LM3477MM National Semiconductor, LM3477MM Datasheet - Page 11

• Manufacturer Part Number: LM3477MM
• Description: Pulse Width Modulation (PWM) Controller IC
• Manufacturer: National Semiconductor

Specifications of LM3477MM

• Input Voltage Primary Min: 2.95V
• Mounting Type: Surface Mount
• Topology: Buck (Step Down)
• Control Mode: Current
• Duty Cycle Max: 93%
• Input Voltage Primary Max: 35V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Functional Description
DEFAULT/ADJUSTABLE SLOPE COMPENSATION
The LM3477/A uses a current mode control scheme. There
are many advantages in a current mode architecture includ-
ing inherent cycle-by-cycle current limiting and simple com-
pensation of the control loop. However, there are conse-
quences to using current mode control that one must be
aware of while selecting circuit components. One of these
consequences is the inherent possibility of subharmonic os-
cillations in the inductor current. This is a form of instability
and should be avoided.
As a brief explanation, consider Figure 4. A lot of information
is shown here. The top portion shows a schematic of the
current sensing loop. The bottom portion shows the pulse
width modulation (PWM) comparator waveforms for two
FIGURE 3. The Feedback Voltage Experiences an
Oscillation if the Input Voltage Crosses the 7.2V
FIGURE 4. The Current Sensing Loop and
Corresponding Waveforms
Internal Bias Threshold
(Continued)
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11
switching cycles. The two solid waveforms shown are the
waveforms compared at the internal pulse width modulator,
used to generate the MOSFET drive signal. The top wave-
form with the slope S
waveform V
the sensed inductor current waveform V
are compared at the PWM comparator. There is a feedback
loop involved here. The inductor current is sensed and fed
back to the PWM comparator, where it is compared to V
The output of the comparator in combination with the R/S
latch determine if the MOSFET is on or off, which effectively
controls the amount of current the inductor receives. While
V
signal, driving the external power MOSFET on. When MOS-
FET is on, the inductor current rises at a constant slope,
generating the sensed voltage V
the PWM comparator signals to drive the MOSFET off, and
the sensed inductor current decreases with a slope S
process begins again when R
oscillator.
The subharmonic oscillation phenomenon is realized when a
load excursion is experienced. The way it is analyzed is to
calculate how the inductor current settles after such an
excursion. Take for example the case when the inductor
current experiences a step increase in its average current,
shown as the dotted line in Figure 4. In the switching period
that the excursion occurs, the inductor current will change by
∆I
have a difference ∆I
original excursion is being propagated each switching cycle.
What is desired is to find out if this propagation is converging
or diverging. It is apparent that the difference in the inductor
current from one cycle to the next is a function of S
S
Hence, if the quantity (S
inductor current diverges and subharmonic oscillations re-
sult. Notice that as S
when the duty cycle is greater than 50%, as the inductance
become less, the factor increases.
The LM3477/A internally generates enough slope compen-
sation S
height of the compensation slope ramp V
the
LM3477/A incorporates a patented scheme to increase S
there is need to use a smaller inductor. With the use of a
single resistor R
increases the compensation slope Se by the amount:
Therefore,
When excursions of the inductor current are divergent, the
current sensing control loop is unstable and produces a
subharmonic oscillation in the inductor current. This oscilla-
tion is viewed as a resonance in the outer voltage control
loop at half the switching frequency. In the inductor section,
C
e
0
, as follows:
. In the following switching period, the inductor current will
is higher than V
ELECTRICAL
e
to allow for the use of reasonable inductances. The
C
. The bottom waveform with slopes S
SL
SEN
, Se can be increased indefinitely. R
e
1
CHARACTERISTICS
increases, the factor decreases. Also,
, the PWM comparator outputs a high
e
from its original starting value. The
f
is the internally generated control
- S
e
)/(S
S
SEN
n
latch is set by an internal
+ S
. When V
e
)is greater than 1, the
SEN
SL
can be found in
. These signals
SEN
section.
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equals V
n
n
and S
, S
f
f
. The
, and
The
f
e
SL
C
C
is
if
.
,