+

-

ISEN

DR

Q

Q

R

S

RSL = 0 (nominal)

VIN

LM3477

RSN

L

Q

D

COUT

VOUT

I0

I1

I2

VC

VSEN

Se

Sn

Sf

PWM Comparator

PWM Comparator

Waveforms

Time

VC

VIN (V)

VFB (V)

7.2V

OVP

(1.315V)

1.265V

t

t

xxxxx

LM3477

SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

www.ti.com

The Feedback Voltage Experiences an Oscillation if the Input Voltage Crosses the 7.2V Internal Bias Threshold

Figure 21.

DEFAULT/ADJUSTABLE SLOPE COMPENSATION

The LM3477/A uses a current mode control scheme. There are many advantages in a current mode architecture

including inherent cycle-by-cycle current limiting and simple compensation of the control loop. However, there are

consequences 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 oscillations in the inductor current. This is a

form of instability and should be avoided.

Figure 22. The Current Sensing Loop and Corresponding Waveforms

As a brief explanation, consider Figure 22. 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 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. 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 VC. 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 VC is higher than VSEN, the PWM comparator outputs a high

signal, driving the external power MOSFET on. When MOSFET is on, the inductor current rises at a constant

slope, generating the sensed voltage VSEN. When VSEN equals VC, the PWM comparator signals to drive the

MOSFET off, and the sensed inductor current decreases with a slope Sf. The process begins again when RS

latch is set by an internal oscillator.

12

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