Datasheet
+
-
ISEN DR
Q
Q
R
S
R
SL
= 0 (nominal)
V
IN
LM3477
R
SN
L
Q
D
C
OUT
V
OUT
I
0
I
1
I
2
V
C
V
SEN
Se
Sn Sf
PWM Comparator
PWM Comparator
Waveforms
Time
Voltage
V
C
VIN (V)
VFB (V)
7.2V
OVP
(1.315V)
1.265V
t
t
LM3477
SNVS141K –OCTOBER 2000–REVISED MARCH 2013
www.ti.com
The Feedback Voltage Experiences an Oscillation if the Input Voltage Crosses the 7.2V Internal Bias Threshold
Figure 21.
DEFAULT/ADJUSTABLE SLOPE COMPENSATION
The LM3477/A uses a current mode control scheme. There are many advantages in a current mode architecture
including inherent cycle-by-cycle current limiting and simple compensation of the control loop. However, there are
consequences 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 oscillations in the inductor current. This is a
form of instability and should be avoided.
Figure 22. The Current Sensing Loop and Corresponding Waveforms
As a brief explanation, consider Figure 22. 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 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 waveform with the slope S
e
is the internally
generated control waveform V
C
. The bottom waveform with slopes S
n
and S
f
is the sensed inductor current
waveform V
SEN
. These signals 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
C
. 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
C
is higher than V
SEN
, the PWM comparator outputs a high
signal, driving the external power MOSFET on. When MOSFET is on, the inductor current rises at a constant
slope, generating the sensed voltage V
SEN
. When V
SEN
equals V
C
, the PWM comparator signals to drive the
MOSFET off, and the sensed inductor current decreases with a slope S
f
. The process begins again when R
S
latch is set by an internal oscillator.
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