Datasheet

Z
p
=
1
C
FF
R
FB1
(
1+
R
FB2
R
FB1
(
(rad/s)
Z
z
=
1
C
FF
R
FB1
(rad/s)
LM2611
www.ti.com SNOS965I –JUNE 2001–REVISED APRIL 2013
A general rule of thumb is to keep f
ESR
> 80kHz for LM2611 Cuk designs. Low ESR tantalum capacitors will
usually be rated for at least 180mA in a voltage rating of 10V or above. However the ESR in a tantalum capacitor
(even in a low ESR tantalum capacitor) is much higher than in a ceramic capacitor and could place f
ESR
low
enough to cause the LM2611 to run unstable.
Improving Transient Response/Compensation
The compensator in the LM2611 is internal. However, a zero-pole pair can be added to the open loop frequency
response by inserting a feed forward capacitor, C
FF
, in parallel to the top feedback resistor (R
FB1
). Phase margin
and bandwidth can be improved with the added zero-pole pair. This inturn will improve the transient response to
a step load change (see Figure 17 and Figure 18). The position of the zero-pole pair is a function of the feedback
resistors and the capacitor value:
(10)
(11)
The optimal position for this zero-pole pair will vary with circuit parameters such as D, I
OUT
, C
OUT
, L1, L2, and
C
CUK
. For most cases, the value for the zero frequency is between 5 kHz to 20 kHz. Notice how the pole
position, ω
p
, is dependant on the feedback resistors R
FB1
and R
FB2
, and therefore also dependant on the output
voltage. As the output voltage becomes closer to −1.26V, the pole moves towards the zero, tending to cancel it
out. If the absolute magnitude of the output voltage is less than 3.3V, adding the zero-pole pair will not have
much effect on the response.
Figure 17. 130mA to 400mA Transient Response Figure 18. 130mA to 400mA Transient Response
of the circuit in 5V to -5V Inverting Converter with of the circuit in 5V to -5V Inverting Converter with
C
FF
= 1nF C
FF
disconnected
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