LM2734
LM2734 Thin SOT23 1A Load Step-Down DC-DC Regulator
Literature Number: SNVS288H
LM2734
June 21, 2010
Thin SOT23 1A Load Step-Down DC-DC Regulator
General Description
The LM2734 regulator is a monolithic, high frequency, PWM
step-down DC/DC converter in a 6-pin Thin SOT23 package.
It provides all the active functions to provide local DC/DC
conversion with fast transient response and accurate regula-
tion in the smallest possible PCB area.
With a minimum of external components and online design
support through WEBENCH®™, the LM2734 is easy to use.
The ability to drive 1A loads with an internal 300mΩ NMOS
switch using state-of-the-art 0.5µm BiCMOS technology re-
sults in the best power density available. The world class
control circuitry allows for on-times as low as 13ns, thus sup-
porting exceptionally high frequency conversion over the en-
tire 3V to 20V input operating range down to the minimum
output voltage of 0.8V. Switching frequency is internally set
to 550kHz (LM2734Y) or 1.6MHz (LM2734X), allowing the
use of extremely small surface mount inductors and chip ca-
pacitors. Even though the operating frequencies are very
high, efficiencies up to 90% are easy to achieve. External
shutdown is included, featuring an ultra-low stand-by current
of 30nA. The LM2734 utilizes current-mode control and inter-
nal compensation to provide high-performance regulation
over a wide range of operating conditions. Additional features
include internal soft-start circuitry to reduce inrush current,
pulse-by-pulse current limit, thermal shutdown, and output
over-voltage protection.
Features
■Thin SOT23-6 package
■3.0V to 20V input voltage range
■0.8V to 18V output voltage range
■1A output current
■550kHz (LM2734Y) and 1.6MHz (LM2734X)
switching frequencies
■300mΩ NMOS switch
■30nA shutdown current
■0.8V, 2% internal voltage reference
■Internal soft-start
■Current-Mode, PWM operation
■WEBENCH® online design tool
■Thermal shutdown
■LM2734XQ/LM2734YQ are AEC-Q100 Grade 1 qualified
and are manufactured on an Automotive Grade Flow
Applications
■Local Point of Load Regulation
■Core Power in HDDs
■Set-Top Boxes
■Battery Powered Devices
■USB Powered Devices
■DSL Modems
■Notebook Computers
■Automotive
Typical Application Circuit
20102301
Efficiency vs Load Current
VIN = 5V, VOUT = 3.3V
20102345
WEBENCH™ is a trademark of Transim.
© 2010 National Semiconductor Corporation 201023 www.national.com
LM2734 Thin SOT23 1A Load Step-Down DC-DC Regulator
Connection Diagrams
20102305
6-Lead TSOT
NS Package Number MK06A
20102360
Pin 1 Indentification
Ordering Information
Order Number Package Type NSC Package
Drawing
Package
Marking
Supplied As Features
LM2734XMK
TSOT-6 MK06A
SFDB 1000 Units on Tape and Reel
LM2734XMKX SFDB 3000 Units on Tape and Reel
LM2734XQMKE SUKB 250 Units on Tape and Reel AEC-Q100 Grade 1
Qualified. Automotive
Grade Production Flow*
LM2734XQMK SUKB 1000 Units on Tape and Reel
LM2734XQMKX SUKB 3000 Units on Tape and Reel
LM2734YMK SFEB 1000 Units on Tape and Reel
LM2734YMKX SFEB 3000 Units on Tape and Reel
LM2734YQMKE SVCB 250 Units on Tape and Reel AEC-Q10-0 Grade 1
Qualified. Automotive
Grade Production Flow*
LM2734YQMK SVCB 1000 Units on Tape and Reel
LM2734YQMKX SVCB 3000 Units on Tape and Reel
*Automotive Grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, including defect detection methodologies.
Reliability qualification is compliant with the requirements and temperature grades defined in the AEC-Q100 standard. Automotive grade products are identified
with the letter Q. For more information go to http://www.national.com/automotive.
Pin Descriptions
Pin Name Function
1 BOOST Boost voltage that drives the internal NMOS control switch. A
bootstrap capacitor is connected between the BOOST and SW pins.
