LM3478_09 Datasheet, PDF (11/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

the scenario is identical to a step change in the load current,
so the response at the output voltage is the same as would
be observed in a step load change. Hence, the output voltage
overshoot here can also trigger OVP. The LM3478 will regu-
late in hysteretic mode for several cycles, or may not recover
and simply stay in hysteretic mode until the load current drops
or Vin is not crossing the 7.2V threshold anymore. Note that
the output is still regulated in hysteretic mode.
Depending on the requirements of the application there is
some influence one has over this effect. The threshold of 7.2V
can be shifted to higher voltages by adding a resistor in series
with Vin. In case Vin is right at the threshold of 7.2V it can
happen that the threshold is crossed over and over due to
some slight ripple on Vin. To minimize the effect on the output
voltage one can filter the Vin pin with an RC filter.
10135511
FIGURE 1. The Feedback Voltage Experiences an
Oscillation if the Input Voltage crosses the 7.2V Internal
Bias Threshold
FIGURE 2. Basic Operation of the PWM Comparator
10135507
SLOPE COMPENSATION RAMP
The LM3478 uses a current mode control scheme. The main
advantages of current mode control are inherent cycle-by-cy-
cle current limit for the switch and simpler control loop char-
acteristics. It is also easy to parallel power stages using
current mode control since current sharing is automatic. How-
ever, current mode control has an inherent instability for duty
cycles greater than 50%, as shown in Figure 3.
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.
10135512
FIGURE 3. Sub-Harmonic Oscillation for D>0.5 and
Compensation Ramp to Avoid Sub-Harmonic Oscillation
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