lm2619atl National Semiconductor Corporation, lm2619atl Datasheet - Page 9

• Manufacturer Part Number: lm2619atl
• Description: 500ma Sub-miniature Step-down Dc-dc Converter
• Manufacturer: National Semiconductor Corporation

Device Information
PWM OPERATION
While in PWM (Pulse Width Modulation) 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. Energy per cycle is set by modulating the PFET
switch on-time pulse-width to control the peak inductor cur-
rent. This is done by comparing the signal from the current-
sense amplifier with a slope compensated error signal from
the voltage-feedback error amplifier. At the beginning of
each cycle, the clock 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
turns off the PFET switch and turns on the NFET synchro-
nous rectifier, ending the first part of the cycle. 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. 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 error 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.
The minimum on time of the PFET in PWM mode is about
200ns.
(Continued)
FIGURE 5. Simplified Functional Diagram
9
A: Inductor Current, 500mA/div
B: SW Pin, 2V/div
C: V
OUT
, 10mV/div, AC Coupled
PWM Mode Switching Waveform
FIGURE 6.
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