LM5001, LM5001-Q1

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SNVS484H – JANUARY 2007 – REVISED JULY 2015

Feature Description (continued)

The current sense signal is reduced to a scale factor of 1.05 V/A for the PWM comparator signal. The signal is

then summed with a 450 mV peak slope compensation ramp. The combined signal provides the PWM

comparator with a control signal that reaches 1.5 V when the MOSFET current is 1 A. 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 450 mV ramp (zero volts when the power MOSFET turns on,

and 450 mV 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.

7.3.7 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 17 should be augmented if soft-start from Thermal Shutdown state is required.

7.3.8 Power MOSFET

The LM5001 switching regulator includes an N-Channel MOSFET with 440-m

Ω on-resistance. The on-resistance

of the LM5001 MOSFET varies with temperature as shown in the Typical 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.

8 Applications and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

This information is intended to provide guidelines for the power supply designer using the LM5001.

8.1.1 VIN

The voltage applied to the VIN pin can vary within the range of 3.1 V to 75 V. 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 14 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 2.8 V with the EN pin voltage greater than 0.45 V, the VCC

regulator is enabled, supplying current into the external capacitor connected to the VCC pin. When the VIN

voltage is between 2.8 V and 6.9 V, the VCC voltage is approximately equal to the VIN voltage. When the

voltage on the VCC pin exceeds 6.9 V, the VCC pin voltage is regulated at 6.9 V. 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 6.9 V 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 significantly improves conversion efficiency. It also reduces the power dissipation within the

LM5001. The externally applied VCC voltage should never exceed 14 V. 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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Product Folder Links: LM5001 LM5001-Q1