Datasheet

f
ESR
=
1
2SC
OUT
ESR
(Hz)
=
I
COUT(RMS)
=
1
2
3
3
'i
L2
V
IN
)
)
f
s
L
2
Vi
V
o
+1
I
CIN(RMS)
=
V
IN
1
2
3
)
)
f
s
L
1
Vi
V
o
+1
i
SW
(A)
I
L1
+ I
L2
t
I
CL
I
SW
'i
L1
+'i
L2
i
SW
i
SW(PEAK)
= i
L1
+ i
L2
= I
L1
+ I
L2
+ 'i
L1
+ 'i
L2
= I
OUT
x


1+
D
(1-D)
+
2
V
IN
x D x T
S
+


1
L
2
1
L
1
x
LM2611
SNOS965I –JUNE 2001–REVISED APRIL 2013 www.ti.com
Switch Current Limit
The LM2611 incorporates a separate current limit comparator, making current limit independent of any other
variables. The current limit comparator measures the switch current versus a reference that represents current
limit. If at any time the switch current surpasses the current limit, the switch opens until the next switching period.
To determine the maximum load for a given set of conditions, both the input and output inductor currents must be
considered. The switch current is equal to i
L1
+ i
L2
, and is drawn in Figure 16. In summary:
(6)
I
SW(PEAK)
must be less than the current limit (1.2A typical), but will also be limited by the thermal resistivity of the
LM2611's SOT23-5 package (θ
JA
= 265°C/W).
Figure 16. Switch Current Waveform in a Cuk Converter. The peak value is equal to the sum of the
average currents through L1 and L2 and the average-to-peak current ripples through L1 and L2.
Input Capacitor
The input current waveform to a Cuk converter is continuous and triangular, as shown in Figure 14. The input
inductor insures that the input capacitor sees fairly low ripple currents. However, as the input inductor gets
smaller, the input ripple goes up. The RMS current in the input capacitor is given by:
(7)
The input capacitor should be capable of handling the RMS current. Although the input capacitor is not so critical
in a Cuk converter, a 10µF or higher value good quality capacitor prevents any impedance interactions with the
input supply. A 0.1µF or 1µF ceramic bypass capacitor is also recommended on the V
IN
pin (pin 5) of the IC. This
capacitor must be connected very close to pin 5 (within 0.2 inches).
Output Capacitor
Like the input current, the output current is also continuous, triangular, and has low ripple (see I
L2
in Figure 15).
The output capacitor must be rated to handle its RMS current:
(8)
For example, I
COUT(RMS)
can range from 30mA to 180mA with 10µH ≤ L
1,2
≤ 22µH, −10V ≤ V
OUT
≤ −3.3V, and
2.7V ≤ V
IN
≤ 30V (V
IN
may be 30V if using separate power and analog supplies, see Split Supply Operation in the
APPLICATIONS section). The worst case conditions are with L
1,2
, V
OUT(MAX)
, and V
IN(MAX)
. Many capacitor
technologies will provide this level of RMS current, but ceramic capacitors are ideally suited for the LM2611.
Ceramic capacitors provide a good combination of capacitance and equivalent series resistance (ESR) to keep
the zero formed by the capacitance and ESR at high frequencies. The ESR zero is calculated as:
(9)
8 Submit Documentation Feedback Copyright © 2001–2013, Texas Instruments Incorporated
Product Folder Links: LM2611