LM2734
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 regu-
lation 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 300mΩNMOS
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
supporting exceptionally high frequency conversion over the
entire 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
capacitors. 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 in-
ternal compensation to provide high-performance regulation
over a wide range of operating conditions. Additional fea-
tures include internal soft-start circuitry to reduce inrush
current, pulse-by-pulse current limit, thermal shutdown, and
output over-voltage protection.
Features
nThin SOT23-6 package
n3.0V to 20V input voltage range
n0.8V to 18V output voltage range
n1A output current
n550kHz (LM2734Y) and 1.6MHz (LM2734X)
switching frequencies
n300mΩNMOS switch
n30nA shutdown current
n0.8V, 2% internal voltage reference
nInternal soft-start
nCurrent-Mode, PWM operation
nWEBENCH®online design tool
nThermal shutdown
Applications
nLocal Point of Load Regulation
nCore Power in HDDs
nSet-Top Boxes
nBattery Powered Devices
nUSB Powered Devices
nDSL Modems
nNotebook Computers
Typical Application Circuit
Efficiency vs Load Current
V
IN
= 5V, V
OUT
= 3.3V
20102301
20102345
WEBENCH™is a trademark of Transim.
January 2005
LM2734 Thin SOT23 1A Load Step-Down DC-DC Regulator
© 2005 National Semiconductor Corporation DS201023 www.national.com
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
LM2734XMK
TSOT-6 MK06A
SFDB 1000 Units on Tape and Reel
LM2734YMK SFEB 1000 Units on Tape and Reel
LM2734XMKX SFDB 3000 Units on Tape and Reel
LM2734YMKX SFEB 3000 Units on Tape and Reel
* Contact the local sales office for the lead-free package.
Pin Description
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 V
IN
+ 0.3V.
5V
IN
Input supply voltage. Connect a bypass capacitor to this pin.
6 SW Output switch. Connects to the inductor, catch diode, and
bootstrap capacitor.
LM2734
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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
-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 (V
IN
+ 0.3V)
Junction Temperature 150˚C
ESD Susceptibility (Note 2) 2kV
Storage Temp. Range -65˚C to 150˚C
Soldering Information
Infrared/Convection Reflow (15sec) 220˚C
Wave Soldering Lead Temp. (10sec) 260˚C
Operating Ratings (Note 1)
V
IN
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 θ
JA
(Note 3) 118˚C/W
Electrical Characteristics
Specifications with standard typeface are for T
J
= 25˚C, and those in boldface type apply over the full Operating Tempera-
ture Range (T
J
= -40˚C to 125˚C). V
IN
= 5V, V
BOOST
-V
SW
= 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
V
FB
Feedback Voltage 0.784 0.800 0.816 V
∆V
FB
/∆V
IN
Feedback Voltage Line
Regulation
V
IN
=3Vto20V 0.01 % / V
I
FB
Feedback Input Bias Current Sink/Source 10 250 nA
UVLO
Undervoltage Lockout V
IN
Rising 2.74 2.90
VUndervoltage Lockout V
IN
Falling 2.0 2.3
UVLO Hysteresis 0.30 0.44 0.62
F
SW
Switching Frequency LM2734X 1.2 1.6 1.9 MHz
LM2734Y 0.40 0.55 0.66
D
MAX
Maximum Duty Cycle LM2734X 85 92 %
LM2734Y 90 96
D
MIN
Minimum Duty Cycle LM2734X 2 %
LM2734Y 1
R
DS(ON)
Switch ON Resistance V
BOOST
-V
SW
= 3V 300 600 mΩ
I
CL
Switch Current Limit V
BOOST
-V
SW
=3V 1.2 1.7 2.5 A
I
Q
Quiescent Current Switching 1.5 2.5 mA
Quiescent Current (shutdown) V
EN
=0V 30 nA
I
BOOST
Boost Pin Current LM2734X (50% Duty Cycle) 2.5 3.5 mA
LM2734Y (50% Duty Cycle) 1.0 1.8
V
EN_TH
Shutdown Threshold Voltage V
EN
Falling 0.4 V
Enable Threshold Voltage V
EN
Rising 1.8
I
EN
Enable Pin Current Sink/Source 10 nA
I
SW
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, 1.5kΩin 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=(T
J(MAX) –T
A)/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.
