LM22678-Q1 [TI]
SIMPLE SWITCHER Step-Down Voltage Regulator With Features;
| 型号： | LM22678-Q1 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | SIMPLE SWITCHER Step-Down Voltage Regulator With Features |
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LM22678, LM22678-Q1
SNVS585L –SEPTEMBER 2008–REVISED NOVEMBER 2014
®
LM22678/-Q1 42-V, 5-A SIMPLE SWITCHER Step-Down Voltage Regulator
With Features
1 Features
3 Description
The LM22678 switching regulator provides all of the
functions necessary to implement an efficient high-
voltage step-down (buck) regulator using a minimum
of external components. This easy-to-use regulator
incorporates a 42-V N-channel MOSFET switch that
can provide up to 5 A of load current. Excellent line
and load regulation along with high efficiency (> 90%)
are featured. Voltage mode control offers short
minimum on-time, allowing the widest ratio between
input and output voltages. Internal loop compensation
means that the user is free from the tedious task of
calculating the loop compensation components. Fixed
5-V output and adjustable output voltage options are
available. A switching frequency of 500 kHz allows for
small external components and good transient
1
•
Wide Input Voltage Range: 4.5 V to 42 V
Internally Compensated Voltage Mode Control
Stable With Low ESR Ceramic Capacitors
100-mΩ N-Channel MOSFET
•
•
•
•
Output Voltage Options:
-ADJ (Outputs as Low as 1.285 V)
-5.0 (Output Fixed to 5 V)
•
•
•
±1.5% Feedback Reference Accuracy
Switching Frequency of 500 kHz
–40°C to 125°C Operating Junction Temperature
Range
•
•
•
•
•
Precision Enable Pin
response.
A
precision enable input allows
Integrated Boot-Strap Diode
simplification of regulator control and system power
sequencing. In shutdown mode the regulator draws
only 25 µA (typical). Built-in soft-start (500 µs, typical)
saves external components. The LM22678 device
also has built-in thermal shutdown, and current
limiting to protect against accidental overloads.
Integrated Soft-Start
Fully WEBENCH® Enabled
LM22678-Q1 is an Automotive-Grade Product
that is AEC-Q100 Grade 1 Qualified (–40°C to
+125°C Operating Junction Temperature)
•
PFM (Exposed Pad) Package
The LM22678 device is
a member of Texas
Instruments' SIMPLE SWITCHER® family. The
SIMPLE SWITCHER concept provides for an easy-to-
use complete design using a minimum number of
external components and the TI WEBENCH design
tool. TI's WEBENCH tool includes features such as
external component calculation, electrical simulation,
thermal simulation, and Build-It boards for easy
design-in.
2 Applications
•
•
•
•
Industrial Control
Telecom and Datacom Systems
Embedded Systems
Conversions from Standard 24-V, 12-V and 5-V
Input Rails
Device Information(1)
PART NUMBER
LM22678
PACKAGE
BODY SIZE (NOM)
TO-263 (7)
10.16 mm x 9.85 mm
LM22678-Q1
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Simplified Application Schematic
VIN
VIN
FB
BOOT
SW
LM22678-ADJ
VOUT
EN
GND
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM22678, LM22678-Q1
SNVS585L –SEPTEMBER 2008–REVISED NOVEMBER 2014
www.ti.com
Table of Contents
7.4 Device Functional Modes........................................ 11
Application and Implementation ........................ 14
8.1 Application Information............................................ 14
8.2 Typical Applications ................................................ 15
Power Supply Recommendations...................... 18
1
2
3
4
5
6
Features.................................................................. 1
Applications ........................................................... 1
Description ............................................................. 1
Revision History..................................................... 2
Pin Configuration and Functions......................... 3
Specifications......................................................... 4
6.1 Absolute Maximum Ratings ...................................... 4
6.2 Handling Ratings: LM22678...................................... 4
6.3 Handling Ratings: LM22678-Q1................................ 4
6.4 Recommended Operating Conditions....................... 4
6.5 Thermal Information.................................................. 4
6.6 Electrical Characteristics........................................... 5
6.7 Typical Characteristics.............................................. 6
Detailed Description .............................................. 8
7.1 Overview ................................................................... 8
7.2 Functional Block Diagram ......................................... 8
7.3 Feature Description................................................... 9
8
9
