Datasheet

LM3481
DR
I
SEN
R
SEN
V
IN
L1
D1
Q1
C
OUT
V
OUT
R
SL
+
C
SEN
Control Signal
Compensation Ramp
without R
SL
-M
C
V
SL
'
V
SL
Control Signal
Compensation Ramp
with R
SL
LM3481
www.ti.com SNVS346E –NOVEMBER 2007–REVISED APRIL 2012
Figure 21. Additional Slope Compensation Added Using External Resistor R
SL
Where,
ΔV
SL
= K x R
SL
K = 40 µA typically and changes slightly as the switching frequency changes. Figure 23 shows the effect the
current K has on ΔV
SL
and different values of R
SL
as the switching frequency changes.
A more general equation for the slope compensation ramp, M
C
, is shown below to include ΔV
SL
caused by the
resistor R
SL
.
M
C
= (V
SL
+ ΔV
SL
) x f
S
It is good design practice to only add as much slope compensation as needed to avoid subharmonic oscillation.
Additional slope compensation minimizes the influence of the sensed current in the control loop. With very large
slope compensation the control loop characteristics are similar to a voltage mode regulator which compares the
error voltage to a saw tooth waveform rather than the inductor current.
Figure 22. Increasing the Slope of the Compensation Ramp
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