2 GND Signal and Power ground pin. Place the bottom resistor of the
feedback network as close as possible to this pin for accurate
regulation.
3 FB Feedback pin. Connect FB to the external resistor divider to set output
voltage.
4 EN Enable control input. Logic high enables operation. Do not allow this
pin to float or be greater than VIN + 0.3V.
5 VIN Input supply voltage. Connect a bypass capacitor to this pin.
6 SW Output switch. Connects to the inductor, catch diode, and bootstrap
capacitor.
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LM2734
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN -0.5V to 24V
SW Voltage -0.5V to 24V
Boost Voltage -0.5V to 30V
Boost to SW Voltage -0.5V to 6.0V
FB Voltage -0.5V to 3.0V
EN Voltage -0.5V to (VIN + 0.3V)
Junction Temperature 150°C
ESD Susceptibility (Note 2) 2kV
Storage Temp. Range -65°C to 150°C
Soldering Information
Reflow Peak Pkg. Temp.(15sec) 260°C
Operating Ratings (Note 1)
VIN 3V to 20V
SW Voltage -0.5V to 20V
Boost Voltage -0.5V to 25V
Boost to SW Voltage 1.6V to 5.5V
Junction Temperature Range −40°C to +125°C
Thermal Resistance θJA (Note 3)118°C/W
Electrical Characteristics
Specifications with standard typeface are for TJ = 25°C, and those in boldface type apply over the full Operating Temperature
Range (TJ = -40°C to 125°C). VIN = 5V, VBOOST - VSW = 5V unless otherwise specified. Datasheet min/max specification limits are
guaranteed by design, test, or statistical analysis.
Symbol Parameter Conditions Min
(Note 4)
Typ
(Note 5)
Max
(Note 4)Units
VFB Feedback Voltage 0.784 0.800 0.816 V
ΔVFB/ΔVIN Feedback Voltage Line Regulation VIN = 3V to 20V 0.01 % / V
IFB Feedback Input Bias Current Sink/Source 10 250 nA
UVLO
Undervoltage Lockout VIN Rising 2.74 2.90
V
Undervoltage Lockout VIN Falling 2.0 2.3
UVLO Hysteresis 0.30 0.44 0.62
FSW Switching Frequency LM2734X 1.2 1.6 1.9 MHz
LM2734Y 0.40 0.55 0.66
DMAX Maximum Duty Cycle LM2734X 85 92 %
LM2734Y 90 96
DMIN Minimum Duty Cycle LM2734X 2 %
LM2734Y 1
RDS(ON) Switch ON Resistance VBOOST - VSW = 3V 300 600 mΩ
ICL Switch Current Limit VBOOST - VSW = 3V 1.2 1.7 2.5 A
IQQuiescent Current Switching 1.5 2.5 mA
Quiescent Current (shutdown) VEN = 0V 30 nA
IBOOST Boost Pin Current LM2734X (50% Duty Cycle) 2.5 3.5 mA
LM2734Y (50% Duty Cycle) 1.0 1.8
VEN_TH
Shutdown Threshold Voltage VEN Falling 0.4 V
Enable Threshold Voltage VEN Rising 1.8
IEN Enable Pin Current Sink/Source 10 nA
ISW Switch Leakage 40 nA
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see Electrical Characteristics.
Note 2: Human body model, 1.5kΩ in series with 100pF.
Note 3: Thermal shutdown will occur if the junction temperature exceeds 165°C. The maximum power dissipation is a function of TJ(MAX) , θJA and TA . The
maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) – TA)/θJA . All numbers apply for packages soldered directly onto a 3” x 3” PC
board with 2oz. copper on 4 layers in still air. For a 2 layer board using 1 oz. copper in still air, θJA = 204°C/W.
Note 4: Guaranteed to National’s Average Outgoing Quality Level (AOQL).
Note 5: Typicals represent the most likely parametric norm.
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LM2734
Typical Performance Characteristics All curves taken at VIN = 5V, VBOOST - VSW = 5V, L1 = 4.7 µH ("X"),
L1 = 10 µH ("Y"), and TA = 25°C, unless specified otherwise.
