LM2734
Thin SOT23 1A Load Step-Down DC-DC Regulator
General Description
Features
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 regulation 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 results 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 internal 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.
n
n
n
n
n
n
n
n
n
n
n
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
Applications
n
n
n
n
n
n
n
Local Point of Load Regulation
Core Power in HDDs
Set-Top Boxes
Battery Powered Devices
USB Powered Devices
DSL Modems
Notebook Computers
Typical Application Circuit
Efficiency vs Load Current
VIN = 5V, VOUT = 3.3V
20102301
20102345
WEBENCH™ is a trademark of Transim.
© 2004 National Semiconductor Corporation
DS201023
www.national.com
LM2734 Thin SOT23 1A Load Step-Down DC-DC Regulator
October 2004
LM2734
Connection Diagram
20102305
6-Lead TSOT
NS Package Number MK06A
Ordering Information
Order Number
Package Type
NSC Package Drawing
Package Marking
LM2734XMK
LM2734YMK
LM2734XMKX
TSOT-6
MK06A
LM2734YMKX
Supplied As
SFDB
1000 Units on Tape and Reel
SFEB
1000 Units on Tape and Reel
SFDB
3000 Units on Tape and Reel
SFEB
3000 Units on Tape and Reel
* Contact the local sales office for the lead-free package.
Pin Description
Pin
Name
1
BOOST
Boost voltage that drives the internal NMOS control switch. A
bootstrap capacitor is connected between the BOOST and SW
pins.
Function
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.
www.national.com
2
Soldering Information
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Infrared/Convection Reflow (15sec)
220˚C
Wave Soldering Lead Temp. (10sec)
260˚C
VIN
-0.5V to 24V
SW Voltage
-0.5V to 24V
Boost Voltage
-0.5V to 30V
VIN
Boost to SW Voltage
-0.5V to 6.0V
SW Voltage
-0.5V to 20V
-0.5V to 3.0V
Boost Voltage
-0.5V to 25V
Boost to SW Voltage
1.6V to 5.5V
FB Voltage
EN Voltage
Operating Ratings (Note 1)
-0.5V to (VIN + 0.3V)
Junction Temperature
Junction Temperature Range
150˚C
ESD Susceptibility (Note 2)
Storage Temp. Range
3V to 20V
−40˚C to +125˚C
Thermal Resistance θJA (Note 3)
2kV
118˚C/W
-65˚C to 150˚C
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
VFB
∆VFB/∆VIN
IFB
UVLO
Parameter
Conditions
Feedback Voltage
Feedback Voltage Line
Regulation
VIN = 3V to 20V
Feedback Input Bias Current
Sink/Source
Undervoltage Lockout
VIN Rising
Undervoltage Lockout
VIN Falling
Switching Frequency
DMAX
Maximum Duty Cycle
DMIN
Minimum Duty Cycle
RDS(ON)
Typ
(Note 5)
Max
(Note 4)
Units
0.784
0.800
0.816
V
0.01
UVLO Hysteresis
FSW
Min
(Note 4)
%/V
10
250
2.74
2.90
2.0
2.3
0.30
0.44
V
0.62
LM2734X
1.2
1.6
1.9
LM2734Y
0.40
0.55
0.66
LM2734X
85
92
LM2734Y
90
96
LM2734X
2
LM2734Y
1
%
VBOOST - VSW = 3V
ICL
Switch Current Limit
VBOOST - VSW = 3V
IQ
Quiescent Current
Switching
Quiescent Current (shutdown)
VEN = 0V
30
LM2734X (50% Duty Cycle)
2.5
3.5
LM2734Y (50% Duty Cycle)
1.0
1.8
IBOOST
Boost Pin Current
Shutdown Threshold Voltage
VEN Falling
Enable Threshold Voltage
VEN Rising
IEN
Enable Pin Current
Sink/Source
ISW
Switch Leakage
VEN_TH
MHz
%
Switch ON Resistance
1.2
nA
300
600
1.7
2.5
A
1.5
2.5
mA
nA
0.4
1.8
mΩ
mA
V
10
nA
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.
3
www.national.com
LM2734
Absolute Maximum Ratings (Note 1)
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
Efficiency vs Load Current - "Y" VOUT = 5V
20102336
20102334
Efficiency vs Load Current - "X" VOUT = 3.3V
Efficiency vs Load Current - "Y" VOUT = 3.3V
20102351
20102352
Efficiency vs Load Current - "X" VOUT = 1.5V
Efficiency vs Load Current - "Y" VOUT = 1.5V
20102337
www.national.com
20102335
4
Oscillator Frequency vs Temperature - "X"
Oscillator Frequency vs Temperature - "Y"
20102327
20102328
Current Limit vs Temperature
VIN = 20V
Current Limit vs Temperature
VIN = 5V
20102329
20102347
VFB vs Temperature
RDSON vs Temperature
20102333
20102330
5
www.national.com
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. (Continued)
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. (Continued)
Line Regulation - "X"
VOUT = 1.5V, IOUT = 500mA
IQ Switching vs Temperature
20102346
20102356
Line Regulation - "X"
VOUT = 3.3V, IOUT = 500mA
Line Regulation - "Y"
VOUT = 1.5V, IOUT = 500mA
20102354
20102355
Line Regulation - "Y"
VOUT = 3.3V, IOUT = 500mA
20102353
www.national.com
6
LM2734
Block Diagram
20102306
FIGURE 1.
forward voltage (VD) of the catch diode. The regulator loop
adjusts the duty cycle (D) to maintain a constant output
voltage.
