Datasheet

Oscillator
Ramp
Generator
Ramp
Summing
Two Direction
Current Sense
11
15
4COMP
CS
RSUM
+
+
2 V
7
GND
CLK
RAMP
VMC
PCM
Cycle-by-Cycle I
LIM
Mode Select
CS_SLOPECOMP
VREF VCM
GND PCM
+
0.85 V
-
+
SUM
2.5 V
me
0.5 R s
æ ö
=
ç ÷
´ m
è ø
UCC28950
SLUSA16B –MARCH 2010– REVISED OCTOBER 2011 www.ti.com
Slope Compensation (RSUM)
Slope compensation is the technique that adds additional ramp signal to the CS signal and applied to the:
• Input of PWM comparator in case of peak current mode control.
• Input of cycle-by-cycle current limit comparator.
This prevents sub-harmonic oscillation at D > 50% (some publications suggest it might happen even at D <
50%). At low duty cycle and light load, the slope compensation ramp reduces noise sensitivity of Peak Current
Mode control.
Too much additional slope compensation ramp reduces benefits of PCM control. In case of cycle-by-cycle
current limit, the average current limit becomes lower and this might reduce the start up capability with the large
output capacitance. The optimal compensation slope varies depending on duty cycle, L
O
and L
M
.
The slope compensation is needed for the controller operating at peak current mode control or during the
cycle-by-cycle current limit at duty cycle above 50%. Placing a resistor from RSUM pin to ground allows the
controller to operate in peak current control mode. Connecting RSUM pin through resistor to VREF switches
controller to the voltage mode control with the internal PWM ramp. However, the resistor value still provides CS
signal compensation for cycle-by-cycle current limit. In other words, in VMC, the slope compensation is applied
only to cycle-by-cycle comparator. While in PCM, the slope compensation applied to both PWM and
cycle-by-cycle current limit comparators.
The operation logic of slope compensation circuit is shown in Figure 12.
Figure 12. The Operation Logic of Slope Compensation Circuit
The slope of the additional ramp, me, added to CS signal by placing a resistor from RSUM pin to the ground is
defined by the following Equation 12.
(12)
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