LM2734
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Typical Performance Characteristics All curves taken at V
IN
= 5V, V
BOOST
-V
SW
= 5V, L1 = 4.7 µH
("X"), L1 = 10 µH ("Y"), and T
A
= 25˚C, unless specified otherwise.
Efficiency vs Load Current - "X" V
OUT
= 5V Efficiency vs Load Current - "Y" V
OUT
=5V
20102336 20102334
Efficiency vs Load Current - "X" V
OUT
= 3.3V Efficiency vs Load Current - "Y" V
OUT
= 3.3V
20102351 20102352
Efficiency vs Load Current - "X" V
OUT
= 1.5V Efficiency vs Load Current - "Y" V
OUT
= 1.5V
20102337 20102335
LM2734
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Typical Performance Characteristics All curves taken at V
IN
= 5V, V
BOOST
-V
SW
= 5V, L1 = 4.7 µH
("X"), L1 = 10 µH ("Y"), and T
A
= 25˚C, unless specified otherwise. (Continued)
Oscillator Frequency vs Temperature - "X" Oscillator Frequency vs Temperature - "Y"
20102327 20102328
Current Limit vs Temperature
V
IN
=5V
Current Limit vs Temperature
V
IN
= 20V
20102329 20102347
V
FB
vs Temperature R
DSON
vs Temperature
20102333 20102330
LM2734
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Typical Performance Characteristics All curves taken at V
IN
= 5V, V
BOOST
-V
SW
= 5V, L1 = 4.7 µH
("X"), L1 = 10 µH ("Y"), and T
A
= 25˚C, unless specified otherwise. (Continued)
I
Q
Switching vs Temperature
Line Regulation - "X"
V
OUT
= 1.5V, I
OUT
= 500mA
20102346 20102356
Line Regulation - "Y"
V
OUT
= 1.5V, I
OUT
= 500mA
Line Regulation - "X"
V
OUT
= 3.3V, I
OUT
= 500mA
20102354 20102355
Line Regulation - "Y"
V
OUT
= 3.3V, I
OUT
= 500mA
20102353
LM2734
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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
operate with small surface mount capacitors and inductors,
resulting in DC/DC converters that require a minimum
amount of board space. The LM2734 is internally compen-
sated, 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
constant 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 control logic turns on the internal NMOS control
switch. During this on-time, the SW pin voltage (V
SW
) swings
up to approximately V
IN
, and the inductor current (I
L
) in-
creases with a linear slope. I
L
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 am-
plifier’s output, which is proportional to the difference be-
tween the feedback voltage and V
REF
. When the PWM
comparator output goes high, the output 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 (V
D
) of the catch diode. The regulator loop
adjusts the duty cycle (D) to maintain a constant output
voltage.
BOOST FUNCTION
Capacitor C
BOOST
and diode D2 in Figure 3 are used to
generate a voltage V
BOOST
.V
BOOST
-V
SW
is the gate drive
voltage to the internal NMOS control switch. To properly
drive the internal NMOS switch during its on-time, V
BOOST
needs to be at least 1.6V greater than V
SW
. Although the
LM2734 will operate with this minimum voltage, it may not
have sufficient gate drive to supply large values of output
Block Diagram
20102306
FIGURE 1.
20102307
FIGURE 2. LM2734 Waveforms of SW Pin Voltage and
Inductor Current
LM2734
www.national.com7
Application Information (Continued)
current. Therefore, it is recommended that V
BOOST
be
greater than 2.5V above V
SW
for best efficiency. V
BOOST
–
V
SW
should not exceed the maximum operating limit of 5.5V.