Efficiency vs Load Current - "X" VOUT = 5V
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Efficiency vs Load Current - "Y" VOUT = 5V
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Efficiency vs Load Current - "X" VOUT = 3.3V
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Efficiency vs Load Current - "Y" VOUT = 3.3V
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Efficiency vs Load Current - "X" VOUT = 1.5V
20102337
Efficiency vs Load Current - "Y" VOUT = 1.5V
20102335
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LM2734
Oscillator Frequency vs Temperature - "X"
20102327
Oscillator Frequency vs Temperature - "Y"
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Current Limit vs Temperature
VIN = 5V
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Current Limit vs Temperature
VIN = 20V
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VFB vs Temperature
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RDSON vs Temperature
20102330
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LM2734
IQ Switching vs Temperature
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Line Regulation - "X"
VOUT = 1.5V, IOUT = 500mA
20102356
Line Regulation - "Y"
VOUT = 1.5V, IOUT = 500mA
20102354
Line Regulation - "X"
VOUT = 3.3V, IOUT = 500mA
20102355
Line Regulation - "Y"
VOUT = 3.3V, IOUT = 500mA
20102353
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LM2734
Block Diagram
20102306
FIGURE 1.
Application Information
THEORY OF OPERATION
The LM2734 is a constant frequency PWM buck regulator IC
that delivers a 1A load current. The regulator has a preset
switching frequency of either 550kHz (LM2734Y) or 1.6MHz
(LM2734X). These high frequencies allow the LM2734 to op-
erate with small surface mount capacitors and inductors,
resulting in DC/DC converters that require a minimum amount
of board space. The LM2734 is internally compensated, so it
is simple to use, and requires few external components. The
LM2734 uses current-mode control to regulate the output
voltage.
The following operating description of the LM2734 will refer
to the Simplified Block Diagram (Figure 1) and to the wave-
forms in Figure 2. The LM2734 supplies a regulated output
voltage by switching the internal NMOS control switch at con-
stant frequency and variable duty cycle. A switching cycle
begins at the falling edge of the reset pulse generated by the
internal oscillator. When this pulse goes low, the output con-
trol logic turns on the internal NMOS control switch. During
this on-time, the SW pin voltage (VSW) swings up to approxi-
mately VIN, and the inductor current (IL) increases with a linear
slope. IL is measured by the current-sense amplifier, which
generates an output proportional to the switch current. The
sense signal is summed with the regulator’s corrective ramp
and compared to the error amplifier’s output, which is propor-
tional to the difference between the feedback voltage and
VREF. When the PWM comparator output goes high, the out-
put switch turns off until the next switching cycle begins.
During the switch off-time, inductor current discharges
through Schottky diode D1, which forces the SW pin to swing
below ground by the forward voltage (VD) of the catch diode.
The regulator loop adjusts the duty cycle (D) to maintain a
constant output voltage.
20102307
FIGURE 2. LM2734 Waveforms of SW Pin Voltage and
Inductor Current
BOOST FUNCTION
Capacitor CBOOST and diode D2 in Figure 3 are used to gen-
erate a voltage VBOOST. VBOOST - VSW is the gate drive voltage
to the internal NMOS control switch. To properly drive the in-
ternal NMOS switch during its on-time, VBOOST needs to be at
least 1.6V greater than VSW. Although the LM2734 will oper-
ate with this minimum voltage, it may not have sufficient gate
drive to supply large values of output current. Therefore, it is
recommended that VBOOST be greater than 2.5V above VSW
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LM2734
for best efficiency. VBOOST – VSW should not exceed the max-
imum operating limit of 5.5V.
5.5V > VBOOST – VSW > 2.5V for best performance.
20102308
FIGURE 3. VOUT Charges CBOOST
When the LM2734 starts up, internal circuitry from the
BOOST pin supplies a maximum of 20mA to CBOOST. This
current charges CBOOST to a voltage sufficient to turn the
switch on. The BOOST pin will continue to source current to
CBOOST until the voltage at the feedback pin is greater than
0.76V.