10 Layout................................................................... 18
10.1 Layout Guidelines ................................................. 18
10.2 Layout Example .................................................... 19
10.3 Thermal Considerations........................................ 19
11 Device and Documentation Support ................. 20
11.1 Documentation Support ........................................ 20
11.2 Related Links ........................................................ 20
11.3 Trademarks........................................................... 20
11.4 Electrostatic Discharge Caution............................ 20
11.5 Glossary................................................................ 20
7
12 Mechanical, Packaging, and Orderable
Information ........................................................... 20
4 Revision History
Changes from Revision K (March 2013) to Revision L
Page
•
Added Pin Configuration and Functions section, Handling Rating table, Feature Description section, Device
Functional Modes, Application and Implementation section, Power Supply Recommendations section, Layout
section, Device and Documentation Support section, and Mechanical, Packaging, and Orderable Information
section ................................................................................................................................................................................... 1
•
Deleted Inverting Regulator Application ............................................................................................................................... 14
2
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SNVS585L –SEPTEMBER 2008–REVISED NOVEMBER 2014
5 Pin Configuration and Functions
PFM Package
7-Pin
Top View
7 EN
6 FB
5 NC
4 GND
3 BOOT
2 VIN
1 SW
Exposed Pad
Connect to GND
Pin Functions
PIN
TYPE
DESCRIPTION
Boot-strap input
APPLICATION INFORMATION
NAME
NO.
BOOT
3
I
I
Provides the gate voltage for the high side NFET.
Used to control regulator start-up and shutdown. See Precision Enable
and UVLO section of data sheet.
EN
7
Enable input
Connect to ground. Provides thermal connection to PCB. See
Application and Implementation.
EP
EP
6
—
I
Exposed Pad
FB
Feedback input
Feedback input to the regulator.
Ground input to regulator;
system common
GND
4
—
System ground pin.
Pin is not electrically connected inside chip. Pin does function as
thermal conductor.
NC
5
—
Not Connected
SW
VIN
1
2
O
I
Switch Pin
Switching output of regulator.
Supply input to the regulator.
Input voltage
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6 Specifications
6.1 Absolute Maximum Ratings(1)(2)
MIN
MAX
UNIT
VIN to GND
43
V
V
V
V
V
EN Pin Voltage
–0.5
-5
6
VIN
(3)
SW to GND
BOOT Pin Voltage
FB Pin Voltage
VSW + 7
7
–0.5
Power Dissipation
Junction Temperature(4)
Internally Limited
150
°C
(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of
device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or
other conditions beyond those indicated in the Recommended Operating Conditions is not implied. The Recommended Operating
Conditions indicate conditions at which the device is functional and should not be operated beyond such conditions.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
(3) The absolute-maximum specification of the ‘SW to GND’ applies to dc voltage. An extended negative voltage limit of –10 V applies to a
pulse of up to 50 ns.
(4) For soldering specifications, refer to the application report Absolute Maximum Ratings for Soldering (SNOA549).
6.2 Handling Ratings: LM22678
MIN
MAX
UNIT
Tstg
Storage temperature range
Electrostatic discharge
–65
150
°C
Human body model (HBM), per ANSI/ESDA/JEDEC JS-001, all
pins(1)
V(ESD)
–2
2
kV
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
6.3 Handling Ratings: LM22678-Q1
MIN
MAX UNIT
Tstg
Storage temperature range
Electrostatic discharge
–65
–2
150
2
°C
kV
V(ESD)
Human body model (HBM), per AEC Q100-002(1)
(1) AEC Q100-002 indicates HBM stressing is done in accordance with the ANSI/ESDA/JEDEC JS-001 specification.
6.4 Recommended Operating Conditions
MIN
MAX UNIT
VIN
Supply Voltage
4.5
42
V
Junction Temperature Range
–40
125
°C
6.5 Thermal Information
LM22678,
LM22678-Q1
THERMAL METRIC(1)(2)
UNIT
NDR
7 PINS
22
RθJA
Junction-to-ambient thermal resistance
°C/W
(1) For more information about traditional and new thermal metrics, see the application report IC Package Thermal Metrics (SPRA953).
(2) The value of RθJA for the PFM package of 22°C/W is valid if package is mounted to 1 square inch of copper. The RθJA value can range
from 20 to 30°C/W depending on the amount of PCB copper dedicated to heat transfer. See application note AN-1797 (SNVA328) for
more information.
