LM2853 [NSC]
3A 550 kHz Synchronous SIMPLE SWITCHER Buck Regulator; 3A 550千赫同步SIMPLE SWITCHER降压稳压器
| 型号： | LM2853 |
| 厂家： | 
National Semiconductor |
| 描述： | 3A 550 kHz Synchronous SIMPLE SWITCHER Buck Regulator |
| 文件： | 总12页 (文件大小：623K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
October 2006
LM2853
3A 550 kHz Synchronous SIMPLE SWITCHER® Buck
Regulator
General Description
Features
The LM2853 synchronous SIMPLE SWITCHER® buck regu-
lator is a 550 kHz step-down switching voltage regulator
capable of driving up to a 3A load with excellent line and load
regulation. The LM2853 accepts an input voltage between
3.0V and 5.5V and delivers a customizable output voltage
that is factory programmable from 0.8V to 3.3V in 100mV
increments. Internal type-three compensation enables a low
component count solution and greatly simplifies external
component selection. The exposed-pad TSSOP-14 package
enhances the thermal performance of the LM2853.
n Input voltage range of 3.0V to 5.5V
n Factory EEPROM set output voltages from 0.8V to 3.3V
in 100 mV increments
n Maximum load current of 3A
n Voltage Mode Control
n Internal type-three compensation
n Switching frequency of 550 kHz
n Low standby current of 12 µA
n Internal 40 mΩ MOSFET switches
n Standard voltage options
0.8/1.0/1.2/1.5/1.8/2.5/3.0/3.3 volts
n Exposed pad TSSOP-14 package
Applications
n Low voltage point of load regulation
n Local solution for FPGA/DSP/ASIC core power
n Broadband networking and communications
infrastructure
Typical Application Circuit
20201502
Efficiency vs Load Current (VOUT = 3.3V)
20201501
SIMPLE SWITCHER® is a Registered Trademark of National Semiconductor Corporation.
© 2006 National Semiconductor Corporation
DS202015
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Connection Diagram
20201503
Ordering Information
Voltage
Option
Package
Marking
Package
Drawing
Order Number
LM2853MH-0.8
LM2853MHX-0.8
LM2853MH-1.0
LM2853MHX-1.0
LM2853MH-1.2
LM2853MHX-1.2
LM2853MH-1.5
LM2853MHX-1.5
LM2853MH-1.8
LM2853MHX-1.8
LM2853MH-2.5
LM2853MHX-2.5
LM2853MH-3.0
LM2853MHX-3.0
LM2853MH-3.3
LM2853MHX-3.3
Package Type
Supplied As
94 Units, Rail
0.8
LM2853-0.8
LM2853-1.0
LM2853-1.2
LM2853-1.5
LM2853-1.8
LM2853-2.5
LM2853-3.0
LM2853-3.3
2500 Units, Tape and Reel
94 Units, Rail
1.0
1.2
1.5
1.8
2.5
3.0
3.3
2500 Units, Tape and Reel
94 Units, Rail
2500 Units, Tape and Reel
94 Units, Rail
2500 Units, Tape and Reel
94 Units, Rail
TSSOP-14 exposed
pad
MXA14A
2500 Units, Tape and Reel
94 Units, Rail
2500 Units, Tape and Reel
94 Units, Rail
2500 Units, Tape and Reel
94 Units, Rail
2500 Units, Tape and Reel
Note: Contact factory for other voltage options.
Pin Descriptions
Pin #
Name
AVIN
EN
Function
1
Input Voltage for Control Circuitry.
Enable.
2
3
SGND
SS
Low noise ground.
Soft-Start Pin.
4
5
6,7
NC
No Connect. This pin must be tied to ground.
Input Voltage for Power Circuitry.
PVIN
SW
8,9
Switch Pin.
10,11
12,13
14
PGND
NC
Power Ground.
No-Connect. These pins must be tied to ground.
Output Voltage Sense Pin.
SNS
EP
Exposed Pad
The exposed pad is internally connected to GND, but it cannot be
used as the primary GND connection. The exposed pad should be
soldered to an external GND plane.
