The LM2698 is a general purpose PWM boost converter.
The 1.9A, 18V, 0.2ohm internal switch enables the LM2698
to provide efficient power conversion to outputs ranging from
2.2V to 17V. It can operate with input voltages as low as 2.2V
and as high as 12V. Current-mode architecture provides
superior line and load regulation and simple frequency compensation over the device’s 2.2V to 12V input voltage range.
The LM2698 sets the standard in power density and is
capable of supplying 12V at 400mA from a 5V input. The
LM2698 can also be used in flyback or SEPIC topologies.
®
The LM2698 SIMPLE SWITCHER
switching frequency of either 600kHz or 1.25MHz. This promotes flexibility in component selection and filtering techniques. A shutdown pin is available to suspend the device
and decrease the quiescent current to 5µA. An external
compensation pin gives the user flexibility in setting frequency compensation, which makes possible the use of
small, low ESR ceramic capacitors at the output. Switchers
Made Simple
and guaranteed design. The LM2698 is available in a low
profile 8-lead MSOP package.
®
software is available to insure a quick, easy
features a pin selectable
Typical Application Circuit
Features
n 1.9A, 0.2Ω, internal switch (typical)
n Operating voltage as low as 2.2V
n 600kHz/1.25MHz adjustable frequency operation
n Switchers Made Simple
n 8-Lead MSOP package
®
software
Applications
n 3.3V to 5V, 5V to 12V conversion
n Distributed Power
n Set-Top Boxes
n DSL Modems
n Diagnostic Medical Instrumentation
n Boost Converters
n Flyback Converters
n SEPIC Converters
®
1.35A Boost Regulator
20012658
SIMPLE SWITCHER®is a registered trademark of National Semiconductor Corporation.
LM2698MM-ADJMSOP-8MUA08A1000 Units, Tape and ReelS22B
LM2698MMX-ADJMSOP-8MUA08A3500 Units, Tape and ReelS22B
Supplied AsPackage ID
Pin Description
PinNameFunction
1V
C
2FBOutput voltage feedback input.
3SHDN
4GNDAnalog and power ground.
5V
6V
SW
IN
7FSLCTSwitching frequency select input. V
8NCConnect to ground.
Compensation network connection. Connected to the output of the voltage error amplifier.
Shutdown control input, active low.
Power switch input. Switch connected between SW pin and GND pin.
Analog power input.
= 1.25MHz. Ground = 600kHz.
IN
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Page 3
Block Diagram
LM2698
20012603
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Page 4
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
LM2698
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
SW Voltage−0.3V ≤ V
FB Voltage−0.3V ≤ V
V
Voltage0.965<V
C
SHDN Voltage
(Note 2)−0.3V ≤ V
FSLCT
(Note 2)−0.3V ≤ V
Maximum Junction
Temperature
Power Dissipation (Note 3)Internally Limited
Lead Temperature300˚C
−0.3V ≤ VIN≤ 12V
C
SHDN
FSLCT
SW
FB
<
≤ 18V
≤ 7V
1.565
≤ 7V
≤ 12V
150˚C
Vapor Phase (60 sec.)215˚C
Infrared (15 sec.)220˚C
ESD Susceptibility
(Note 4)
Human Body Model
(Note 5)2kV
Machine Model200V
Operating Conditions
Operating Junction
Temperature Range
(Note 6)−40˚C to +125˚C
Storage Temperature−65˚C to +150˚C
Supply Voltage2.2V to 12V
SW Voltage0 ≤ V
Electrical Characteristics
Specifications in standard type face are for TJ= 25˚C and those with boldface type apply over the full Operating Temperature Range (T
SymbolParameterConditions
I
Q
V
FB
I
CL
%V
/∆VINFeedback Voltage Line
FB
I
B
V
IN
g
m
A
V
D
MAX
D
MIN
f
S
I
SHDN
I
L
R
DS(ON)
TH
SHDN
UVPOn Threshold1.952.052.2V
= −40˚C to +125˚C)Unless otherwise specified. VIN=2.2V and IL= 0A, unless otherwise specified.