There are various methods to derive VBOOST:
1. From the input voltage (VIN)
2. From the output voltage (VOUT)
3. From an external distributed voltage rail (VEXT)
4. From a shunt or series zener diode
In the Simplifed Block Diagram of Figure 1, capacitor
CBOOST and diode D2 supply the gate-drive current for the
NMOS switch. Capacitor CBOOST is charged via diode D2 by
VIN. During a normal switching cycle, when the internal NMOS
control switch is off (TOFF) (refer to Figure 2), VBOOST equals
VIN minus the forward voltage of D2 (VFD2), during which the
current in the inductor (L) forward biases the Schottky diode
D1 (VFD1). Therefore the voltage stored across CBOOST is
VBOOST - VSW = VIN - VFD2 + VFD1
When the NMOS switch turns on (TON), the switch pin rises
to
VSW = VIN – (RDSON x IL),
forcing VBOOST to rise thus reverse biasing D2. The voltage at
VBOOST is then
VBOOST = 2VIN – (RDSON x IL) – VFD2 + VFD1
which is approximately
2VIN - 0.4V
for many applications. Thus the gate-drive voltage of the
NMOS switch is approximately
VIN - 0.2V
An alternate method for charging CBOOST is to connect D2 to
the output as shown in Figure 3. The output voltage should
be between 2.5V and 5.5V, so that proper gate voltage will be
applied to the internal switch. In this circuit, CBOOST provides
a gate drive voltage that is slightly less than VOUT.
In applications where both VIN and VOUT are greater than
5.5V, or less than 3V, CBOOST cannot be charged directly from
these voltages. If VIN and VOUT are greater than 5.5V,
CBOOST can be charged from VIN or VOUT minus a zener volt-
age by placing a zener diode D3 in series with D2, as shown
in Figure 4. When using a series zener diode from the input,
ensure that the regulation of the input supply doesn’t create
a voltage that falls outside the recommended VBOOST voltage.
(VINMAX – VD3) < 5.5V
(VINMIN – VD3) > 1.6V
20102309
FIGURE 4. Zener Reduces Boost Voltage from VIN
An alternative method is to place the zener diode D3 in a
shunt configuration as shown in Figure 5. A small 350mW to
500mW 5.1V zener in a SOT-23 or SOD package can be used
for this purpose. A small ceramic capacitor such as a 6.3V,
0.1µF capacitor (C4) should be placed in parallel with the
zener diode. When the internal NMOS switch turns on, a pulse
of current is drawn to charge the internal NMOS gate capac-
itance. The 0.1 µF parallel shunt capacitor ensures that the
VBOOST voltage is maintained during this time.
Resistor R3 should be chosen to provide enough RMS current
to the zener diode (D3) and to the BOOST pin. A recom-
mended choice for the zener current (IZENER) is 1 mA. The
current IBOOST into the BOOST pin supplies the gate current
of the NMOS control switch and varies typically according to
the following formula for the X version:
IBOOST = 0.56 x (D + 0.54) x (VZENER – VD2) mA
IBOOST can be calculated for the Y version using the following:
IBOOST = 0.22 x (D + 0.54) x (VZENER - VD2) µA
where D is the duty cycle, VZENER and VD2 are in volts, and
IBOOST is in milliamps. VZENER is the voltage applied to the
anode of the boost diode (D2), and VD2 is the average forward
voltage across D2. Note that this formula for IBOOST gives typ-
ical current. For the worst case IBOOST, increase the current
by 40%. In that case, the worst case boost current will be
IBOOST-MAX = 1.4 x IBOOST
R3 will then be given by
R3 = (VIN - VZENER) / (1.4 x IBOOST + IZENER)
For example, using the X-version let VIN = 10V, VZENER = 5V,
VD2 = 0.7V, IZENER = 1mA, and duty cycle D = 50%. Then
IBOOST = 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA
R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11kΩ
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LM2734
20102348
FIGURE 5. Boost Voltage Supplied from the Shunt Zener
on VIN
ENABLE PIN / SHUTDOWN MODE
The LM2734 has a shutdown mode that is controlled by the
enable pin (EN). When a logic low voltage is applied to EN,
the part is in shutdown mode and its quiescent current drops
to typically 30nA. Switch leakage adds another 40nA from the
input supply. The voltage at this pin should never exceed
VIN + 0.3V.
