LM2852
LM2852 2A 500/1500kHz Synchronous SIMPLE SWITCHER® Buck Regulator
Literature Number: SNVS325C
LM2852
2A 500/1500kHz Synchronous SIMPLE SWITCHER®Buck
Regulator
General Description
The LM2852 SIMPLE SWITCHER®synchronous buck regu-
lator is a high frequency step-down switching voltage regu-
lator capable of driving up to a 2A load with excellent line and
load regulation. The LM2852 can accept an input voltage
between 2.85V and 5.5V and deliver an output voltage that is
factory programmable from 0.8V to 3.3V in 100mV incre-
ments. The LM2852 is available with a choice of two switch-
ing frequencies - 500kHz (LM2852Y) or 1.5MHz (LM2852X).
It also features internal, type-three compensation to deliver a
low component count solution. The exposed-pad TSSOP-14
package enhances the thermal performance of the LM2852.
Features
nInput voltage range of 2.85 to 5.5V
nFactory EEPROM set output voltages from 0.8V to 3.3V
in 100mV increments
nMaximum load current of 2A
nVoltage Mode Control
nInternal type-three compensation
nSwitching frequency of 500kHz or 1.5MHz
nLow standby current of 10µA
nInternal 60 mMOSFET switches
nStandard voltage options 0.8/1.0/1.2/1.5/1.8/2.5/3.3 volts
Applications
nLow voltage point of load regulation
nLocal solution for FPGA/DSP/ASIC core power
nBroadband networking and communications
infrastructure
nPortable computing
Typical Application Circuit
20127001
20127002
SIMPLE SWITCHER®
SIMPLE SWITCHERreg; is a registered trademark of National Semiconductor Corporation
October 2006
LM2852 2A 500/1500kHz Synchronous SIMPLE SWITCHER
®
Buck Regulator
© 2006 National Semiconductor Corporation DS201270 www.national.com
Connection Diagram
TOP VIEW
20127003
MXA14A
ETSSOP-14
Pin Descriptions
AVIN (Pin 1): Chip bias input pin. This provides power to the
logic of the chip. Connect to the input voltage or a separate
rail.
EN (Pin 2): Enable. Connect this pin to ground to disable the
chip; connect to AVIN or leave floating to enable the chip;
enable is internally pulled up.
SGND (Pin 3): Signal ground.
SS (Pin 4): Soft-start pin. Connect this pin to a small capaci-
tor to control startup. The soft-start capacitance range is
restricted to values 1 nF to 50 nF.
NC (Pins 5, 12 and 13): No connect. These pins must be
tied to ground or left floating in the application.
PVIN (Pins 6, 7): Input supply pin. PVIN is connected to the
input voltage. This rail connects to the source of the internal
power PFET.
SW (Pins 8, 9): Switch pin. Connect to the output inductor.
PGND (Pins 10, 11): Power ground. Connect this to an
internal ground plane or other large ground plane.
SNS (Pin 14): Output voltage sense pin. Connect this pin to
the output voltage as close to the load as possible.
Exposed Pad: Connect to ground.