J
Min
(Note 6)
Typ
(Note 7)
Max
(Note 6)
Quiescent CurrentFB = 0V (Not Switching)1.32.0mA
V
=0V510µA
SHDN
Feedback Voltage1.22851.261.2915V
Switch Current LimitVIN= 2.7V (Note 8)1.351.92.4A
2.2V ≤ VIN≤ 12.0V0.0130.1%/V
Regulation
FB Pin Bias Current
0.520nA
(Note 9)
Input Voltage Range2.212V
Error Amp Transconductance ∆I = 5µA40135290µmho
Error Amp Voltage Gain120V/V
Maximum Duty CycleFSLCT = Ground7885%
Minimum Duty CycleFSLCT = Ground15%
FSLCT = V
IN
30
Switching FrequencyFSLCT = Ground480600720kHz
Shutdown Pin CurrentV
FSLCT = V
SHDN
V
SHDN
IN
=V
IN
=0V−0.5-1
11.251.5MHz
0.010.1µA
Switch Leakage CurrentVSW= 18V0.013µA
Switch R
DS(ON)
VIN= 2.7V, ISW= 1A0.20.4Ω
SHDN Threshold VoltageOutput High0.60.9V
Output Low0.30.6V
Off Threshold1.851.952.1V
SW
≤ 17.5V
Units
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Page 5
Electrical Characteristics (Continued)
Specifications in standard type face are for TJ= 25˚C and those with boldface type apply over the full Operating Temperature Range (T
SymbolParameterConditions
θ
JA
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: Shutdown and voltage frequency select should not exceed V
Note 3: The maximum allowable power dissipation is a function of the maximum junction temperature, T
and the ambient temperature, T
at any ambient temperature is calculated using: P
temperature, and the regulator will go into thermal shutdown.
Note 4: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF capacitor discharged
directly into each pin.
Note 5: ESD susceptibility using the human body model is 500V for V
Note 6: 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 7: Typical numbers are at 25˚C and represent the most likely norm.
Note 8: This is the switch current limit at 0% duty cycle. The switch current limit will change as a function of duty cycle. See Typical performance Characteristics
section for I
Note 9: Bias current flows into FB pin.
Note 10: Junction to ambient thermal resistance (no external heat sink) for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit.
See "Scenario ’A’" in the Power Dissipation section.
Note 11: Junction to ambient thermal resistance for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit and approximately
0.0191 sq. in. of copper heat sinking. See "Scenario ’B’" in the Power Dissipation section.
Note 12: Junction to ambient thermal resistance for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit and approximately
0.0465 sq. in. of copper heat sinking. See "Scenario ’C’" in the Power Dissipation section.
Note 13: Junction to ambient thermal resistance for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit and approximately
0.2523 sq. in. of copper heat sinking. See "Scenario ’D’" in the Power Dissipation section.
Note 14: Junction to ambient thermal resistance for the MSO8 package with minimal trace widths (0.010 inches) from the pins to the circuit and approximately
0.0098 sq. in. of copper heat sinking on the top layer and 0.0760 sq. in. of copper heat sinking on the bottom layer, with three 0.020 in. vias connecting the planes.
See "Scenario ’E’" in the Power Dissipation section.
= −40˚C to +125˚C)Unless otherwise specified. VIN=2.2V and IL= 0A, unless otherwise specified.
J
Min
(Note 6)
Thermal ResistanceJunction to Ambient
Typ
(Note 7)
235˚C/W
(Note 10)
Junction to Ambient
225
(Note 11)
Junction to Ambient
220
(Note 12)
Junction to Ambient
200
(Note 13)
Junction to Ambient
195
(Note 14)
.
IN
. See the Electrical Characteristics table for the thermal resistance of various layouts. The maximum allowable power dissipation
A
vs. V
CL
IN
(MAX) = (T
D
J(MAX)−TA
.
C
)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die
(MAX), the junction-to-ambient thermal resistance, θJA,
Switching Frequency vs VIN(600kHz)Switching Frequency vs VIN(1.25MHz)
ICLvs. Ambient Temperature
V
IN
= 3.3V, V
=8VICLvs. V
OUT
2001262020012623
IN
2001264120012642
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Page 8
Operation
LM2698
Continuous Conduction Mode
FIGURE 1. Simplified Boost Converter Diagram
(a) First Cycle of Operation (b) Second Cycle Of Operation
The LM2698 is a current-mode, PWM boost regulator. A
boost regulator steps the input voltage up to a higher output
voltage. In continuous conduction mode (when the inductor
current never reaches zero at steady state), the boost regulator operates in two cycles.
In the first cycle of operation, shown in Figure 1 (a), the
transistor is closed and the diode is reverse biased. Energy
is collected in the inductor and the load current is supplied by
.
