LM2611
LM2611 1.4MHz Cuk Converter
Literature Number: SNOS965F
LM2611
1.4MHz Cuk Converter
General Description
The LM2611 is a current mode, PWM inverting switching
regulator. Operating from a 2.7 - 14V supply, it is capable of
producing a regulated negative output voltage of up to −(36-
V
IN(MAX)
). The LM2611 utilizes an input and output inductor,
which enables low voltage ripple and RMS current on both
the input and the output. With a switching frequency of
1.4MHz, the inductors and output capacitor can be physi-
cally small and low cost. High efficiency is achieved through
the use of a low R
DS(ON)
FET.
The LM2611 features a shutdown pin, which can be acti-
vated when the part is not needed to lower the Iq and save
battery life. A negative feedback (NFB) pin provides a simple
method of setting the output voltage, using just two resistors.
Cycle-by-cycle current limiting and internal compensation
further simplify the use of the LM2611.
The LM2611 is available is a small SOT23-5 package. It
comes in two grades:
Grade A Grade B
Current Limit 1.2A 0.9A
R
DS(ON)
0.50.7
Features
n1.4MHz switching frequency
nLow R
DS(ON)
DMOS FET
n1mVp-p output ripple
n−5V at 300mA from 5V input
nBetter regulation than a charge pump
nUses tiny capacitors and inductors
nWide input range: 2.7V to 14V
nLow shutdown current: <1uA
n5-lead SOT-23 package
Applications
nMR Head Bias
nDigital camera CCD bias
nLCD bias
nGaAs FET bias
nPositive to negative conversion
Typical Application Circuit
20018117
January 2005
LM2611 1.4MHz Cuk Converter
© 2005 National Semiconductor Corporation DS200181 www.national.com
Connection Diagram
Top View
20018115
5-lead SOT-23 Package
NS Package Number MF05A
Ordering Information
Order Number Package Type NSC Package
Drawing
Supplied As Package ID
LM2611AMF
SOT23-5 MF05A
1K Tape and Reel S40A
LM2611AMFX 3K Tape and Reel S40A
LM2611BMF 1K Tape and Reel S40B
LM2611BMFX 3K Tape and Reel S40B
Pin Description
Pin Name Function
1 SW Drain of internal switch. Connect at the node of the input inductor and Cuk capacitor.
2 GND Analog and power ground.
3 NFB Negative feedback. Connect to output via external resistor divider to set output voltage.
4 SHDN Shutdown control input. V
IN
= Device on. Ground = Device in shutdown.
5V
IN
Analog and power input. Filter out high frequency noise with a 0.1 µF ceramic capacitor
placed close to the pin.
Block Diagram
20018101
LM2611
www.national.com 2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
14.5V
SW Voltage −0. 4V to 36V
NFB Voltage +0. 4V to −6V
SHDN Voltage −0. 4V to 14.5V
Maximum Junction
Temperature
125˚C
Power Dissipation (Note 2) Internally Limited
Lead Temperature 300˚C
ESD Susceptibility (Note 3)
Human Body Model 2kV
Machine Model 200V
Operating Conditions
Operating Junction
Temperature Range −40˚C to +125˚C
Storage Temperature −65˚C to +150˚C
Supply Voltage 2.7V to 14V
θ
JA
256˚C/W
Electrical Characteristics
Specifications in standard type face are for T
J
= 25˚C and those with boldface type apply over the Temperature Range T
J
=
−40˚C to +85˚C, unless otherwise specified. V
IN
= 5.0V and I
L
= 0A, unless otherwise specified.
Symbol Parameter Conditions Min
(Note 4)
Typ
(Note 5)
Max
(Note 4) Units
V
IN
Input Voltage 2.7 14 V
I
SW
Switch Current Limit Grade A 11.2 2A
Grade B 0.7 0.9
R
DSON
Switch ON Resistance Grade A 0.5 0.65
Grade B 0.7 0.9
SHDN
TH
Shutdown Threshold Device enabled 1.5 V
Device disabled 0.50
I
SHDN
Shutdown Pin Bias Current V
SHDN
= 0V 0.0 µA
V
SHDN
= 5V 0.0 1.0
NFB Negative Feedback
Reference
V
IN
=3V −1.205 −1.23 −1.255 V
I
NFB
NFB Pin Bias Current V
NFB
=−1.23V −2.7 −4.7 −6.7 µA
I
q
Quiescent Current V
SHDN
= 5V, Switching 1.8 3.5 mA
V
SHDN
= 5V, Not Switching 270 500 µA
V
SHDN
= 0V 0.024 1µA
%V
OUT
/
V
IN
Reference Line Regulation 2.7V V
IN
14V 0.02 %/V
f
S
Switching Frequency 1.0 1.4 1.8 MHz
D
MAX
Maximum Duty Cycle 82 88 %
I
L
Switch Leakage Not Switching
V
SW
=5V
A
Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to
be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA,
and the ambient temperature, TA. See the Electrical Characteristics table for the thermal resistance of various layouts. The maximum allowable power dissipation
at any ambient temperature is calculated using: PD(MAX) = (TJ(MAX) −T
A)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die
temperature, and the regulator will go into thermal shutdown.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5kresistor into each pin. The machine model is a 200pF capacitor discharged
directly into each pin.