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2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
14-Pin Exposed Pad TSSOP Package
Infrared (15 sec)
220˚C
215˚C
260˚C
Vapor Phase (60 sec)
Soldering (10 sec)
AVIN, PVIN, EN, SNS, SW, SS
ESD Susceptibility (Note 2)
Power Dissipation
−0.3V to 6.0V
2kV
Operating Ratings (Note 1)
PVIN to GND
Internally Limited
−65˚C to +150˚C
150˚C
1.5V to 5.5V
3.0V to 5.5V
Storage Temperature Range
Maximum Junction Temp.
AVIN to GND
Junction Temperature
−40˚C to +125˚C
Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those in bold face
type apply over the full Junction Temperature Range (−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 TJ = 25˚C and are provided
for reference purposes only. Unless otherwise specified AVIN = PVIN = 5V.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
SYSTEM PARAMETERS
VOUT
Voltage Tolerance (Note 3)
VOUT = 0.8V option
0.782
0.9775
1.1730
1.4663
1.7595
2.4437
2.9325
3.2257
0.8
1.0
1.2
1.5
1.8
2.5
3.0
3.3
0.2
0.818
1.0225
1.227
VOUT = 1.0V option
VOUT = 1.2V option
VOUT = 1.5V option
VOUT = 1.8V option
VOUT = 2.5V option
VOUT = 3.0V option
VOUT = 3.3V option
VOUT = 0.8V, 1.0V, 1.2V, 1.5V,
1.8V or 2.5V
1.5337
1.8405
2.5563
3.0675
3.3743
1.1
V
∆VOUT/∆AVIN Line Regulation (Note 3)
%
%
3.0V ≤ AVIN ≤ 5.5V
VOUT = 3.0V or 3.3V
3.5V ≤ AVIN ≤ 5.5V
Normal operation
Rising
0.2
1.1
∆VOUT/∆IO
Load Regulation
2
mV/A
V
VON
UVLO Threshold (AVIN)
2.47
155
40
3.0
260
120
100
Falling Hysteresis
Isw = 3A
50
mV
mΩ
mΩ
kΩ
A
RDS(ON)-P
RDS(ON)-N
RSS
PFET On Resistance
NFET On Resistance
Soft-Start Resistance
Peak Current Limit Threshold
Operating Current
Isw = 3A
32
450
5
ICL
3.6
IQ
Non-switching
EN = 0V
0.85
12
2
mA
µA
ISD
Shutdown Quiescent Current
Sense Pin Resistance
50
RSNS
PWM
fosc
432
kΩ
Switching Frequency
Duty Cycle Range
.
325
0
550
725
100
kHz
%
Drange
ENABLE CONTROL (Note 4)
VIH
VIL
IEN
EN Pin Minimum High Input
75
% of
AVIN
% of
AVIN
µA
EN Pin Maximum Low Input
EN Pin Pullup Current
25
EN = 0V
1.5
3
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Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those in bold face type
apply over the full Junction Temperature Range (−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 TJ = 25˚C and are provided for
reference purposes only. Unless otherwise specified AVIN = PVIN = 5V. (Continued)
Symbol
Parameter
Conditions
Min
Typ
Max
Units
THERMAL CONTROLS
TSD
Thermal Shutdown Threshold
Hysteresis for Thermal
Shutdown
165
10
˚C
˚C
TSD-HYS
THERMAL RESISTANCE
θJA Junction to Ambient
MXA14A
38
˚C/W
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 specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Test Method is per JESD22-AI14.
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 LM2853 is automatically enabled unless an external enable voltage is applied.
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4
Typical Performance Characteristics Unless otherwise specified, the following conditions apply: VIN
= AVIN = PVIN = 5V, TJ = 25˚C.
Efficiency vs. ILOAD
VOUT = 1.8V
NFET RDS(ON) vs. Temperature
20201507
20201509
20201508
20201505
Efficiency vs. ILOAD
VOUT = 2.5V
PFET RDS(ON) vs. Temperature
20201504
Efficiency vs. ILOAD
VOUT = 3.3V
Switching Frequency vs. Temperature
20201506
5
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Typical Performance Characteristics Unless otherwise specified, the following conditions apply: VIN
= AVIN = PVIN = 5V, TJ = 25˚C. (Continued)
IQ vs. VIN and Temperature
ISD vs. VIN and Temperature
20201510
20201511
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6
Block Diagram
20201512
Applications Information
The LM2853 is a DC-DC buck regulator belonging to Na-
tional Semiconductor’s synchronous 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.
load regulation and transient performance, the use of a small
1 µF ceramic capacitor is also recommended as a local
bypass for the AVIN pin.