LM2852
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Ordering Information
Order Number Frequency
Voltage
Option Package Type
Package
Drawing Supplied As
LM2852YMXA-0.8
500kHz
0.8
TSSOP-14 exposed
pad MXA14A
94 Units, Rail
LM2852YMXAX-0.8 2500 Units, Tape and Reel
LM2852YMXA-1.0 1.0 94 Units, Rail
LM2852YMXAX-1.0 2500 Units, Tape and Reel
LM2852YMXA-1.2 1.2 94 Units, Rail
LM2852YMXAX-1.2 2500 Units, Tape and Reel
LM2852YMXA-1.5 1.5 94 Units, Rail
LM2852YMXAX-1.5 2500 Units, Tape and Reel
LM2852YMXA-1.8 1.8 94 Units, Rail
LM2852YMXAX-1.8 2500 Units, Tape and Reel
LM2852YMXA-2.5 2.5 94 Units, Rail
LM2852YMXAX-2.5 2500 Units, Tape and Reel
LM2852YMXA-3.0 3.0 94 Units, Rail
LM2852YMXAX-3.0 2500 Units, Tape and Reel
LM2852YMXA-3.3 3.3 94 Units, Rail
LM2852YMXAX-3.3 2500 Units, Tape and Reel
LM2852XMXA-0.8
1500kHz
0.8 94 Units, Rail
LM2852XMXAX-0.8 2500 Units, Tape and Reel
LM2852XMXA-1.0 1.0 94 Units, Rail
LM2852XMXAX-1.0 2500 Units, Tape and Reel
LM2852XMXA-1.2 1.2 94 Units, Rail
LM2852XMXAX-1.2 2500 Units, Tape and Reel
LM2852XMXA-1.5 1.5 94 Units, Rail
LM2852XMXAX-1.5 2500 Units, Tape and Reel
LM2852XMXA-1.8 1.8 94 Units, Rail
LM2852XMXAX-1.8 2500 Units, Tape and Reel
LM2852XMXA-2.5 2.5 94 Units, Rail
LM2852XMXAX-2.5 2500 Units, Tape and Reel
LM2852XMXA-3.0 3.0 94 Units, Rail
LM2852XMXAX-3.0 2500 Units, Tape and Reel
LM2852XMXA-3.3 3.3 94 Units, Rail
LM2852XMXAX-3.3 2500 Units, Tape and Reel
Note: Contact factory for other voltage options.
LM2852
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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.
PVIN, AVIN, EN, SNS −0.3V to 6.5V
ESD Susceptibility (Note 2) 2kV
Power Dissipation Internally Limited
Storage Temperature Range −65˚C to +150˚C
Maximum Junction Temp. 150˚C
14-Pin Exposed Pad TSSOP
Package
Infrared (15 sec)
Vapor Phase (60 sec)
Soldering (10 sec)
220˚C
215˚C
260˚C
Operating Ratings
PVIN to GND 1.5V to 5.5V
AVIN to GND 2.85V to 5.5V
Junction Temperature −40˚C to +125˚C
θ
JA
38˚C/W
Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column.
Limits in standard type are for T
J
= 25˚C only; limits in boldface type apply over the junction temperature (T
J
) range of -40˚C
to +125˚C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values repre-
sent the most likely parametric norm at T
J
= 25˚C, and are provided for reference purposes only.
Symbol Parameter Conditions Min Typ Max Units
SYSTEM PARAMETERS
V
OUT
Voltage Tolerance
3
V
OUT
= 0.8V option 0.782 0.818 V
V
OUT
= 1.0V option 0.9775 1.0225
V
OUT
= 1.2V option 1.1730 1.2270
V
OUT
= 1.5V option 1.4663 1.5337
V
OUT
= 1.8V option 1.7595 1.8405
V
OUT
= 2.5V option 2.4437 2.5563
V
OUT
= 3.0V option 2.9325 3.0675
V
OUT
= 3.3V option 3.2257 3.3743
V
OUT
/AVIN Line Regulation
3
V
OUT
= 0.8V, 1.0V, 1.2V, 1.5V, 1.8V or
2.5V
2.85V AVIN 5.5V
0.2 0.6 %
V
OUT
= 3.3V
3.5V AVIN 5.5V
0.2 0.6 %
V
OUT
/I
O
Load Regulation Normal operation 8 mV/A
V
ON
UVLO Threshold
(AVIN)
Rising 2.47 2.85 V
Falling Hysteresis 85 150 210 mV
r
DSON-P
PFET On
Resistance
Isw=2A 75 140 m
r
DSON-N
NFET On
Resistance
Isw=2A 55 120 m
R
SS
Soft-start
resistance
400 k
I
CL
Peak Current Limit
Threshold
LM2852X 2.75 44.95 A
LM2852Y 2.25 33.65
I
Q
Operating Current Non-switching 0.85 2mA
I
SD
Shutdown
Quiescent Current
EN=0V 10 25 µA
R
SNS
Sense pin
resistance
400 k
PWM
f
osc
LM2852X 1500kHz option. 1050 1500 1825 kHz
LM2852Y 500kHz option. 325 500 625 kHz
LM2852
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Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column.