SOFT-START
This function forces VOUT to increase at a controlled rate dur-
ing start up. During soft-start, the error amplifier’s reference
voltage ramps from 0V to its nominal value of 0.8V in approx-
imately 200µs. This forces the regulator output to ramp up in
a more linear and controlled fashion, which helps reduce in-
rush current. Under some circumstances at start-up, an out-
put voltage overshoot may still be observed. This may be due
to a large output load applied during start up. Large amounts
of output external capacitance can also increase output volt-
age overshoot. A simple solution is to add a feed forward
capacitor with a value between 470pf and 1000pf across the
top feedback resistor (R1). See Figure 7 for further detail.
OUTPUT OVERVOLTAGE PROTECTION
The overvoltage comparator compares the FB pin voltage to
a voltage that is 10% higher than the internal reference Vref.
Once the FB pin voltage goes 10% above the internal refer-
ence, the internal NMOS control switch is turned off, which
allows the output voltage to decrease toward regulation.
UNDERVOLTAGE LOCKOUT
Undervoltage lockout (UVLO) prevents the LM2734 from op-
erating until the input voltage exceeds 2.74V(typ).
The UVLO threshold has approximately 440mV of hysteresis,
so the part will operate until VIN drops below 2.3V(typ). Hys-
teresis prevents the part from turning off during power up if
VIN is non-monotonic.
CURRENT LIMIT
The LM2734 uses cycle-by-cycle current limiting to protect
the output switch. During each switching cycle, a current limit
comparator detects if the output switch current exceeds 1.7A
(typ), and turns off the switch until the next switching cycle
begins.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the output switch when the IC junction temperature exceeds
165°C. After thermal shutdown occurs, the output switch
doesn’t turn on until the junction temperature drops to ap-
proximately 150°C.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (VO) to input voltage (VIN):
The catch diode (D1) forward voltage drop and the voltage
drop across the internal NMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
VSW can be approximated by:
VSW = IO x RDS(ON)
The diode forward drop (VD) can range from 0.3V to 0.7V de-
pending on the quality of the diode. The lower VD is, the higher
the operating efficiency of the converter.
The inductor value determines the output ripple current. Low-
er inductor values decrease the size of the inductor, but
increase the output ripple current. An increase in the inductor
value will decrease the output ripple current. The ratio of ripple
current (ΔiL) to output current (IO) is optimized when it is set
between 0.3 and 0.4 at 1A. The ratio r is defined as:
One must also ensure that the minimum current limit (1.2A)
is not exceeded, so the peak current in the inductor must be
calculated. The peak current (ILPK) in the inductor is calculated
by:
ILPK = IO + ΔIL/2
If r = 0.5 at an output of 1A, the peak current in the inductor
will be 1.25A. The minimum guaranteed current limit over all
operating conditions is 1.2A. One can either reduce r to 0.4
resulting in a 1.2A peak current, or make the engineering
judgement that 50mA over will be safe enough with a 1.7A
typical current limit and 6 sigma limits. When the designed
maximum output current is reduced, the ratio r can be in-
creased. At a current of 0.1A, r can be made as high as 0.9.
The ripple ratio can be increased at lighter loads because the
net ripple is actually quite low, and if r remains constant the
inductor value can be made quite large. An equation empiri-
cally developed for the maximum ripple ratio at any current
below 2A is:
r = 0.387 x IOUT-0.3667
Note that this is just a guideline.
The LM2734 operates at frequencies allowing the use of ce-
ramic output capacitors without compromising transient re-
sponse. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
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LM2734
capacitor section for more details on calculating output volt-
age ripple.
Now that the ripple current or ripple ratio is determined, the
inductance is calculated by:
where fs is the switching frequency and IO is the output cur-
rent. When selecting an inductor, make sure that it is capable
of supporting the peak output current without saturating. In-
ductor saturation will result in a sudden reduction in induc-
tance and prevent the regulator from operating correctly.
Because of the speed of the internal current limit, the peak
current of the inductor need only be specified for the required
maximum output current. For example, if the designed maxi-
mum output current is 0.5A and the peak current is 0.7A, then
the inductor should be specified with a saturation current limit
of >0.7A. There is no need to specify the saturation or peak
current of the inductor at the 1.7A typical switch current limit.
The difference in inductor size is a factor of 5. Because of the
operating frequency of the LM2734, ferrite based inductors
are preferred to minimize core losses. This presents little re-
striction since the variety of ferrite based inductors is huge.