C
OUT
The second cycle is shown in Figure 1 (b). During this cycle,
the transistor is open and the diode is forward biased. The
energy stored in the inductor is transferred to the load and
output capacitor.
The ratio of these two cycles determines the output voltage.
The output voltage is defined as:
where D is the duty cycle of the switch.
20012602
Inductor
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20012605
FIGURE 2. (a) Inductor Current (b) Diode Current
The inductor is one of the two energy storage elements in a
boost converter. Figure 2 shows how the inductor current
varies during a switching cycle. The current through an
inductor is quantified as:
Page 9
Operation (Continued)
If V
is constant, diL/ dt must be constant, thus the current
L(t)
in the inductor changes at a constant rate. This is the case in
DC/DC converters since the voltages at the input and output
can be approximated as a constant. The current through the
inductor of the LM2698 boost converter is shown in Figure2(a). The important quantities in determining a proper inductance value are I
(the inductor current ripple). If ∆iLis larger than I
inductor current will drop to zero for a portion of the cycle and
the converter will operate in discontinuous conduction mode.
is smaller than I
If ∆i
L
above zero and the converter will operate in continuous
conduction mode (CCM). All the analysis in this datasheet
assumes operation in continuous conduction mode. To operate in CCM:
(the average inductor current) and ∆i
L(AVG)
, the inductor current will stay
L(AVG)
>
I
L(AVG)
∆i
L
L(AVG)
, the
LM2698
Current Limit
The current limit in the LM2698 is referenced to the peak
switch current. The peak currents in the switch of a boost
converter will always be higher than the average current
supplied to the load. To determine the maximum average
output current that the LM2698 can supply, use:
I
OUT(MAX)
Where ICLis the switch current limit (see Electrical Charateristics table and Typical Performance Curves). Hence, as
increases, the maximum current that can be supplied to
V
IN
the load increases, as shown in Figure 3.
L
=(ICL− ∆iL)*(1−D) = (ICL− ∆iL)*VIN/V
OUT
Choose the minimum I
CCM operation. A common choice is to set ∆i
.
I
L(AVG)
to determine the minimum L for
OUT
L
to 30% of
The inductance value will also affect the stability of the
converter. Because the LM2698 utilizes current mode control, the inductor value must be carefully chosen. See the
COMPENSATION section for recommended inductance values.
Choosing an appropriate core size for the inductor involves
calculating the average and peak currents expected through
the inductor. In a boost converter,
and
I
L(Peak)=IL(AVG)
+ ∆iL,
where
A core size with ratings higher than these values should be
chosen. If the core is not properly rated, saturation will
dramatically reduce overall efficiency.
20012673
FIGURE 3. Maximum Output Current vs Input Voltage
Diode
The diode in a boost converter such as the LM2698 acts as
a switch to the output. During the first cycle, when the
transistor is closed, the diode is reverse biased and current
is blocked; the load current is supplied by the output capacitor. In the second cycle, the transistor is open and the diode
is forward biased; the load current is supplied by the inductor.
Observation of the boost converter circuit shows that the
average current through the diode is the average load current, and the peak current through the diode is the peak
current through the inductor. The diode should be rated to
handle more than its peak current. To improve efficiency, a
low forward drop Schottky diode is recommended.
Input Capacitor
Due to the presence of an inductor at the input of a boost
converter, the input current waveform is continuous and
triangular. The inductor ensures that the input capacitor sees
fairly low ripple currents. However, as the inductor gets
smaller, the input ripple increases. 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 boost
applications, a 10 µF or higher value, good quality capacitor
prevents any impedance interactions with the input supply.
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Page 10
Operation (Continued)
A 0.1µF or 1µF ceramic bypass capacitor is also recom-
LM2698
mended on the V
be connected very close to pin 6 to effectively filter high
frequency noise. When operating at 1.25 MHz switching
frequency, a minimum bypass capacitance of 0.22 µF is
recommended.