5.5V >V
BOOST
–V
SW
>2.5V for best performance.
When the LM2734 starts up, internal circuitry from the
BOOST pin supplies a maximum of 20mA to C
BOOST
. This
current charges C
BOOST
to a voltage sufficient to turn the
switch on. The BOOST pin will continue to source current to
C
BOOST
until the voltage at the feedback pin is greater than
0.76V.
There are various methods to derive V
BOOST
:
1. From the input voltage (V
IN
)
2. From the output voltage (V
OUT
)
3. From an external distributed voltage rail (V
EXT
)
4. From a shunt or series zener diode
In the Simplifed Block Diagram of Figure 1, capacitor
C
BOOST
and diode D2 supply the gate-drive current for the
NMOS switch. Capacitor C
BOOST
is charged via diode D2 by
V
IN
. During a normal switching cycle, when the internal
NMOS control switch is off (T
OFF
) (refer to Figure 2), V
BOOST
equals V
IN
minus the forward voltage of D2 (V
FD2
), during
which the current in the inductor (L) forward biases the
Schottky diode D1 (V
FD1
). Therefore the voltage stored
across C
BOOST
is
V
BOOST
-V
SW
=V
IN
-V
FD2
+V
FD1
When the NMOS switch turns on (T
ON
), the switch pin rises
to
V
SW
=V
IN
–(R
DSON
xI
L
),
forcing V
BOOST
to rise thus reverse biasing D2. The voltage
at V
BOOST
is then
V
BOOST
=2V
IN
–(R
DSON
xI
L
)–V
FD2
+V
FD1
which is approximately
2V
IN
- 0.4V
for many applications. Thus the gate-drive voltage of the
NMOS switch is approximately
V
IN
- 0.2V
An alternate method for charging C
BOOST
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, C
BOOST
provides a gate drive voltage that is slightly less than V
OUT
.
In applications where both V
IN
and V
OUT
are greater than
5.5V, or less than 3V, C
BOOST
cannot be charged directly
from these voltages. If V
IN
and V
OUT
are greater than 5.5V,
C
BOOST
can be charged from V
IN
or V
OUT
minus a zener
voltage 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 V
BOOST
voltage.
(V
INMAX
–V
D3
)<5.5V
(V
INMIN
–V
D3
)>1.6V
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
capacitance. The 0.1 µF parallel shunt capacitor ensures
that the V
BOOST
voltage is maintained during this time.
Resistor R3 should be chosen to provide enough RMS cur-
rent to the zener diode (D3) and to the BOOST pin. A
recommended choice for the zener current (I
ZENER
)is1mA.
The current I
BOOST
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:
I
BOOST
= 0.56 x (D + 0.54) x (V
ZENER
–V
D2
)mA
I
BOOST
can be calculated for the Y version using the follow-
ing:
I
BOOST
= 0.22 x (D + 0.54) x (V
ZENER
-V
D2
)µA
where D is the duty cycle, V
ZENER
and V
D2
are in volts, and
I
BOOST
is in milliamps. V
ZENER
is the voltage applied to the
anode of the boost diode (D2), and V
D2
is the average
forward voltage across D2. Note that this formula for I
BOOST
gives typical current. For the worst case I
BOOST
, increase the
current by 40%. In that case, the worst case boost current
will be
I
BOOST-MAX
=1.4xI
BOOST
R3 will then be given by
R3=(V
IN
-V
ZENER
)/(1.4xI
BOOST
+I
ZENER
)
For example, using the X-version let V
IN
= 10V, V
ZENER
=5V,
V
D2
= 0.7V, I
ZENER
= 1mA, and duty cycle D = 50%. Then
I
BOOST
= 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
20102308
FIGURE 3. V
OUT
Charges C
BOOST
20102309
FIGURE 4. Zener Reduces Boost Voltage from V
IN
LM2734
www.national.com 8
Application Information (Continued)
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
V
IN
+ 0.3V.