Lastly, inductors with lower series resistance (DCR) will pro-
vide better operating efficiency. For recommended inductors
see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that VIN does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage,
RMS current rating, and ESL (Equivalent Series Inductance).
The recommended input capacitance is 10µF, although 4.7µF
works well for input voltages below 6V. The input voltage rat-
ing is specifically stated by the capacitor manufacturer. Make
sure to check any recommended deratings and also verify if
there is any significant change in capacitance at the operating
input voltage and the operating temperature. The input ca-
pacitor maximum RMS input current rating (IRMS-IN) must be
greater than:
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle, D, is closest to 0.5.
The ESL of an input capacitor is usually determined by the
effective cross sectional area of the current path. A large
leaded capacitor will have high ESL and a 0805 ceramic chip
capacitor will have very low ESL. At the operating frequencies
of the LM2734, certain capacitors may have an ESL so large
that the resulting impedance (2πfL) will be higher than that
required to provide stable operation. As a result, surface
mount capacitors are strongly recommended. Sanyo
POSCAP, Tantalum or Niobium, Panasonic SP or Cornell
Dubilier ESR, and multilayer ceramic capacitors (MLCC) are
all good choices for both input and output capacitors and have
very low ESL. For MLCCs it is recommended to use X7R or
X5R dielectrics. Consult capacitor manufacturer datasheet to
see how rated capacitance varies over operating conditions.
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired out-
put ripple and transient response. The initial current of a load
transient is provided mainly by the output capacitor. The out-
put ripple of the converter is:
When using MLCCs, the ESR is typically so low that the ca-
pacitive ripple may dominate. When this occurs, the output
ripple will be approximately sinusoidal and 90° phase shifted
from the switching action. Given the availability and quality of
MLCCs and the expected output voltage of designs using the
LM2734, there is really no need to review any other capacitor
technologies. Another benefit of ceramic capacitors is their
ability to bypass high frequency noise. A certain amount of
switching edge noise will couple through parasitic capaci-
tances in the inductor to the output. A ceramic capacitor will
bypass this noise while a tantalum will not. Since the output
capacitor is one of the two external components that control
the stability of the regulator control loop, most applications will
require a minimum at 10 µF of output capacitance. Capaci-
tance can be increased significantly with little detriment to the
regulator stability. Like the input capacitor, recommended
multilayer ceramic capacitors are X7R or X5R. Again, verify
actual capacitance at the desired operating voltage and tem-
perature.
Check the RMS current rating of the capacitor. The RMS cur-
rent rating of the capacitor chosen must also meet the follow-
ing condition:
CATCH DIODE
The catch diode (D1) conducts during the switch off-time. A
Schottky diode is recommended for its fast switching times
and low forward voltage drop. The catch diode should be
chosen so that its current rating is greater than:
ID1 = IO x (1-D)
The reverse breakdown rating of the diode must be at least
the maximum input voltage plus appropriate margin. To im-
prove efficiency choose a Schottky diode with a low forward
voltage drop.
BOOST DIODE
A standard diode such as the 1N4148 type is recommended.
For VBOOST circuits derived from voltages less than 3.3V, a
small-signal Schottky diode is recommended for greater effi-
ciency. A good choice is the BAT54 small signal diode.
BOOST CAPACITOR
A ceramic 0.01µF capacitor with a voltage rating of at least
6.3V is sufficient. The X7R and X5R MLCCs provide the best
performance.
OUTPUT VOLTAGE
The output voltage is set using the following equation where
R2 is connected between the FB pin and GND, and R1 is
connected between VO and the FB pin. A good value for R2
is 10kΩ.
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LM2734
PCB Layout Considerations
When planning layout there are a few things to consider when
trying to achieve a clean, regulated output. The most impor-
tant consideration when completing the layout is the close
coupling of the GND connections of the CIN capacitor and the
catch diode D1. These ground ends should be close to one
another and be connected to the GND plane with at least two
through-holes. Place these components as close to the IC as
possible. Next in importance is the location of the GND con-
nection of the COUT capacitor, which should be near the GND
connections of CIN and D1.
There should be a continuous ground plane on the bottom
layer of a two-layer board except under the switching node
island.
