LM3679

SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

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Figure 23. Typical PWM Operation

Internal Synchronous Rectification

While in PWM mode, the LM3679 uses an internal NFET as a synchronous rectifier to reduce rectifier forward

voltage drop and associated power loss. Synchronous rectification provides a significant improvement in

efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier

diode.

Current Limiting

A current limit feature allows the LM3679 to protect itself and external components during overload conditions.

PWM mode implements current limiting using an internal comparator that trips at 920 mA (typ). If the output is

shorted to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration

until the inductor current falls below a low threshold, ensuring inductor current has more time to decay, thereby

preventing runaway.

PFM OPERATION

At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply

current to maintain high efficiency.

The part will automatically transition into PFM mode when either of the following conditions occurs for a duration

of 32 or more clock cycles:

a. The NFET current reaches zero.

Figure 24. Typical PFM Operation

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