SOFT-START
This function forces V
OUT
to increase at a controlled rate
during start up. During soft-start, the error amplifier’s refer-
ence voltage ramps from 0V to its nominal value of 0.8V in
approximately 200µs. This forces the regulator output to
ramp up in a more linear and controlled fashion, which helps
reduce inrush current.
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
operating until the input voltage exceeds 2.74V(typ).
The UVLO threshold has approximately 440mV of hyster-
esis, so the part will operate until V
IN
drops below 2.3V(typ).
Hysteresis prevents the part from turning off during power up
if V
IN
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 ex-
ceeds 165˚C. After thermal shutdown occurs, the output
switch doesn’t turn on until the junction temperature drops to
approximately 150˚C.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
O
) to input voltage (V
IN
):
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 follow-
ing formula:
V
SW
can be approximated by:
V
SW
=I
O
xR
DS(ON)
The diode forward drop (V
D
) can range from 0.3V to 0.7V
depending on the quality of the diode. The lower V
D
is, the
higher the operating efficiency of the converter.
The inductor value determines the output ripple current.
Lower 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 (∆i
L
) to output current (I
O
) 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 (I
LPK
) in the inductor is calcu-
lated by:
I
LPK
=I
O
+∆I
L
/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
empirically developed for the maximum ripple ratio at any
current below 2A is:
r = 0.387 x I
OUT-0.3667
Note that this is just a guideline.
20102348
FIGURE 5. Boost Voltage Supplied from the Shunt
Zener on V
IN
LM2734
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Design Guide (Continued)
The LM2734 operates at frequencies allowing the use of
ceramic output capacitors without compromising transient
response. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
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 f
s
is the switching frequency and I
O
is the output
current. When selecting an inductor, make sure that it is
capable of supporting the peak output current without satu-
rating. Inductor saturation will result in a sudden reduction in
inductance and prevent the regulator from operating cor-
rectly. 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 de-
signed maximum output current is 0.5A and the peak current
is 0.7A, then the inductor should be specified with a satura-
tion 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 restriction since the variety of ferrite
based inductors is huge. Lastly, inductors with lower series
resistance (DCR) will provide better operating efficiency. For
recommended inductors see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
IN
does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, volt-
age, RMS current rating, and ESL (Equivalent Series Induc-
tance). The recommended input capacitance is 10µF, al-
though 4.7µF works well for input voltages below 6V. The
input voltage rating is specifically stated by the capacitor
manufacturer. Make sure to check any recommended derat-
ings and also verify if there is any significant change in
capacitance at the operating input voltage and the operating
temperature. The input capacitor maximum RMS input cur-
rent rating (I
RMS-IN
) must be greater than:
It can be shown from the above equation that maximum
RMS capacitor current occurs when D = 0.5. Always calcu-
late 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 ce-
ramic 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 ca-
pacitors and have very low ESL. For MLCCs it is recom-
mended 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
output ripple and transient response. The initial current of a
load transient is provided mainly by the output capacitor. The
output ripple of the converter is:
When using MLCCs, the ESR is typically so low that the
capacitive ripple may dominate. When this occurs, the out-
put 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 capaci-
tors is their ability to bypass high frequency noise. A certain
amount of switching edge noise will couple through parasitic
capacitances in the inductor to the output. A ceramic capaci-
tor 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 appli-
cations will require a minimum at 10 µF of output capaci-
tance. Capacitance 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 operat-
ing voltage and temperature.
Check the RMS current rating of the capacitor. The RMS
current rating of the capacitor chosen must also meet the
following 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:
I
D1
=I
O
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 V
BOOST
circuits derived from voltages less than 3.3V, a
small-signal Schottky diode is recommended for greater ef-
ficiency. 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.
LM2734
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Design Guide (Continued)
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 V
O
and the FB pin. A good value for R2
is 10kΩ.