Note 4: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% tested
or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods.
All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 5: Typical numbers are at 25˚C and represent the expected value of the parameter.
LM2611
www.national.com3
Typical Performance Characteristics
R
DS(ON)
vs V
IN
R
DS(ON)
Vs. Ambient Temperature
V
IN
=5V
20018112 20018145
Switch Current Limit vs. V
IN
Switch Current Limit vs Ambient Temperature
V
IN
=5V
20018111 20018143
Oscillator Frequency vs V
IN
Oscillator Frequency vs Ambient Temperature
V
IN
=5V
20018119 20018116
LM2611
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Typical Performance Characteristics (Continued)
V
NFB
vs V
IN
T
A
= 25˚C, V
OUT
= −5V
V
NFB
vs Ambient Temperature
V
IN
=5V
20018107 20018124
I
NFB
vs V
IN
T
A
= 25˚C, V
OUT
= −5V
I
NFB
vs Ambient Temperature
V
IN
= 3.5V, V
OUT
= −5V
20018108 20018109
I
q
vs Ambient Temperature (No Load)
V
SHUTDOWN
vs Ambient Temperature
V
IN
=5V
20018144 20018110
LM2611
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Operation
Cuk Converter
The LM2611 is a current mode, fixed frequency PWM
switching regulator with a −1.23V reference that makes it
ideal for use in a Cuk converter. The Cuk converter inverts
the input and can step up or step down the absolute value.
Using inductors on both the input and output, the Cuk con-
verter produces very little input and output current ripple.
This is a significant advantage over other inverting topolo-
gies such as the buck-boost and flyback.
The operating states of the Cuk converter are shown in
Figure 1. During the first cycle, the transistor switch is closed
and the diode is open. L1 is charged by the source and L2 is
charged by C
CUK
, while the output current is provided by L2.
In the second cycle, L1 charges C
CUK
and L2 discharges
through the load. By applying the volt-second balance to
either of the inductors, the relationship of V
OUT
to the duty
cycle (D) is found to be:
The following sections review the steady-state design of the
LM2611 Cuk converter.
Output and Input Inductor
Figure 2 and Figure 3 show the steady-state voltage and
current waveforms for L1 and L2, respectively. Referring to
Figure 1 (a), when the switch is closed, V
IN
is applied across
L1. In the next cycle, the switch opens and the diode be-
comes forward biased, and V
OUT
is applied across L1 (the
voltage across C
CUK
is V
IN
−V
OUT
.
The voltage and current waveforms of inductor L2 are shown
in Figure 3. During the first cycle of operation, when the
switch is closed, V
IN
is applied across L2. When the switch
opens, V
OUT
is applied across L2.
20018105
FIGURE 1. Operating Cycles of a Cuk Converter
20018103
FIGURE 2. Voltage and Current Waveforms in Inductor
L1 of a Cuk Converter
20018104
FIGURE 3. Voltage and Current Waveforms in Inductor
L2 of a Cuk Converter
LM2611
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Operation (Continued)
The following equations define values given in Figure 2 and
Figure 3:
I
L2
=I
OUT
Use these equations to choose correct core sizes for the
inductors. The design of the LM2611’s internal compensa-
tion assumes L1 and L2 are equal to 10 - 22 µH, thus it is
recommended to stay within this range.
Switch Current Limit
The LM2611 incorporates a separate current limit compara-
tor, 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 maxi-
mum 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 4.In
summary:
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).
Input Capacitor
The input current waveform to a Cuk converter is continuous
and triangular, as shown in Figure 2. The input inductor
insures that the input capacitor sees fairly low ripple cur-
rents. However, as the input inductor gets smaller, the input
ripple goes up. The RMS current in the input capacitor is
given by:
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 recom-
mended 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 3). The output
capacitor must be rated to handle its RMS current:
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 APPLI-
CATIONS 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 re-
sistance (ESR) to keep the zero formed by the capacitance
and ESR at high frequencies. The ESR zero is calculated as:
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 capaci-
tor 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 par-
allel 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 5 and Figure 6). The position of the
zero-pole pair is a function of the feedback resistors and the
capacitor value:
20018102
FIGURE 4. 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.
