LED7707 datasheet(17/48 Pages) STMICROELECTRONICS

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LED7707

Operation description

17/48

Figure 11.

External sync waveforms

5.1.6

Slope compensation

The constant frequency, peak current-mode topology has the advantage of very easy loop

compensation with output ceramic caps (reduced cost and size of the application) and fast

transient response. In addition, the intrinsic peak-current measurement simplifies the

current limit protection, avoiding undesired saturation of the inductor.

On the other side, this topology has a drawback: there is an inherent open loop instability

when operating with a duty-ratio greater than 0.5. This phenomenon is known as “Sub-

Harmonic Instability” and can be avoided by adding an external ramp to the one coming

from the sensed current. This compensating technique, based on the additional ramp, is

called “slope compensation”. In Figure 12, where the switching duty-cycle is higher than 0.5,

the small perturbation

ΔI

L dies away in subsequent cycles thanks to the slope compensation

and the system reverts to a stable situation.

The SLOPE pin allows to properly set the amount of slope compensation connecting a

simple resistor RSLOPE between the SLOPE pin and the output. The compensation ramp

starts at 35% (typ.) of each switching period and its slope is given by the following equation:

Equation 5

Where KS = 5.8 ⋅10

10 s-1, V

BE = 2 V (typ.) and SE is the slope ramp in [A/s].

To avoid sub-harmonic instability, the compensating slope should be at least half the slope

of the inductor current during the off-phase when the duty-cycle is greater than 50%. The

value of

RSLOPE can be calculated according to equation 6.

Slave SYNC pin voltage

270mV threshold

FSW pin voltage (ext. sync)

Slave LX pin voltage

270ns minimum

AM00588v1

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

SLOPE

OUT

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