Datasheet

PWM Comparator resets
the RS latch
PWM
Comparator
Blank-Out prevents false
reset
325 ns Blank-Out time
Oscillator Sets
the RS Latch
+
-
92 mV
typ
VIN (V)
VFB (V)
7.2V
OVP
(1.31V)
1.26V
t
t
LM3478
LM3478-Q1
SNVS085V –JULY 2000–REVISED FEBRUARY 2013 www.ti.com
Figure 24. The Feedback Voltage Experiences an Oscillation if the Input Voltage crosses the 7.2V
Internal Bias Threshold
Figure 25. Basic Operation of the PWM Comparator
SLOPE COMPENSATION RAMP
The LM3478 uses a current mode control scheme. The main advantages of current mode control are inherent
cycle-by-cycle current limit for the switch and simpler control loop characteristics. It is also easy to parallel power
stages using current mode control since current sharing is automatic. However, current mode control has an
inherent instability for duty cycles greater than 50%, as shown in Figure 26.
A small increase in the load current causes the switch current to increase by ΔI
0
. The effect of this load change
is ΔI
1
.
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
.
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