LM2612 National Semiconductor Corporation, LM2612 Datasheet - Page 13

• Manufacturer Part Number: LM2612
• Description: LM2612 - Miniature, Programmable, Step-down DC-DC Converter For Ultra Low-voltage Circuits, Package: Microsmd, Pin Nb=10
• Manufacturer: National Semiconductor Corporation

Circuit Operation
PWM Operation
The LM2612 can be set to current-mode PWM operation by
connecting the SYNC/MODE pin to VDD. While in PWM
(Pulse Width Modulation) mode, the output voltage is regu-
lated by switching at a constant frequency and then modu-
lating the energy per cycle to control power to the load.
Energy per cycle is set by modulating the PFET switch
on-time pulse-width to control the peak inductor current. This
is done by controlling the PFET switch using a flip-flop driven
by an oscillator and a comparator that compares a ramp
from the current-sense amplifier with an error signal from a
voltage-feedback error amplifier. At the beginning of each
cycle, the oscillator sets the flip-flop and turns on the PFET
switch, causing the inductor current to ramp up. When the
current sense signal ramps past the error amplifier signal,
(Continued)
FIGURE 2. Simplified Functional Diagram
13
the PWM comparator resets the flip-flop and turns off the
PFET switch, ending the first part of the cycle. The NFET
synchronous rectifier turns on until the next clock pulse or
the inductor current ramps to zero. If an increase in load
pulls the output voltage down, the error amplifier output
increases, which allows the inductor current to ramp higher
before the comparator turns off the PFET switch. This in-
creases the average current sent to the output and adjusts
for the increase in the load.
Before going to the PWM comparator, the current sense
signal is summed with a slope compensation ramp from the
oscillator for stability of the current feedback loop. During the
second part of the cycle, a zero crossing detector turns off
the NFET synchronous rectifier if the inductor current ramps
to zero.
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