Datasheet

Operation description LED7707
16/47
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 R
SLOPE
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 K
S
= 5.8 ⋅10
10
s
-1
, V
BE
= 2 V (typ.) and S
E
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
R
SLOPE
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
BEINOUT
SE
R
VVV
KS