Output Capacitor
The output capacitor in a boost converter provides all the
output current when the switch is closed and the inductor is
charging. As a result, it sees very large ripple currents. The
output capacitor should be capable of handling the maximum RMS current. The RMS current in the output capacitor
is:
where,
and
The ESR and ESL of the output capacitor directly control the
output ripple. Use capacitors with low ESR and ESL at the
output for high efficiency and low ripple voltage. Surface
mount tantalums, surface mount polymer electrolytic, and
polymer tantalum, Sanyo OS-CON, or multi-layer ceramic
capacitors are recommended at the output.
pin (pin 6) of the IC. This capacitor must
IN
D=(V
OUT-VIN
)/V
OUT
Compensation
This section presents a step-by-step procedure to design the
compensation network at pin 1 (V
) of the LM2698. These
c
design methods will produce a conservative and stable control loop.
There is a minimum inductance requirement in any current
mode converter. This is a function of V
, duty cycle, and
OUT
switching frequency, among other things. The graphs below
plot the recommended inductance range vs. duty cycle for
= 12V. The two lines represent the upper and lower
V
OUT
bounds of the recommended inductance range. The simplified compensation procedure that follows assumes that the
inductance never drops below the Q = 5 line. Figure 4 plots
the equation:
(1)
where,
= 0.15,
R
DSON
*
Se = 0.072
fS,
and Q = 0.5 and 5
Use Q = 5 to calculate the minimum inductance recom-
mended for a stable design. Choosing an inductor between
the Q = 0.5 and Q = 5 values provides a good tradeoff
between size and stability. Note that as V
5V, R
Characteristics section (R
case R
increases, as shown in the Typical Performance
DS(ON)
DS(ON)
should be used when choosing the induc-
vs.VINcurve). The worst
DS(ON)
drops less than
IN
tance. To view plots for different Vout, multiply the Y axis by
a factor of V
/12, or plot Equation (1) for the respective
OUT
output voltage.
20012654
FIGURE 4. Minimum Inductance Requirements for (a) fS= 600kHz and (b) fS= 1.25MHz
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20012653
Page 11
Operation (Continued)
The goal of the compensation network is to provide the best
static and dynamic performance while insuring stability over
line and load variations. The relationship of stability and
LM2698
performance can be best analyzed by plotting the magnitude
and phase of the open loop frequency response in the form
of a bode plot. A typical bode plot of the LM2698 open loop
frequency response is shown in Figure 5.
FIGURE 5. Bode plot of the LM2698 Frequency Response using the Typical Application Circuit
Poles are marked with an ’X’, and zeros are marked with a
’O’. The bolded ’O’ labeled ’f
’ is a right-half plane zero.
RHP
Right half plane zeros act like normal zeros to the magnitude
(+20dB/decade slope influence) and like poles to the phase
(−90˚ shift). Three curves are shown. The powerstage curve
is the frequency response of the powerstage, which includes
the switch, diode, inductor, output capacitor, and load. The
compensator curve is the frequency response of the compensator, which is the error amp combined with the compensation network. T is the product of the powerstage and the
compensator and is the complete open loop frequency response. The power stage response is fixed by line and load
constraints, while the compensator is set by the external
compensation network at pin 1. The compensator can be
designed in a few simple steps as follows.
Quick Compensator Design
Calculate:
where,
where R
Choose C
Choose
OUT
C1
20012657
= 875kΩ
= 4.7nF
Where,
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Page 12
Operation (Continued)
LM2698
If the output capacitor is of high ESR (0.1Ω or higher), it may
be necessary to use C
1/(2πC
used. Choose C
where R
ESR) (Hz) is lower than fS/2 (Hz), CC2should be
OUT
(R
OUT
such that:
C2
C+ROUT
= output impedance of the error amp (875 kΩ).
Improving Transient Response Time
The above compensator design provides a loop gain with
high phase margin for a large stability margin. The transient
response time of this loop is limited by the lower midfrequency gain necessary to achieve a high phase margin. If
it is desired to increase the transient response time, C
be decreased. Decreasing C
increasingly shorter transient response times, however the
loop phase margin will become progressively lower as C
decreased. When optimizing the loop gain for transient response time, it is recommended to keep the phase margin
above 40˚.
. A rule of thumb is that if
C2
)(C
ESR) / (RCR
OUT
by 2x, 4x, and 6x will yield
C1
OUT
) (F)
Additional Comments on the Open Loop Frequency Response
The procedure used here to pick the compensation network
will provide a good starting point. In most cases, these
values will be sufficient for a stable design. It is always
recommended to check the design in a real test setup. This
is easy to do with the aid of a dynamic load. Set the high and
low load values to your system requirements and switch
between the two at about 1kHz. View the output voltage with
an oscilloscope using AC coupling, and zoom in enough to
see the waveform react to the load change. Use the following table to determine if your design is stable. Remember to
use worst case conditions (V
IN(MIN),ROUT(MIN),ROUT(MAX)
).
