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LM3103

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SNVS523G – SEPTEMBER 2007 – REVISED JANUARY 2018

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Feature Description (continued)

Figure 20. Inductor Current - Current Limit Operation

7.2.8 N-Channel MOSFET and Driver

The LM3103 integrates an N-Channel main MOSFET and an associated floating high voltage main MOSFET

gate driver. The gate drive circuit works in conjunction with an external bootstrap capacitor CBST and an internal

high voltage diode. CBST connected between the BST and SW pins powers the main MOSFET gate driver during

the main MOSFET on-time. During each off-time, the voltage of the SW pin falls to approximately –1 V, and CBST

charges from VCC through the internal diode. The minimum off-time of 240 ns provides enough time for charging

CBST in each cycle.

7.2.9 Soft-Start

The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing

startup stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold and a 180 µs

fixed delay, a 70 µA internal current source charges an external capacitor CSS connecting to the SS pin. The

ramping voltage at the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the

output voltage VOUT in a controlled manner. An internal switch grounds the SS pin if any of the following three

cases happen: (i) VCC is below the under-voltage lockout threshold; (ii) a thermal shutdown occurs; or (iii) the EN

pin is grounded. Alternatively, the output voltage can be shut off by connecting the SS pin to the ground using an

external switch. Releasing the switch allows the voltage of the SS pin to ramp up and the output voltage to return

to normal. The shutdown configuration is shown in Figure 21.

Figure 21. Alternate Shutdown Implementation

7.2.10 Thermal Protection

The junction temperature of the LM3103 should not exceed the maximum limit. Thermal protection is

implemented by an internal Thermal Shutdown circuit, which activates (typically) at 165°C to make the controller

enter a low power reset state by disabling the main MOSFET, disabling the on-timer, and grounding the SS pin.

Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction

temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation

resumes.