Limits in standard type are for T
J
= 25˚C only; limits in boldface type apply over the junction temperature (T
J
) range of -40˚C
to +125˚C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values
represent the most likely parametric norm at T
J
= 25˚C, and are provided for reference purposes only. (Continued)
Symbol Parameter Conditions Min Typ Max Units
D
range
Duty Cycle Range 0 100 %
ENABLE CONTROL
4
V
IH
EN Pin Minimum
High Input
75 %of
AVIN
V
IL
EN Pin Maximum
Low Input
25 %of
AVIN
I
EN
EN Pin Pullup
Current
EN = 0V 1.2 µA
THERMAL CONTROLS
T
SD
T
J
for Thermal
Shutdown
165 ˚C
T
SD-HYS
Hysteresis for
Thermal Shutdown
10 ˚C
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating Range indicates conditions for which the device is
intended to be functional, but does not guarantee specfic performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: Human body model: 1.5kin series with 100pF. SW and PVIN pins are derated to 1.5kV
Note 3: VOUT measured in a non-switching, closed-loop configuration at the SNS pin.
Note 4: The enable pin is internally pulled up, so the LM2852 is automatically enabled unless an external enable voltage is applied.
LM2852
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LM2852Y Typical Performance Characteristics (500kHz)
Efficiency vs I
Load
V
OUT
= 1.5V
Efficiency vs I
Load
V
OUT
= 2.5V
20127004
20127024
Efficiency vs I
Load
V
OUT
= 3.3V Frequency vs Temperature
20127006 20127009
LM2852
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LM2852X Typical Performance Characteristics (1500kHz)
Efficiency vs I
Load
V
OUT
= 1.5V
Efficiency vs I
Load
V
OUT
= 2.5V
20127025 20127026
Efficiency vs I
Load
V
OUT
= 3.3V Frequency vs Temperature
20127027
20127028
LM2852
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LM2852 Typical Performance Characteristics (Both Y and X Versions)
Shutdown Current vs V
IN
Quiescent Current (Non-Switching) vs V
IN
20127008 20127007
NMOS Switch R
DSON
vs Temperature PMOS Switch R
DSON
vs Temperature
20127010 20127011
LM2852
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Block Diagram
20127012
Applications Information
The LM2852 is a DC-DC synchronous buck regulator be-
longing to National Semiconductor’s SIMPLE SWITCHER®
family. Integration of the PWM controller, power switches
and compensation network greatly reduces the component
count required to implement a switching power supply. A
typical application requires only four components: an input
capacitor, a soft-start capacitor, an output filter capacitor and
an output filter inductor.
INPUT CAPACITOR (C
IN
)
Fast switching of large currents in the buck converter places
a heavy demand on the voltage source supplying PVIN. The
input capacitor, C
IN
, supplies extra charge when the switcher
needs to draw a burst of current from the supply. The RMS
current rating and the voltage rating of the C
IN
capacitor are
therefore important in the selection of C
IN
. The RMS current
specification can be approximated by:
where D is the duty cycle, V
OUT
/V
IN
.C
IN
also provides
filtering of the supply. Trace resistance and inductance de-
grade the benefits of the input capacitor, so C
IN
should be
placed very close to PVIN in the layout. A 22 µF or 47 µF
ceramic capacitor is typically sufficient for C
IN
. In parallel
with the large input capacitance a smaller capacitor may be
added such as a 1µF ceramic for higher frequency filtering.
SOFT-START CAPACITOR (C
SS
)
The DAC that sets the reference voltage of the error amp
sources a current through a resistor to set the reference
voltage. The reference voltage is one half of the output
voltage of the switcher due to the 200kdivider connected
to the SNS pin. Upon start-up, the output voltage of the
switcher tracks the reference voltage with a two to one ratio
as the DAC current charges the capacitance connected to
the reference voltage node. Internal capacitance of 20pF is
permanently attached to the reference voltage node which is
also connected to the soft-start pin, SS. Adding a soft-start
capacitor externally increases the time it takes for the output
voltage to reach its final level.
