VC

VSEN

Sn

PWM Comparator

Waveforms

Time

Sf

Se

'I0

'I1

'I2

VIN (V)

VFB (V)

7.2V

OVP

(1.31V)

1.26V

t

t

13

LM3478

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SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Feature Description (continued)

Depending on the requirements of the application, there is some influence one has over this effect. The threshold

of 7.2 V can be shifted to higher voltages by adding a resistor in series with VIN. In case VIN is right at the

threshold of 7.2 V, the threshold could cross 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.

Figure 21. The Feedback Voltage Experiences an Oscillation if the Input Voltage crosses the 7.2-V

Internal Bias Threshold

7.3.2 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 22.

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 Se is the internally generated control

waveform VC. The bottom waveform with slopes Sn and Sf is the sensed inductor current waveform VSEN.

Figure 22. Sub-Harmonic Oscillation for D>0.5 and Compensation Ramp to Avoid Sub-Harmonic

Oscillation

Sub-harmonic Oscillation can be easily understood as a geometric problem. If the control signal does not have

slope, the slope representing the inductor current ramps up until the control signal is reached and then slopes

down again. If the duty cycle is above 50%, any perturbation will not converge but diverge from cycle to cycle

and causes sub-harmonic oscillation.

It is apparent that the difference in the inductor current from one cycle to the next is a function of Sn, Sf and Se as

shown in Equation 1.