LM2611
www.national.com7
Operation (Continued)
(1)
(2)
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 depen-
dant 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. 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.
20018120
) LEVEL 3
FIGURE 5. 130mA to 400mA Transient Response
of the circuit in Figure 9 with C
FF
= 1nF
20018122
) LEVEL 3
FIGURE 6. 130mA to 400mA Transient Response
of the circuit in Figure 9 with C
FF
disconnected
20018121
FIGURE 7. The LM2611 in PWM mode has very low
ripple
LM2611
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Operation (Continued)
Thermal Shutdown
If the junction temperature of the LM2611 exceeds 163˚C, it
will enter thermal shutdown. In thermal shutdown, the part
deactivates the driver and the switch turns off. The switch
remains off until the junction temperature drops to 155˚C, at
which point the part begins switching again. It will typically
take 10ms for the junction temperature to drop from 163˚C to
155˚C with the switch off.
20018123
FIGURE 8. At low loads, the LM2611 enters a
pulse-skipping mode. The output ripple
slightly increases in this mode.
LM2611
www.national.com9
Application Circuits
5V to -5V Inverting Converter
20018117
Efficiency vs Load Current
20018158 20018160
FIGURE 9. The Maximum Output Current vs Output Voltage (Adjust R
FB2
to Set a Different Output Voltage) When The
Input Voltage is 5V
LM2611
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Application Circuits (Continued)
9V to -5V Inverting Converter
20018164
20018161
FIGURE 10. The Maximum Output Current vs Output
Voltage (Adjust R
FB2
to Set a Different Output Voltage)
When The Input Voltage is 9V
12V to -5V Inverting Converter
20018163
20018162
FIGURE 11. The Maximum Output Current vs Output
Voltage (Adjust R
FB2
to Set a Different Output Voltage)
When The Input Voltage is 12V
LM2611
www.national.com11
Application Circuits (Continued)
Split Supply Operation
The LM2611 may be operated with separate power and bias
supplies. In the circuit shown in Figure 12.V
IN
is the power
supply that the regulated voltage is derived from, and V
DD
is
a low current supply used to bias the LM2611. Conditions for
the supplies are:
2.7V V
DD
14V
0V V
IN
(36-IV
OUT
I) V
As the input voltage increases, the maximum output current
capacbility increases. Using a separate, higher voltage sup-
ply for power conversion enables the LM2611 to provide
higher output currents than it would with a single supply that
is limited in voltage by V
IN(MAX)
.
Shutdown/Soft Start
A soft start circuit is used in switching power supplies to limit
the input inrush current upon start-up. Without a soft-start
circuit, the inrush current can be several times the steady-
state load current, and thus apply unnecessary stress to the
input source. The LM2611 does not have soft-start circuitry,
but implementing the circuit in Figure 13 will lower the peak
inrush current. The SHDN pin is coupled to the output
through C
SS
. The LM2611 is toggled between shutdown and
run states while the output slowly decreases to its steady-
state value. The energy required to reach steady-state is
spread over a longer time and the input current spikes
decrease (see Figure 14 and Figure 15).
20018114
FIGURE 12. LM2611 Operating with Separate Power and Biasing Supplies
LM2611
www.national.com 12
Application Circuits (Continued)
20018125
FIGURE 13. LM2611 Soft Start Circuit
20018142
FIGURE 14. Start-Up Waveforms with Soft Start Circuit
20018141
FIGURE 15. Start-Up Waveforms without Soft Start Circuit
LM2611
www.national.com13
Application Circuits (Continued)
High Duty Cycle/Load Current Operation
The circuit in Figure 16 is used for high duty cycles (D >0.5)
and high load currents. The duty cycle will begin to increase
beyond 50% as the input voltage drops below the absolute
magnitude of the output voltage. R
FB3
and C
FF2
are added to
the feedback network to introduce a low frequency lag com-
pensation (pole-zero pair) necessary to stabilize the circuit
under the combination of high duty cycle and high load
currents.
20018129
FIGURE 16. LM2611 High Current Schematic
LM2611
www.national.com 14
Physical Dimensions inches (millimeters)
unless otherwise noted
5-lead SOT-23 Package
NS Package Number MF05A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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LM2611 1.4MHz Cuk Converter
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