The charging time required for the reference voltage can be
estimated using the RC time constant of the DAC resistor
and the capacitance connected to the SS pin. Three RC time
constant periods are needed for the reference voltage to
reach 95% of its final value. The actual start-up time will vary
with differences in the DAC resistance and higher-order
effects.
If little or no soft-start capacitance is connected, then the
start-up time may be determined by the time required for the
current limit current to charge the output filter capacitance.
The capacitor charging equationI=CV/t can be used to
estimate the start-up time in this case. For example, a part
with a 3V output, a 100 µF output capacitance and a 3A
current limit threshold would require a time of 100 µs:
Since it is undesirable for the power supply to start up in
current limit, a soft-start capacitor must be chosen to force
the LM2852 to start up in a more controlled fashion based on
the charging of the soft-start capacitance. In this example,
supposea3msstart time is desired. Three time constants
are required for charging the soft-start capacitor to 95% of
the final reference voltage. So in this case RC=1ms. The
DAC resistor, R, is 400 kso C can be calculated to be
2.5nF. A 2.7nF ceramic capacitor can be chosen to yield
approximately a 3ms start-up time.
SOFT-START CAPACITOR (C
SS
) AND FAULT
CONDITIONS
Various fault conditions such as short circuit and UVLO of
the LM2852 activate internal circuitry designed to control the
voltage on the soft-start capacitor. For example, during a
short circuit current limit event, the output voltage typically
LM2852
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Applications Information (Continued)
falls to a low voltage. During this time, the soft-start voltage
is forced to track the output so that once the short is re-
moved, the LM2852 can restart gracefully from whatever
voltage the output reached during the short circuit event. The
range of soft-start capacitors is therefore restricted to values
1nF to 50nF.
COMPENSATION
The LM2852 provides a highly integrated solution to power
supply design. The compensation of the LM2852, which is
type-three, is included on-chip. The benefit to integrated
compensation is straightforward, simple power supply de-
sign. Since the output filter capacitor and inductor values
impact the compensation of the control loop, the range of L,
C and C
ESR
values is restricted in order to ensure stability.
OUTPUT FILTER VALUES
Table 1 details the recommended inductor and capacitor
ranges for the LM2852 that are suggested for various typical
output voltages. Values slightly different than those recom-
mended may be used, however the phase margin of the
power supply may be degraded.
TABLE 1. Output Filter Values
Frequency
Option V
OUT
(V) PVIN (V)
L (µH) C (µF) C
ESR
(m)
Min Max Min Max Min Max
LM2852Y
(500kHz)
0.8 3.3 10 15 100 220 70 200
0.8 5.0 10 15 100 120 70 200
1.0 3.3 10 15 100 180 70 200
1.0 5.0 10 15 100 180 70 200
1.2 3.3 10 15 100 180 70 200
1.2 5.0 15 22 100 120 70 200
1.5 3.3 10 15 100 120 70 200
1.5 5.0 22 22 100 120 70 200
1.8 3.3 10 15 100 120 100 200
1.8 5.0 22 33 100 120 100 200
2.5 3.3 6.8 10 68 120 95 275
2.5 5.0 15 22 68 120 95 275
3.3 5.0 15 22 68 100 100 275
LM2852X
(1500kHz)
0.8 3.3
110
The 1500kHz version is
designed for ceramic output
capacitors which typically
have very low ESR (<10m.)
