LM3679

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SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the

input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the

The output filter stores charge when the inductor current is high, and releases it when inductor current is low,

smoothing the voltage across the load.

The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the

load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and

synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The

output voltage is equal to the average voltage at the SW pin.

PWM OPERATION

During PWM operation, the converter operates as a voltage-mode controller with input voltage feed forward. This

allows the converter to achieve good load and line regulation. The DC gain of the power stage is proportional to

the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is

introduced.

While in PWM mode, the output voltage is regulated by switching at a constant frequency and then modulating

the energy per cycle to control power to the load. At the beginning of each clock cycle the PFET switch is turned

on and the inductor current ramps up until the comparator trips and the control logic turns off the switch. The

current limit comparator can also turn off the switch in case the current limit of the PFET is exceeded. Then the

NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by the clock turning off

the NFET and turning on the PFET.

Figure 5. 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:

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