LM3691
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SNVS506J –MAY 2008–REVISED DECEMBER 2015
8.3 Feature Description
8.3.1 Circuit Operation
The LM3691 operates as follows. During the first portion of each switching cycle, the control block in the LM3691
turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output
filter capacitor and load. The inductor limits the current to a ramp with a slope of (VIN – VOUT)/L, by storing energy
in a magnetic field. 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 NFET to the output filter capacitor and load, which ramps the inductor current down with a
slope of –VOUT/L.
The output filter stores charge when the inductor current is high, and releases it when 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.
8.3.2 PWM Operation
During PWM operation, the converter operates as a voltage-mode controller with input-voltage feed forward. This
allows the converter to achieve excellent 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 42. Typical PWM Operation
8.3.2.1 Internal Synchronous Rectification
While in PWM mode, the LM3691 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.
8.3.2.2 Current Limiting
A current limit feature allows the LM3691 to protect itself and external components during overload conditions.
PWM mode implements current limit using an internal comparator that trips at 1500 mA (typical). If the output is
shorted to ground, and the output voltage becomes lower than 0.3V (typical), the device enters a timed current-
limit mode where the switching frequency is one fourth, and NFET synchronous rectifier is disabled, thus
preventing excess current and thermal runaway.
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