LM3478MA_15 Datasheet, PDF (10/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
VIN (V)
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7.2V
t
VFB (V)
OVP
(1.31V)
1.26V
t
Figure 23. The Feedback Voltage Experiences an Oscillation if the Input Voltage crosses the 7.2V
Internal Bias Threshold
92 mV
typ
Blank-Out prevents false
reset
PWM Comparator resets
the RS latch
Oscillator Sets
the RS Latch
325 ns Blank-Out time
Figure 24. Basic Operation of the PWM Comparator
-
+
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 25.
A small increase in the load current causes the switch current to increase by ΔI₀. The effect of this load change
is ΔI₁.
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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