This is particularly important when VIN is operated close to the maximum 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 connected 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 applications, 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 regulator. The current requirements from this winding are
relatively 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 applied 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.
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