0.8 5.0
1.0 3.3
1.0 5.0
1.2 3.3
1.2 5.0
1.5 3.3
1.5 5.0
1.8 3.3
1.8 5.0
2.5 3.3
2.5 5.0
3.3 5.0
LM2852
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Applications Information (Continued)
CHOOSING AN INDUCTANCE VALUE
The current ripple present in the output filter inductor is
determined by the input voltage, output voltage, switching
frequency and inductance according to the following equa-
tion:
where I
L
is the peak-to-peak current ripple, D is the duty
cycle V
OUT
/V
IN
,V
IN
is the input voltage applied to the PVIN
pin, V
OUT
is the output voltage of the switcher, f is the
switching frequency and L is the inductance of the output
filter inductor. Knowing the current ripple is important for
inductor selection since the peak current through the induc-
tor is the load current plus one half the ripple current. Care
must be taken to ensure the peak inductor current does not
reach a level high enough to trip the current limit circuitry of
the LM2852.
As an example, consider a 5V to 1.2V conversion and a
500kHz switching frequency. According to Table 1, a 15µH
inductor may be used. Calculating the expected peak-to-
peak ripple,
The maximum inductor current for a 2A load would therefore
be 2A plus 60.8 mA, 2.0608A. As shown in the ripple equa-
tion, the current ripple is inversely proportional to induc-
tance.
OUTPUT FILTER INDUCTORS
Once the inductance value is chosen, the key parameter for
selecting the output filter inductor is its saturation current
(I
sat
) specification. Typically I
sat
is given by the manufacturer
as the current at which the inductance of the coil falls to a
certain percentage of the nominal inductance. The I
sat
of an
inductor used in an application should be greater than the
maximum expected inductor current to avoid saturation. Be-
low is a table of inductors that may be suitable in LM2852
applications.
TABLE 2. LM2852 Output Filter Inductors
Inductance (µH) Part Number Vendor
1 DO1608C-102 Coilcraft
1 DO1813P-102HC Coilcraft
6.8 DO3316P-682 Coilcraft
7 MSS1038-702NBC Coilcraft
10 DO3316P-103 Coilcraft
10 MSS1038-103NBC Coilcraft
12 MSS1038-123NBC Coilcraft
15 D03316P-153 Coilcraft
15 MSS1038-153NBC Coilcraft
18 MSS1038-183NBC Coilcraft
22 DO3316P-223 Coilcraft
22 MSS1038-223NBC Coilcraft
22 DO3340P-223 Coilcraft
27 MSS1038-273NBC Coilcraft
33 MSS1038-333NBC Coilcraft
33 DO3340P-333 Coilcraft
LM2852
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Applications Information (Continued)
OUTPUT FILTER CAPACITORS
The capacitors that may be used in the output filter with the
LM2852 are limited in value and ESR range according to
Table 1. Below are some examples of capacitors that can
typically be used in an LM2852 application.
TABLE 3. LM2852 Output Filter Capacitors
Capacitance (µF) Part Number Chemistry Vendor
10 GRM31MR61A106KE19 Ceramic Murata
10 GRM32DR61E106K Ceramic Murata
68 595D686X_010C2T Tantalum Vishay - Sprague
68 595D686X_016D2T Tantalum Vishay - Sprague
100 595D107X_6R3C2T Tantalum Vishay - Sprague
100 595D107X_016D2T Tantalum Vishay - Sprague
100 NOSC107M004R0150 Niobium Oxide AVX
100 NOSD107M006R0100 Niobium Oxide AVX
120 595D127X_004C2T Tantalum Vishay - Sprague
120 595D127X_010D2T Tantalum Vishay - Sprague
150 595D157X_004C2T Tantalum Vishay - Sprague
150 595D157X_016D2T Tantalum Vishay - Sprague
150 NOSC157M004R0150 Niobium Oxide AVX
150 NOSD157M006R0100 Niobium Oxide AVX
220 595D227X_004D2T Tantalum Vishay - Sprague
220 NOSD227M004R0100 Niobium Oxide AVX
220 NOSE227M006R0100 Niobium Oxide AVX
SPLIT-RAIL OPERATION
The LM2852 can be powered using two separate voltages
for AVIN and PVIN. AVIN is the supply for the control logic;
PVIN is the supply for the power FETs. The output filter
components need to be chosen based on the value of PVIN.
For PVIN levels lower than 3.3V, use output filter component
values recommended for 3.3V. PVIN must always be equal
to or less than AVIN.
