LM2852XMXAX-1.0 [NSC]
2A 500/1500kHz SIMPLE SYNCHRONOUS Buck Regulator; 2A 500 / 1500kHz简单的同步降压稳压器型号: | LM2852XMXAX-1.0 |
厂家: | National Semiconductor |
描述: | 2A 500/1500kHz SIMPLE SYNCHRONOUS Buck Regulator |
文件: | 总13页 (文件大小:617K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
January 2005
LM2852
™
2A 500/1500kHz SIMPLE SYNCHRONOUS Buck
Regulator
General Description
Features
n Input voltage range of 2.85 to 5.5V
n Factory EEPROM set output voltages from 0.8V to 3.3V
in 100mV increments
™
The LM2852 SIMPLE SYNCHRONOUS buck regulator is
a high frequency step-down switching voltage regulator ca-
pable of driving up to a 2A load with excellent line and load
regulation. The LM2852 can accept an input voltage be-
tween 2.85V and 5.5V and deliver a customizable output
voltage that is factory programmable from 0.8V to 3.3V in
100mV increments. The LM2852 is available with a choice of
two switching frequencies - 500kHz (LM2852Y) or 1.5MHz
(LM2852X). It also features internal compensation to deliver
n Maximum Load Current of 2A
n Voltage Mode Control
n Internal type three compensation
n Switching frequency of 500kHz or 1.5MHz
n Low standby current of 10µA
n Internal 60 mΩ MOSFET switches
n Standard voltage options 1.0/1.2/1.5/1.8/2.5/3.3 volts
a
low component count solution. The exposed-pad
TSSOP-14 package enhances the thermal performance of
the LM2852.
Applications
n Low voltage point of load regulators
n Local solution for FPGA/DSP/ASIC core power
n Broadband networking and communications
infrastructure
n Portable computing
Typical Application Circuit
20127001
20127002
© 2005 National Semiconductor Corporation
DS201270
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Connection Diagram
TOP VIEW
20127003
MXA14A
NC (Pins 5, 12 and 13): No-connect. These pins must be
tied to ground or left floating in the application.
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.
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.
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.
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.
SGND (Pin 3): Low-noise ground.
SNS (Pin 14): Output voltage sense pin. Connect this pin to
the output voltage as close to the load as possible.
SS (Pin 4): Soft-start pin. Connect this pin to a small capaci-
tor to control startup and soften inrush current. The soft-start
capacitance range is restricted to values 1 nF to 50 nF.
Exposed Pad: Connect to ground.
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Ordering Information
Order Number
LM2852YMXA-1.0
LM2852YMXAX-1.0
Frequency
Voltage Option
Package Type
Package Drawing
Supplied As
94 Units, Rail
2500 Units, Tape and
Reel
1.0
LM2852YMXA-1.2
LM2852YMXAX-1.2
1.2
1.5
1.8
2.5
3.3
94 Units, Rail
2500 Units, Tape and
Reel
LM2852YMXA-1.5
LM2852YMXAX-1.5
94 Units, Rail
2500 Units, Tape and
Reel
500kHz
LM2852YMXA-1.8
LM2852YMXAX-1.8
94 Units, Rail
2500 Units, Tape and
Reel
LM2852YMXA-2.5
LM2852YMXAX-2.5
94 Units, Rail
2500 Units, Tape and
Reel
TSSOP-14 exposed pad
MXA14A
LM2852YMXA-3.3
LM2852YMXAX-3.3
94 Units, Rail
2500 Units, Tape and
Reel
LM2852XMXA-1.0
LM2852XMXAX-1.0
LM2852XMXA-1.2
LM2852XMXAX-1.2
LM2852XMXA-1.5
LM2852XMXAX-1.5
LM2852XMXA-1.8
LM2852XMXAX-1.8
LM2852XMXA-2.5
LM2852XMXAX-2.5
LM2852XMXA-3.3
LM2852XMXAX-3.3
1.0
1.2
1.5
1.8
2.5
3.3
1500kHz
Coming Soon
Note: Contact factory for other voltage options.
3
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Absolute Maximum Ratings (Note 1)
Operating Ratings
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
PVIN to GND
AVIN to GND
Junction Temperature
θJA
1.5V to 5.5V
2.85V to 5.5V
−40˚C to +125˚C
37.6˚C/W
PVIN, AVIN, EN, SNS
ESD Susceptibility (Note 2)
Power Dissipation
−0.3V to 6.5V
2kV
Internally Limited
−65˚C to +150˚C
150˚C
Storage Temperature Range
Maximum Junction Temp.
14-Pin Exposed Pad TSSOP
Package
220˚C
215˚C
260˚C
Infrared (15 sec)
Vapor Phase (60 sec)
Soldering (10 sec)
Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column.