SOFT-START CAPACITOR (CSS
)
The DAC that sets the reference voltage of the error ampli-
fier 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 200 kΩ 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 20 pF 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.
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 by:
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 should
be added such as a 1 µF ceramic for higher frequency
filtering. Ceramic capacitors with high quality dielectrics such
as X5R or X7R should be used to provide a constant capaci-
tance across temperature and line variations. For improved
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 5A
current limit threshold would require a time of 60 µs:
7
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voltage the output reached during the short circuit event. The
range of soft-start capacitors is therefore restricted to values
1 nF to 50 nF.
Applications Information (Continued)
COMPENSATION
The LM2853 provides a highly integrated solution to power
supply design. The compensation of the LM2853, which is
type-three, is included on-chip. The benefit of 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 LO,
CO and CESR values is restricted in order to ensure stability.
Since it is undesirable for the power supply to start up in
current limit, a soft-start capacitor must be chosen to force
the LM2853 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 = 1 ms. The
DAC resistor, R, is 450 kΩ so C can be calculated to be 2.2
nF. A 2.2 nF ceramic capacitor can be chosen to yield
approximately a 3 ms start-up time.
OUTPUT FILTER VALUES
Table 1 details the recommended inductor and capacitor
ranges for the LM2853 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. For best performance when
output voltage ripple is a concern, ESR values near the
minimum of the recommended range should be paired with
capacitance values near the maximum. If a minimum output
voltage ripple solution from a 5V input voltage is desired, a
6.8 µH inductor can be paired with a 220 µF (50 mΩ)
capacitor without degraded phase margin.
SOFT-START CAPACITOR (CSS) AND FAULT
CONDITIONS
Various fault conditions such as short circuit and UVLO of
the LM2853 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 LM2853 can restart gracefully from whatever
TABLE 1. Recommended LO and CO Values
LO (µH) CO (µF)
CESR (mΩ)
Max
VOUT (V)
0.8
VIN (V)
Min
4.7
4.7
Max
6.8
Min
120
150
Max
220
220
Min
70
5
100
100
0.8
3.3
4.7
50
1
1
5
4.7
4.7
6.8
4.7
120
150
220
220
70
50
100
100
3.3
1.2
1.2
5
4.7
4.7
6.8
4.7
120
120
220
220
70
60
100
100
3.3
1.5
1.5
5
4.7
4.7
6.8
4.7
120
120
220
220
70
60
100
100
3.3
1.8
1.8
5
4.7
4.7
6.8
4.7
120
100
220
220
70
70
120
120
3.3
2.5
2.5
5
4.7
4.7
6.8
4.7
120
100
220
220
70
80
150
150
3.3
3.0
3.0
5
4.7
4.7
6.8
4.7
120
100
220
220
70
80
150
150
3.3
3.3
5
4.7
6.8
120
220
70
150
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8
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:
The maximum inductor current for a 3A load would therefore
be 3A plus 177 mA, 3.177A. As shown in the ripple equation,
the current ripple is inversely proportional to inductance.
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 manufac-
turer 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 are suitable in LM2853
applications.
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 LO 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 LM2853. As an example, consider a 5V to 1.2V conver-
sion and a 550 kHz switching frequency. According to Table
1, a 4.7 µH inductor may be used. Calculating the expected
peak-to-peak ripple,
TABLE 2. Recommended Inductors
Inductance
4.7 µF
Part Number
Vendor
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
DO3308P-472ML
DO3316P-472ML
MSS1260-472ML
MSS1038-522NL
MSS1260-562ML
DO3316P-682ML
MSS1260-682ML
4.7 µF
4.7 µF
5.2 µF
5.6 µF
6.8 µF
6.8 µF
OUTPUT FILTER CAPACITORS
Below are some examples of capacitors that can typically be
used in an LM2853 application.