The FB pin is a high impedance node and care should be
taken to make the FB trace short to avoid noise pickup and
inaccurate regulation. The feedback resistors should be
placed as close as possible to the IC, with the GND of R2
placed as close as possible to the GND of the IC. The VOUT
trace to R1 should be routed away from the inductor and any
other traces that are switching.
High AC currents flow through the VIN, SW and VOUT traces,
so they should be as short and wide as possible. However,
making the traces wide increases radiated noise, so the de-
signer must make this trade-off. Radiated noise can be de-
creased by choosing a shielded inductor.
The remaining components should also be placed as close
as possible to the IC. Please see Application Note AN-1229
for further considerations and the LM2734 demo board as an
example of a four-layer layout.
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LM2734
LM2734X Circuit Examples
20102342
FIGURE 6. LM2734X (1.6MHz)
VBOOST Derived from VIN
5V to 1.5V/1A
Bill of Materials for Figure 6
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734X National Semiconductor
C1, Input Cap 10µF, 6.3V, X5R C3216X5ROJ106M TDK
C2, Output Cap 10µF, 6.3V, X5R C3216X5ROJ106M TDK
C3, Boost Cap 0.01uF, 16V, X7R C1005X7R1C103K TDK
D1, Catch Diode 0.3VF Schottky 1A, 10VR MBRM110L ON Semi
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
L1 4.7µH, 1.7A, VLCF4020T- 4R7N1R2 TDK
R1 8.87kΩ, 1% CRCW06038871F Vishay
R2 10.2kΩ, 1% CRCW06031022F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
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LM2734
20102343
FIGURE 7. LM2734X (1.6MHz)
VBOOST Derived from VOUT
12V to 3.3V/1A
Bill of Materials for Figure 7
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator NSC LM2734X
C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK
C2, Output Cap 22µF, 6.3V, X5R C3216X5ROJ226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
CFF 1000pF 25V C0603X5R1E102K TDK
D1, Catch Diode 0.34VF Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
L1 4.7µH, 1.7A VLCF4020T- 4R7N1R2 TDK
R1 31.6kΩ, 1% CRCW06033162F Vishay
R2 10kΩ, 1% CRCW06031002F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
13 www.national.com
LM2734
20102344
FIGURE 8. LM2734X (1.6MHz)
VBOOST Derived from VSHUNT
18V to 1.5V/1A
Bill of Materials for Figure 8
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734X National Semiconductor
C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK
C2, Output Cap 22µF, 6.3V, X5R C3216X5ROJ226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
C4, Shunt Cap 0.1µF, 6.3V, X5R C1005X5R0J104K TDK
D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
D3, Zener Diode 5.1V 250Mw SOT-23 BZX84C5V1 Vishay
L1 6.8µH, 1.6A, SLF7032T-6R8M1R6 TDK
R1 8.87kΩ, 1% CRCW06038871F Vishay
R2 10.2kΩ, 1% CRCW06031022F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
R4 4.12kΩ, 1% CRCW06034121F Vishay
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LM2734
20102349
FIGURE 9. LM2734X (1.6MHz)
VBOOST Derived from Series Zener Diode (VIN)
15V to 1.5V/1A
Bill of Materials for Figure 9
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734X National Semiconductor
C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK
C2, Output Cap 22µF, 6.3V, X5R C3216X5ROJ226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
D3, Zener Diode 11V 350Mw SOT-23 BZX84C11T Diodes, Inc.
L1 6.8µH, 1.6A, SLF7032T-6R8M1R6 TDK
R1 8.87kΩ, 1% CRCW06038871F Vishay
R2 10.2kΩ, 1% CRCW06031022F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
15 www.national.com
LM2734
20102350
FIGURE 10. LM2734X (1.6MHz)
VBOOST Derived from Series Zener Diode (VOUT)
15V to 9V/1A
Bill of Materials for Figure 10
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734X National Semiconductor
C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK
C2, Output Cap 22µF, 16V, X5R C3216X5R1C226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
D3, Zener Diode 4.3V 350mw SOT-23 BZX84C4V3 Diodes, Inc.
