LM2611_05 NSC [National Semiconductor], LM2611_05 Datasheet - Page 8

• Manufacturer Part Number: LM2611_05
• Description: 1.4MHz Cuk Converter
• Manufacturer: NSC [National Semiconductor]

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Operation
The optimal position for this zero-pole pair will vary with
circuit parameters such as D, I
For most cases, the value for the zero frequency is between
5 kHz to 20 kHz. Notice how the pole position, ω
dant on the feedback resistors R
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 5. 130mA to 400mA Transient Response
of the circuit in Figure 9 with C
(Continued)
OUT
FB1
, C
and R
OUT
, L1, L2, and C
FB2
FF
= 1nF
, and therefore
20018120
p
, is depen-
CUK
(1)
(2)
) LEVEL 3
.
8
Hysteretic Mode
As the output current decreases, there will come a point
when the energy stored in the Cuk capacitor is more than the
energy required by the load. The excess energy is absorbed
by the output capacitor, causing the output voltage to in-
crease out of regulation. The LM2611 detects when this
happens and enters a pulse skipping, or hysteretic mode. In
hysteretic mode, the output voltage ripple will increase, as
illustrated in Figure 7 and Figure 8.
FIGURE 7. The LM2611 in PWM mode has very low
FIGURE 6. 130mA to 400mA Transient Response
of the circuit in Figure 9 with C
ripple
FF
disconnected
20018121
20018122
) LEVEL 3