LM3477 datasheet(12/23 Pages) NSC

12 / 23 page

Functional Description (Continued)

Default/Adjustable Slope Compensation

The LM3477/A uses a current mode control scheme. There

are many advantages in a current mode architecture includ-

ing inherent cycle-by-cycle current limiting and simple com-

pensation of the control loop. However, there are conse-

quences 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 os-

cillations in the inductor current. This is a form of instability

and should be avoided.

As a brief explanation, consider Figure 4. 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 wave-

form with the slope S

e is the internally generated control

waveform V

C. The bottom waveform with slopes Sn and Sf 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

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

C is higher than VSEN, the PWM comparator outputs a high

signal, driving the external power MOSFET on. When MOS-

FET is on, the inductor current rises at a constant slope,

generating the sensed voltage V

SEN. When VSEN equals VC,

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.

The subharmonic oscillation phenomenon is realized when a

load excursion is experienced. The way it is analyzed is to

calculate how the inductor current settles after such an

excursion. Take for example the case when the inductor

current experiences a step increase in its average current,

shown as the dotted line in Figure 4. In the switching period

that the excursion occurs, the inductor current will change by

∆I

0. In the following switching period, the inductor current will

have a difference

∆I

1 from its original starting value. The

original excursion is being propagated each switching cycle.

What is desired is to find out if this propagation is converging

or diverging. It is apparent that the difference in the inductor

current from one cycle to the next is a function of S

n,Sf, and

e, as follows:

Hence, if the quantity (S

f -Se)/(Sn +Se)is greater than 1, the

inductor current diverges and subharmonic oscillations re-

sult. Notice that as S

e increases, the factor decreases. Also,

when the duty cycle is greater than 50%, as the inductance

become less, the factor increases.

The LM3477/A internally generates enough slope compen-

sation S

e to allow for the use of reasonable inductances. The

height of the compensation slope ramp V

SL can be found in

the

ELECTRICAL

CHARACTERISTICS

section.

The

LM3477/A incorporates a patented scheme to increase S

e if

there is need to use a smaller inductor. With the use of a

single resistor R

SL, Se can be increased indefinitely. RSL

increases the compensation slope Se by the amount:

Therefore,

When excursions of the inductor current are divergent, the

current sensing control loop is unstable and produces a

subharmonic oscillation in the inductor current. This oscilla-

tion is viewed as a resonance in the outer voltage control

loop at half the switching frequency. In the inductor section,

200033J6

FIGURE 3. The Feedback Voltage Experiences an

Oscillation if the Input Voltage Crosses the 7.2V

Internal Bias Threshold

200033C2

FIGURE 4. The Current Sensing Loop and

Corresponding Waveforms

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