PCB Layout Considerations
When planning layout there are a few things to consider
when trying to achieve a clean, regulated output. The most
important consideration when completing the layout is the
close coupling of the GND connections of the C
IN
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 connection of the C
OUT
capacitor, which should be
near the GND connections of C
IN
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 V
OUT
trace to R1 should be routed away from the inductor and any
other traces that are switching.
High AC currents flow through the V
IN
, SW and V
OUT
traces,
so they should be as short and wide as possible. However,
making the traces wide increases radiated noise, so the
designer must make this trade-off. Radiated noise can be
decreased 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.
LM2734
www.national.com11
LM2734X Circuit Examples
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 0.3V
F
Schottky 1A, 10VR MBRM110L ON Semi
D2, Boost Diode 1V
F
@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
20102342
FIGURE 6. LM2734X (1.6MHz)
V
BOOST
Derived from V
IN
5V to 1.5V/1A
LM2734
www.national.com 12
LM2734X Circuit Examples (Continued)
Bill of Materials for Figure 7
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 0.34V
F
Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 0.6V
F
@30mA Diode BAT17 Vishay
L1 4.7µH, 1.7A, VLCF4020T- 4R7N1R2 TDK
R1 31.6kΩ, 1% CRCW06033162F Vishay
R2 10.0 kΩ, 1% CRCW06031002F Vishay
R3 100kΩ, 1% CRCW06031003F Vishay
20102343
FIGURE 7. LM2734X (1.6MHz)
V
BOOST
Derived from V
OUT
12V to 3.3V/1A
LM2734
www.national.com13
LM2734X Circuit Examples (Continued)
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 0.4V
F
Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1V
F
@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
20102344
FIGURE 8. LM2734X (1.6MHz)
V
BOOST
Derived from V
SHUNT
18V to 1.5V/1A
LM2734
www.national.com 14
LM2734X Circuit Examples (Continued)
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 0.4V
F
Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1V
F
@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
20102349
FIGURE 9. LM2734X (1.6MHz)
V
BOOST
Derived from Series Zener Diode (V
IN
)
15V to 1.5V/1A
LM2734
www.national.com15
LM2734X Circuit Examples (Continued)
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 0.4V
F
Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1V
F
@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
20102350
FIGURE 10. LM2734X (1.6MHz)
V
BOOST
Derived from Series Zener Diode (V
OUT
)
15V to 9V/1A
LM2734
www.national.com 16
LM2734Y Circuit Examples
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 0.3V
F
Schottky 1A, 10VR MBRM110L ON Semi
D2, Boost Diode 1V
F
@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
20102342
FIGURE 11. LM2734Y (550kHz)
V
BOOST
Derived from V
IN
5V to 1.5V/1A
LM2734
www.national.com17
LM2734Y Circuit Examples (Continued)
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 0.34V
F
Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 0.6V
F
@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
20102343
FIGURE 12. LM2734Y (550kHz)
V
BOOST
Derived from V
OUT
12V to 3.3V/1A
LM2734
www.national.com 18
LM2734Y Circuit Examples (Continued)
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 0.4V
F
Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1V
F
@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
20102344
FIGURE 13. LM2734Y (550kHz)
V
BOOST
Derived from V
SHUNT
18V to 1.5V/1A
LM2734
www.national.com19
LM2734Y Circuit Examples (Continued)
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 0.4V
F
Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1V
F
@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
20102349
FIGURE 14. LM2734Y (550kHz)
V
BOOST
Derived from Series Zener Diode (V
IN
)
15V to 1.5V/1A
LM2734
www.national.com 20
LM2734Y Circuit Examples (Continued)
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 0.4V
F
Schottky 1A, 30VR SS1P3L Vishay
D2, Boost Diode 1V
F
@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
20102350
FIGURE 15. LM2734Y (550kHz)
V
BOOST
Derived from Series Zener Diode (V
OUT
)
15V to 9V/1A
LM2734
www.national.com21
Physical Dimensions inches (millimeters) unless otherwise noted
6-Lead TSOT Package
NS Package Number MK06A
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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LM2734 Thin SOT23 1A Load Step-Down DC-DC Regulator