L1 6.8µH, 1.6A, SLF7032T-6R8M1R6 TDK
R1 102kΩ, 1% CRCW06031023F Vishay
R2 10.2kΩ, 1% CRCW06031022F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
www.national.com 16
LM2734
LM2734Y Circuit Examples
20102342
FIGURE 11. LM2734Y (550kHz)
VBOOST Derived from VIN
5V to 1.5V/1A
Bill of Materials for Figure 11
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734Y National Semiconductor
C1, Input Cap 10µF, 6.3V, X5R C3216X5ROJ106M TDK
C2, Output Cap 22µF, 6.3V, X5R C3216X5ROJ226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
D1, Catch Diode 0.3VF Schottky 1A, 10VR MBRM110L ON Semi
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
L1 10µH, 1.6A, SLF7032T-100M1R4 TDK
R1 8.87kΩ, 1% CRCW06038871F Vishay
R2 10.2kΩ, 1% CRCW06031022F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
17 www.national.com
LM2734
20102343
FIGURE 12. LM2734Y (550kHz)
VBOOST Derived from VOUT
12V to 3.3V/1A
Bill of Materials for Figure 12
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734Y National Semiconductor
C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK
C2, Output Cap 22µF, 6.3V, X5R C3216X5ROJ226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
D1, Catch Diode 0.34VF Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 0.6VF @ 30mA Diode BAT17 Vishay
L1 10µH, 1.6A, SLF7032T-100M1R4 TDK
R1 31.6kΩ, 1% CRCW06033162F Vishay
R2 10.0 kΩ, 1% CRCW06031002F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
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LM2734
20102344
FIGURE 13. LM2734Y (550kHz)
VBOOST Derived from VSHUNT
18V to 1.5V/1A
Bill of Materials for Figure 13
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734Y National Semiconductor
C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK
C2, Output Cap 22µF, 6.3V, X5R C3216X5ROJ226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
C4, Shunt Cap 0.1µF, 6.3V, X5R C1005X5R0J104K TDK
D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
D3, Zener Diode 5.1V 250Mw SOT-23 BZX84C5V1 Vishay
L1 15µH, 1.5A SLF7045T-150M1R5 TDK
R1 8.87kΩ, 1% CRCW06038871F Vishay
R2 10.2kΩ, 1% CRCW06031022F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
R4 4.12kΩ, 1% CRCW06034121F Vishay
19 www.national.com
LM2734
20102349
FIGURE 14. LM2734Y (550kHz)
VBOOST Derived from Series Zener Diode (VIN)
15V to 1.5V/1A
Bill of Materials for Figure 14
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734Y National Semiconductor
C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK
C2, Output Cap 22µF, 6.3V, X5R C3216X5ROJ226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
D3, Zener Diode 11V 350Mw SOT-23 BZX84C11T Diodes, Inc.
L1 15µH, 1.5A, SLF7045T-150M1R5 TDK
R1 8.87kΩ, 1% CRCW06038871F Vishay
R2 10.2kΩ, 1% CRCW06031022F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
www.national.com 20
LM2734
20102350
FIGURE 15. LM2734Y (550kHz)
VBOOST Derived from Series Zener Diode (VOUT)
15V to 9V/1A
Bill of Materials for Figure 15
Part ID Part Value Part Number Manufacturer
U1 1A Buck Regulator LM2734Y National Semiconductor
C1, Input Cap 10µF, 25V, X7R C3225X7R1E106M TDK
C2, Output Cap 22µF, 16V, X5R C3216X5R1C226M TDK
C3, Boost Cap 0.01µF, 16V, X7R C1005X7R1C103K TDK
D1, Catch Diode 0.4VF Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1VF @ 50mA Diode 1N4148W Diodes, Inc.
D3, Zener Diode 4.3V 350mw SOT-23 BZX84C4V3 Diodes, Inc.
L1 22µH, 1.4A, SLF7045T-220M1R3-1PF TDK
R1 102kΩ, 1% CRCW06031023F Vishay
R2 10.2kΩ, 1% CRCW06031022F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
21 www.national.com
LM2734
Physical Dimensions inches (millimeters) unless otherwise noted
6-Lead TSOT Package
NS Package Number MK06A
www.national.com 22
LM2734
Notes
23 www.national.com
LM2734
Notes
LM2734 Thin SOT23 1A Load Step-Down DC-DC Regulator
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