Typicals and limits appearing in plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over full Oper-
ating Junction Temperature Range (−40˚C to +125˚C). Datasheet min/max specification limits are guaranteed by design, test,
or statistical analysis.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
SYSTEM PARAMETERS
VOUT
Voltage Tolerance3 VOUT = 1.0V option
VOUT = 1.2V option
0.9775
1.1730
1.4663
1.7595
2.4437
3.2257
1.0225
1.2270
1.5337
1.8405
2.5563
3.3743
0.6
V
VOUT = 1.5V option
VOUT = 1.8V option
VOUT = 2.5V option
VOUT = 3.3V option
Line Regulation3 VOUT = 0.8V, 1.0V, 1.2V, 1.5V, 1.8V or
∆VOUT/∆AVIN
0.2
0.2
%
%
2.5V
2.85V ≤ AVIN ≤ 5.5V
VOUT = 3.3V
0.6
3.5V ≤ AVIN ≤ 5.5V
∆VOUT/∆IO
Load Regulation Normal operation
UVLO Threshold Rising
8
mV/A
V
VON
2.47
150
75
2.85
210
140
(AVIN)
Falling Hysteresis
Isw = 2A
85
mV
mΩ
rDSON-P
rDSON-N
RSS
PFET On
Resistance
NFET On
Isw = 2A
55
400
3
120
mΩ
kΩ
A
Resistance
Soft-start
resistance
ICL
Peak Current Limit
Threshold
2.25
3.65
IQ
Operating Current Non-switching
0.85
10
2
mA
µA
ISD
Shut Down
Quiescent Current
Sense pin
EN = 0V
25
RSNS
400
kΩ
resistance
PWM
fosc
LM2852X
LM2852Y
1500kHz option.
500kHz option.
TBD
325
0
1500
500
TBD
625
100
kHz
kHz
%
Drange
Duty Cycle Range
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Electrical Characteristics AVIN = PVIN = 5V unless otherwise indicated under the Conditions column.
Typicals and limits appearing in plain type apply for TA = TJ = +25˚C. Limits appearing in boldface type apply over full
Operating Junction Temperature Range (−40˚C to +125˚C). Datasheet min/max specification limits are guaranteed by design,
test, or statistical analysis. (Continued)
Symbol
ENABLE CONTROL4
Parameter
Conditions
Min
75
Typ
Max
25
Units
VIH
VIL
IEN
EN Pin Minimum
High Input
% of
AVIN
% of
AVIN
µA
EN Pin Maximum
Low Input
EN Pin Pullup
Current
EN = 0V
1.2
THERMAL CONTROLS
TSD
TJ for Thermal
165
10
˚C
˚C
Shutdown
Hysteresis for
TSD-hys
Thermal Shutdown
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: 1.5kΩ in series with 100pF. SW and PVIN pins are derated to 1.5kV
Note 3: V
measured in a non-switching, closed-loop configuration at the SNS pin.
OUT
Note 4: The enable pin is internally pulled up, so the LM2852 is automatically enabled unless an external enable voltage is applied.
5
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Typical Performance Characteristics
Efficiency vs ILoad
VOUT = 1.5V
Efficiency vs ILoad
VOUT = 2.5V
20127024
20127004
Efficiency vs ILoad
VOUT = 3.3V
Quiescent Current (Non-Switching) vs VIN and Temp.
20127006
20127007
Shut-Down Current vs VIN and Temp.
Frequency vs Temperature and VIN
20127009
20127008
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Typical Performance Characteristics (Continued)
NMOS Switch RDSON vs Temperature and PVIN
PMOS Switch RDSON vs Temperature and PVIN
20127010
20127011
7
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Block Diagram
20127012
Applications Information
The LM2852 is a DC-DC buck converter belonging to Na-
tional Semiconductor’s SIMPLE SYNCHRONOUS® family.
Integration of the PWM controller, power switches and com-
pensation network greatly reduces the component count
required to implement a switching power supply. A typical
application requires only four components: an input capaci-
tor, a soft-start capacitor, an output filter capacitor and an
output filter inductor.
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.
INPUT CAPACITOR (CIN
)
Fast switching of large currents in the buck converter places
a heavy demand on the voltage source supplying PVIN. The
input capacitor, CIN, 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 CIN capacitor are
therefore important in the selection of CIN. The RMS current
specification can be approximated to be the load current
times the square root of the duty cycle:
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 equation I = C ∆V/∆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:
where D is the duty cycle, VOUT/VIN. CIN also provides
filtering of the supply. Trace resistance and inductance de-
grade the benefits of the input capacitor, so CIN should be
placed very close to PVIN in the layout. A 22 µF or 47 µF
ceramic capacitor is typically sufficient for CIN. In parallel
with the large input capacitance a smaller capacitor may be
added such as a 1µF ceramic for higher frequency filtering.
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,
suppose a 3 ms start 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 kΩ so 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 (CSS
)
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 200kΩ divider 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
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type three, is included on-chip. The benefit to integrated
compensation is straight-forward, 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 CESR values is restricted in order to ensure stability.
Applications Information (Continued)
SOFT-START CAPACITOR (CSS) 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
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.
