LM5002

SNVS496D – JANUARY 2007 – REVISED MARCH 2013

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Current Amplifier and Slope Compensation

The LM5002 employs peak current mode control which also provides a cycle-by-cycle over current protection

feature. An internal 100 m

Ω current sense resistor measures the current in the power MOSFET source. The

sense resistor voltage is amplified 30 times to provide a 3V/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

0.5A. 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 2.1V/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 0.5A. 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 LM5002 switching regulator includes an N-Channel MOSFET with 850 m

Ω on-resistance. The on-resistance

of the LM5002 MOSFET varies with temperature as shown in the Typical Performance Characteristics graph.

The typical total gate charge for the MOSFET is 2.4 nC which is supplied from the VCC pin when the MOSFET is

turned on.

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