10

Current mode converters can exhibit subharmonic oscillations when operating at a duty cycle greater than 50%

with continuous inductor current. This instability is independent of the regulators closed–loop characteristics and

is caused by the simultaneous operating conditions of fixed frequency and peak current detecting. Figure 9.A

Sense Input

reaches the threshold established by the control voltage.

unstable condition can be shown if a pertubation is added to the control voltage, and resulting in a small DI

(dashed line). With a fixed oscillator period, the current decay time is reduced and the minimum current at switch

the inductor current at switch turn–on. Several oscillator cycles may be required before the inductor current

will be unstable. Figure 9.B shows that by adding an artificial ramp, that is synchronized with the PWM clock to

inductor current follows the control voltage yielding true current mode operation. The compensating ramp can be

Control Voltage

DI

Inductor

Current

DI + DI

m2

m2

m1

m1

m2

m1

(DI + DI

) (

)

m2

m1

Oscillator Period

A

B

Control Voltage

DI

Oscillator Period

m1

m2

m3

Inductor

Current

Figure 9. Continuous Current Waveforms