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.
COMPENSATION
The LM2852 provides a highly integrated solution to power
supply design. The compensation of the LM2852, which is
TABLE 1. Output Filter Values
L (µH)
C (µF)
CESR (mΩ)
Min
Frequency Option
VOUT (V)
0.8
0.8
1
PVIN (V)
Min
10
10
10
10
10
15
10
22
10
22
6.8
15
15
Max
15
15
15
15
15
22
15
22
15
33
10
22
22
Min
100
100
100
100
100
100
100
100
100
100
68
Max
220
120
180
180
180
120
120
120
120
120
120
120
100
Max
200
200
200
200
200
200
200
200
200
200
275
275
275
3.3
5
70
70
3.3
5
70
1
70
1.2
1.2
1.5
1.5
1.8
1.8
2.5
2.5
3.3
3.3
5
70
70
LM2852Y
(500kHz)
3.3
5
70
70
3.3
5
100
100
95
3.3
5
68
95
5
68
100
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:
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.
where ∆IL is the peak to peak current ripple, D is the duty
cycle VOUT/VIN, VIN is the input voltage applied to the output
stage, VOUT 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.
OUTPUT FILTER INDUCTORS
Once the inductance value is chosen, the key parameter for
selecting the output filter inductor is its saturation current
(Isat) specification. Typically Isat 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 Isat 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.
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,
9
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Applications Information (Continued)
TABLE 2. (LM2852Y Output Filter Inductors (500kHz)
Inductance (µH)
Part Number
DO3316P-682
Vendor
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
6.8
7
MSS1038-702NBC
DO3316P-103
10
10
12
15
15
18
22
22
22
27
33
33
MSS1038-103NBC
MSS1038-123NBC
D03316P-153
MSS1038-153NBC
MSS1038-183NBC
DO3316P-223
MSS1038-223NBC
DO3340P-223
MSS1038-273NBC
MSS1038-333NBC
DO3340P-333
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. LM2852Y Output Filter Capacitors (500kHz)
Capacitance (µF)
Part Number
Chemistry
Tantalum
Vendor
68
595D686X_010C2T
595D686X_016D2T
595D017X_6R3C2T
595D107X_016D2T
NOSC107M004R0150
NOSD107M006R0100
595D127X_004C2T
595D127X_010D2T
595D157X_004C2T
595D157X_016D2T
NOSC157M004R0150
NOSD157M006R0100
595D227X_004D2T
NOSD227M004R0100
NOSE227M006R0100
Vishay - Sprague
Vishay - Sprague
Vishay - Sprague
Vishay - Sprague
AVX
68
Tantalum
100
100
100
100
120
120
150
150
150
150
220
220
220
Tantalum
Tantalum
Niobium Oxide
Niobium Oxide
Tantalum
AVX
Vishay - Sprague
Vishay - Sprague
Vishay - Sprague
Vishay - Sprague
AVX
Tantalum
Tantalum
Tantalum
Niobium Oxide
Niobium Oxide
Tantalum
AVX
Vishay - Sprague
AVX
Niobium Oxide
Niobium Oxide
AVX
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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.
Applications Information (Continued)
SPLIT-RAIL OPERATION
The LM2852 can be powered using two separate voltages
for AVIN and PVIN. AVIN is the supply for the control logic;
20127014
Layout Hints
node) to minimize interference.
These are several guidelines to follow while designing the
PCB layout for an LM2852 application.
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 VOUT assume the same trade off between
low resistance and EMI.
1. The input bulk capacitor, CIN, 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.
5. Allow area under the chip to solder the entire exposed
die attach pad to ground. Lab measurements show im-
proved regulation performance when the exposed pad is
well grounded.
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
LM2852Y Example Circuit Schematic (500kHz)
20127020
FIGURE 1.
Bill of Materials for 3.3VIN to 1.8 VOUT Conversion
ID
U1
Part Number
LM2852YMXA-1.8
Type
Size
Parameters
Qty
1
Vendor
2A Buck
Inductor
Capacitor
Capacitor
Capacitor
Capacitor
Resistor
Capacitor
ETSSOP-14
NSC
Coilcraft
LO
DO3316P-153
15 µH
1
CO*
CIN
CINX
CSS
Rf
595D107X_6R3C2T
GRM32ER60J476ME20B
GRM21BR71C105KA01B
VJ0805Y272KXXA
Case Code “C”
1210
100 µF 20%
47µF/X5R/6.3V
1µF/X7R/16V
2.7nF 10%
10Ω 10%
1
Vishay-Sprague
Murata
1
0805
1
Murata
0805
1
Vishay-Vitramon
Vishay-Dale
Murata
CRCW060310R0F
0603
1
Cf
GRM21BR71C105KA01B
0805
1µF/X7R/16V
1
* If a “non-tantalum” solution is desired use an NOSC107M004R0150, 100 µF capacitor from AVX for C
.
O
11
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12
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.
For the most current product information visit us at www.national.com.
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