20127014
SWITCH NODE PROTECTION
The LM2852 includes protection circuitry that monitors the
voltage on the switch pin. Under certain conditions, switch-
ing is disabled in order to protect the switching devices. One
result of the protection circuitry may be observed when
power to the LM2852 is applied with no or light load on the
output. The output regulates to the rated voltage, but no
switching may be observed. As soon as the output is loaded,
the LM2852 begins normal switching operation.
LM2852
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Applications Information (Continued)
LAYOUT HINTS
These are several guidelines to follow while designing the
PCB layout for an LM2852 application.
1. The input bulk capacitor, C
IN
, should be placed very
close to the PVIN pin to keep the resistance as low as
possible between the capacitor and the pin. High current
levels will be present in this connection.
2. All ground connections must be tied together. Use a
broad ground plane, for example a completely filled back
plane, to establish the lowest resistance possible be-
tween all ground connections.
3. The sense pin connection should be made as close to
the load as possible so that the voltage at the load is the
expected regulated value. The sense line should not run
too close to nodes with high EMI (such as the switch
node) to minimize interference.
4. The switch node connections should be low resistance
to reduce power losses. Low resistance means the trace
between the switch pin and the inductor should be wide.
However, the area of the switch node should not be too
large since EMI increases with greater area. So connect
the inductor to the switch pin with a short, but wide trace.
Other high current connections in the application such
as PVIN and V
OUT
assume the same trade off between
low resistance and EMI.
5. Allow area under the chip to solder the entire exposed
die attach pad to ground for improved thermal and elec-
trical performance.
LM2852 Example Circuit Schematic
Bill of Materials for 500kHz (LM2852Y) 3.3V
IN
to 1.8 V
OUT
Conversion
ID Part Number Type Size Parameters Qty Vendor
U
1
LM2852YMXA-1.8 2A Buck ETSSOP-14 1 NSC
L
O
DO3316P-153 Inductor 15 µH 1 Coilcraft
C
O
* 595D107X_6R3C2T Capacitor Case Code “C” 100 µF ±20% 1 Vishay-Sprague
C
IN
GRM32ER60J476ME20B Capacitor 1210 47µF/X5R/6.3V 1 Murata
C
INX
GRM21BR71C105KA01B Capacitor 0805 1µF/X7R/16V 1 Murata
C
SS
VJ0805Y272KXXA Capacitor 0805 2.7nF ±10% 1 Vishay-Vitramon
R
f
CRCW060310R0F Resistor 0603 10±10% 1 Vishay-Dale
C
f
GRM21BR71C105KA01B Capacitor 0805 1µF/X7R/16V 1 Murata
* If a “non-tantalum” solution is desired use an NOSC107M004R0150, 100 µF capacitor from AVX for CO.
Bill of Materials for 1500kHz (LM2852X) 3.3V to 1.8V Conversion
ID Part Number Type Size Parameters Qty Vendor
U
1
LM2852XMXA-1.8 2A Buck ETSSOP-14 1 NSC
L
0
DO1813P-102HC Inductor 1 µH 1 Coilcraft
C
0
GRM32DR61E106K Capacitor 1210 10 µF/X5R/25V 1 Murata
C
IN
GRM32ER60J476ME20B Capacitor 1210 47µF/X5R/6.3V 1 Murata
C
INX
GRM21BR71C105KA01B Capacitor 0805 1µF/X7R/16V 1 Murata
C
SS
VJ0805Y272KXXA Capacitor 0805 2.7nF ±10% 1 Vishay-Vitramon
R
f
CRCW060310R0F Resistor 0603 10±10% 1 Vishay-Dale
C
f
GRM21BR71C105KA01B Capacitor 0805 1µF/X7R/16V 1 Murata
20127020
FIGURE 1.
LM2852
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Physical Dimensions inches (millimeters) unless otherwise noted
14-Lead ETSSOP Package
NS Package Number MXA14A
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.
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LM2852 2A 500/1500kHz Synchronous SIMPLE SWITCHER
®
Buck Regulator
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