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 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 waveforms 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 (VSW) swings
up to approximately VIN, and the inductor current (IL) increases with a linear slope. IL is measured by the currentsense 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 proportional to the difference between the feedback voltage and VREF. 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
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
generate a voltage VBOOST. VBOOST - VSW is the gate drive
voltage to the internal NMOS control switch. To properly
drive the internal NMOS switch during its on-time, VBOOST
needs to be at least 1.6V greater than VSW. Although the
LM2734 will operate with this minimum voltage, it may not
have sufficient gate drive to supply large values of output
7
www.national.com
LM2734
Application Information
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.
(Continued)
current. Therefore, it is recommended that VBOOST be
greater than 2.5V above VSW for best efficiency. VBOOST –
VSW should not exceed the maximum operating limit of 5.5V.
(VINMAX – VD3) < 5.5V
(VINMIN – VD3) > 1.6V
5.5V > VBOOST – VSW > 2.5V for best performance.
20102308
20102309
FIGURE 3. VOUT Charges CBOOST
FIGURE 4. Zener Reduces Boost Voltage from VIN
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
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 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
recommended 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:
IBOOST = 0.56 x (D + 0.54) x (VZENER – VD2) mA
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 typical 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, 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
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
voltage by placing a zener diode D3 in series with D2, as
www.national.com
8
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
approximately 150˚C.
(Continued)
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (VO) to input voltage (VIN):
20102348
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:
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.
VSW can be approximated by:
VSW = IO x RDS(ON)
The diode forward drop (VD) can range from 0.3V to 0.7V
depending on the quality of the diode. The lower VD is, the
higher the operating efficiency of the converter.
SOFT-START
This function forces VOUT to increase at a controlled rate
during start up. During soft-start, the error amplifier’s reference 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.
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 (∆iL) to output current (IO) is optimized when it
is set between 0.3 and 0.4 at 1A. The ratio r is defined as:
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 reference, the internal NMOS control switch is turned off, which
allows the output voltage to decrease toward regulation.
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 + ∆IL/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 increased. 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 IOUT-0.3667
Note that this is just a guideline.
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 hysteresis, so the part will operate until VIN drops below 2.3V(typ).
Hysteresis 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.
9
www.national.com
LM2734
Application Information
LM2734
Design Guide
pacitors 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.
(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 voltage ripple.
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:
Now that the ripple current or ripple ratio is determined, the
inductance is calculated by:
When using MLCCs, the ESR is typically so low that the
capacitive 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
capacitances 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. 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 operating 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:
where fs is the switching frequency and IO is the output
current. When selecting an inductor, make sure that it is
capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in
inductance 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 maximum 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 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 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 rating 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 capacitor maximum RMS input current rating (IRMS-IN) must be greater than:
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 improve efficiency choose a Schottky diode with a low forward
voltage drop.
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 cawww.national.com
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 efficiency. 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.
10
GND connection 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.
(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 VO and the FB pin. A good value for R2
is 10kΩ.
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
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.
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 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
11
www.national.com
LM2734
Design Guide
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
www.national.com
12
LM2734
LM2734X Circuit Examples
(Continued)
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
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
0.34VF Schottky 1A, 30VR
SS1P3L
Vishay
D2, Boost Diode
0.6VF @ 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
13
Manufacturer
www.national.com
LM2734
LM2734X Circuit Examples
(Continued)
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
www.national.com
14
LM2734
LM2734X Circuit Examples
(Continued)
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
LM2734X Circuit Examples
(Continued)
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
LM2734Y Circuit Examples
(Continued)
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
www.national.com
18
LM2734
LM2734Y Circuit Examples
(Continued)
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
LM2734Y Circuit Examples
(Continued)
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
LM2734Y Circuit Examples
(Continued)
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 Thin SOT23 1A Load Step-Down DC-DC Regulator
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.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship
Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned
Substances’’ as defined in CSP-9-111S2.
National Semiconductor
Americas Customer
Support Center
Email: [email protected]
Tel: 1-800-272-9959
www.national.com
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: [email protected]
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
Asia Pacific Customer
Support Center
Email: [email protected]
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: [email protected]
Tel: 81-3-5639-7560