ResponseConclusionWhat to
Change
Underdamped,
weak attenuation
Underdamped,
Nearing instabilityMake C
larger
StableNothing
C1
strong attenuation
Critically dampedStableNothing
may
C1
is
C1
OverdampedStableNothing
Application Information
FIGURE 6. 3.3V to 10V Boost Converter
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20012668
Page 13
Application Information (Continued)
1.25MHz Boost Converter
Figure 6 shows the LM2698 boosting 3.3V to 10V at 300mA.
As discussed in the COMPENSATION section, the R
of the internal FET in the LM2698 raises as the input voltage
drops below 5V (see Typical Performance Characteristics).
The minimum input voltage for this application is 2.5V, at
DS(ON)
which point the R
is approximately 200mΩ. Substitut-
DS(ON)
ing these values in for Equation (1), it is found that either a
10 µH (1.25MHz operation) or a 22 µH (600kHz operation) is
necessary for a stable design. The circuit is operated at
1.25MHz to allow for a smaller inductance. From the Compensator Design equations, R
is calculated to be 18.6kΩ,
C
and a 20kΩ resistor is used.
LM2698
FIGURE 7. 3.3V SEPIC Converter
3.3V SEPIC
The LM2698 can be used to implement a SEPIC technology.
The advantages of the SEPIC topology are that it can step
up or step down an input voltage, and it has low input current
ripple.
The conversion ratio for the SEPIC is :
where
D’ = 1−D
Solving for D yeilds:
To avoid subharmonic oscillations, it is recommended that
inductors L1 and L2 be the same inductance. Currents conducted by the inductors are:
20012631
I
1=IOUT(VOUT/VIN
1=VIN
D/(2*L1*fs)
∆i
I
2=IOUT
)
∆i1=VIND/(2*L2*fs)
The switch sees a maximum current of I
1+I2
+ ∆i1+ ∆i2.If
L1 = L2 = L, the maximum switch current is given by:
I
OUT
(1+V
OUT/VIN
)+VIND/(L*fs)
The maximum load current is limited by this relationship to
the switch current.
The polarity of C
will change between each cycle, so a
SEPIC
ceramic capacitor should be used here. A high quality, low
ESR capacitor will directly improve efficiency because all the
load current passes through C
should be chosen using the same relationship as in the
C
IN
boost converter (see the C
IN
.
SEPIC
section). CINmust be able to
provide the necessary RMS current.
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Page 14
Application Information (Continued)
LM2698
FIGURE 8. Level-Shifted SEPIC Converter
Level-Shifted SEPIC
The circuit shown in Figure 8 is similar to the SEPIC shown
in Figure 7, except that it is level shifted to provide a negative
output voltage. This is achieved by connecting the ground of
the LM2698 to the output. The circuit analysis for the levelshifted SEPIC is the same as the SEPIC. The voltage at the
input of the LM2698 will need to be clamped if the absolute
value of the output voltage plus the input voltage exceeds
12V, the absolute maximum rating for the V
pin. The sim-
IN
plest way to do this is with a zener diode, as shown in Figure
8. Likewise, if the FSLCT pin is pulled high to operate at 1.25
MHz, its voltage must not exceed 12V. To prevent any high
frequency noise from entering the LM2698’s internal circuitry, a high frequency bypass capacitor must be placed as
close to pin 6 as possible. A good choice for this capacitor is
a 0.1µF ceramic capacitor.
Layout Consideration
The GND pin and the NC pin is recommended to be connected by a short trace as shown below.
Power Dissipation
The output power of the LM2698 is limited by its maximum
power dissipation. The maximum power dissipation is determined by the formula
=(T
P
D
jmax-TA
where T
(125˚C), T
is the maximum specified junction temperature
jmax
is the ambient temperature, and θJAis the ther-
A
mal resistance of the package. θ
)/θ
JA
is dependant on the
JA
layout of the board as shown below.
20012643
20012611
20012612
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Page 15
Application Information (Continued)
20012613
LM2698
20012615
20012614
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Page 16
Physical Dimensions inches (millimeters)
unless otherwise noted
1.35A Boost Regulator
®
8-Lead Plastic MSOP
NS Package Number MUA08A
LM2698 SIMPLE SWITCHER
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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