The recommended capacitors that may be used in the output
filter with the LM2853 are limited in value and ESR range
according to Table 1.
TABLE 3. Recommended Capacitors
Part Number Chemistry
Capacitance (µF)
Vendor
Vishay-Sprague
Vishay-Sprague
AVX
100
100
100
100
100
120
150
150
150
150
150
150
220
220
220
220
594D107X_010C2T
593D107X_010D2_E3
TPSC107M006#0075
NOSD107M006#0080
NOSC107M004#0070
594D127X_6R3C2T
594D157X_010C2T
Tantalum
Tantalum
Tantalum
Niobium Oxide
Niobium Oxide
Tantalum
AVX
AVX
Vishay-Sprague
Vishay-Sprague
Vishay-Sprague
Vishay-Sprague
AVX
Tantalum
595D157X_010D2T
Tantalum
591D157X_6R3C2_20H
TPSD157M006#0050
TPSC157M004#0070
NOSD157M006#0070
594D227X_6R3D2T
591D227X_6R3D2_20H
591D227X_010D2_20H
593D227X_6R3D2_E3
Tantalum
Tantalum
Tantalum
AVX
Niobium Oxide
Tantalum
AVX
Vishay-Sprague
Vishay-Sprague
Vishay-Sprague
Vishay-Sprague
Tantalum
Tantalum
Tantalum
9
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Applications Information (Continued)
TABLE 3. Recommended Capacitors (Continued)
Capacitance (µF)
Part Number
Chemistry
Tantalum
Vendor
AVX
220
220
TPSD227M006#0050
NOSD227M0040060
Niobium Oxide
AVX
SPLIT-RAIL OPERATION
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.
The LM2853 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
20201513
SWITCH NODE PROTECTION
tween all ground connections.
The LM2853 includes protection circuitry that monitors the
voltage on the switch pin. Under certain fault conditions,
switching is disabled in order to protect the switching de-
vices. One side effect of the protection circuitry may be
observed when power to the LM2853 is applied with no or
light load on the output. The output will regulate to the rated
voltage, but no switching may be observed. As soon as the
output is loaded, the LM2853 will begin normal switching
operation.
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 dV/dt or dl/dt (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 VOUT assume the same trade off between
low resistance and EMI.
LAYOUT GUIDELINES
These are several guidelines to follow while designing the
PCB layout for an LM2853 application.
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.
5. Allow area under the chip to solder the entire exposed
die attach pad to ground for improved thermal perfor-
mance. Lab measurements also show improved regula-
tion performance when the exposed pad is well
grounded.
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-
LM2853 Example Circuit Schematic
20201514
FIGURE 1.
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10
LM2853 Example Circuit Schematic (Continued)
Bill of Materials for 5V to 3.3V Conversion
Type Size Parameters
3A Buck ETSSOP-14
ID
U1
Part Number
LM2853MH-3.3
Qty
1
Vendor
NSC
3.3V
47 µF
1 µF
CIN
CBYP
CSS
LO
GRM31CR60J476ME19
GRM21BR71C105KA01
VJ0805Y222KXXA
DO3316P-682
Capacitor
Capacitor
Capacitor
Inductor
1206
0805
1
Murata
1
Murata
0603
2.2 nF
6.8 µH
120µF
(85mΩ)
1
Vishay-Vitramon
Coilcraft
DO3316P
C Case
1
CO
594D127X06R3C2T
Capacitor
1
Vishay-Sprague
Bill of Materials for 3.3V to 1.2V Conversion
ID
U1
Part Number
LM2853MH-1.2
Type
Size
ETSSOP-14
1206
Parameters
1.2V
Qty
1
Vendor
NSC
3A Buck
Capacitor
Capacitor
Capacitor
Inductor
Capacitor
CIN
CBYP
CSS
LO
GRM31CR60J476ME19
GRM21BR71C105KA01
VJ0805Y222KXXA
DO3316P-472
47 µF
1
Murata
0805
1 µF
1
Murata
0603
2.2 nF
4.7 µH
150 µF
(70 mΩ)
1
Vishay-Vitramon
Coilcraft
AVX
DO3316P
D Case
1
CO
NOSD157M006R0070
1
11
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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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