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LM2618 Arkusz danych(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numer części LM2618
Szczegółowy opis  400mA Sub-miniature, High Efficiency, Synchronous PWM & PFM Programmable DC-DC Converter
Download  15 Pages
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Producent  NSC [National Semiconductor (TI)]
Strona internetowa  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2618 Arkusz danych(HTML) 10 Page - National Semiconductor (TI)

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Circuit Operation (Continued)
PWM Operation
The LM2618 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,
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.
20036401
FIGURE 2. Simplified Functional Diagram
PWM Mode Switching Waveform
PFM Mode Switching Waveform
20036410
20036411
FIGURE 3.
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