4
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6.6 Electrical Characteristics
Typical values represent the most likely parametric norm at TA = TJ = 25°C, and are provided for reference purposes only.
Unless otherwise specified: VIN = 12 V.
PARAMETER
TEST CONDITIONS
MIN(1)
TYP(2)
MAX(1)
UNIT
LM22678-5.0
VIN = 8 V to 42 V
4.925
4.9
5.0
5.075
5.1
VFB
Feedback Voltage
V
VIN = 8 V to 42 V, –40°C ≤ TJ ≤ 125°C
LM22678-ADJ
VIN = 4.7 V to 42 V
1.266
1.259
1.285
3.4
1.304
1.311
VFB
Feedback Voltage
V
VIN = 4.7 V to 42 V, –40°C ≤ TJ ≤ 125°C
ALL OUTPUT VOLTAGE VERSIONS
VFB = 5 V
IQ
Quiescent Current
Standby Quiescent Current
Current Limit
mA
µA
A
VFB = 5 V, –40°C ≤ TJ ≤ 125°C
EN Pin = 0 V
6
40
ISTDBY
ICL
25
6.0
7.1
8.4
8.75
2
–40°C ≤ TJ ≤ 125°C
5.75
VIN = 42 V, EN Pin = 0 V, VSW = 0 V
VSW = –1 V
0.2
0.1
0.1
µA
µA
IL
Output Leakage Current
Switch On-Resistance
Oscillator Frequency
Minimum Off-time
3
0.14
0.2
RDS(ON)
Ω
kHz
ns
–40°C ≤ TJ ≤ 125°C
–40°C ≤ TJ ≤ 125°C
–40°C ≤ TJ ≤ 125°C
500
200
fO
400
100
600
300
TOFFMIN
TONMIN
IBIAS
Minimum On-time
100
230
1.6
ns
Feedback Bias Current
VFB = 1.3 V (ADJ Version Only)
Falling
nA
VEN
Enable Threshold Voltage
V
Falling, –40°C ≤ TJ ≤ 125°C
1.3
1.9
VENHYST
IEN
Enable Threshold Hysteresis
Enable Input Current
0.6
6
V
EN Input = 0 V
µA
°C
TSD
Thermal Shutdown Threshold
150
(1) MIN and MAX limits are 100% production tested at 25°C. Limits over the operating temperature range are ensured through correlation
using Statistical Quality Control (SQC) methods. Limits are used to calculate TI's Average Outgoing Quality Level (AOQL).
(2) Typical values represent most likely parametric norms at the conditions specified and are not ensured.
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6.7 Typical Characteristics
Vin = 12 V, TJ = 25°C (unless otherwise specified)
Figure 1. Efficiency vs IOUT and VIN, VOUT = 3.3 V
Figure 2. Normalized Switching Frequency vs Temperature
Figure 4. Normalized RDS(ON) vs Temperature
Figure 3. Current Limit vs Temperature
Figure 5. Feedback Bias Current vs Temperature
Figure 6. Normalized Enable Threshold Voltage vs
Temperature
6
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Typical Characteristics (continued)
Vin = 12 V, TJ = 25°C (unless otherwise specified)
Figure 8. Normalized Feedback Voltage vs Temperature
Figure 7. Standby Quiescent Current vs Input Voltage
Figure 9. Normalized Feedback Voltage vs Input Voltage
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7 Detailed Description
7.1 Overview
The LM22678 incorporates a voltage mode constant frequency PWM architecture. In addition, input voltage feed-
forward is used to stabilize the loop gain against variations in input voltage. This allows the loop compensation to
be optimized for transient performance. The power MOSFET, in conjunction with the diode, produces a
rectangular waveform at the switch pin, which swings from about zero volts to VIN. The inductor and output
capacitor average this waveform to become the regulator output voltage. By adjusting the duty cycle of this
waveform, the output voltage can be controlled. The error amplifier compares the output voltage with the internal
reference and adjusts the duty cycle to regulate the output at the desired value.
The internal loop compensation of the -ADJ option is optimized for outputs of 5 V and below. If an output voltage
of 5 V or greater is required, the -5.0 option can be used with an external voltage divider. The minimum output
voltage is equal to the reference voltage, that is, 1.285 V (typ).
7.2 Functional Block Diagram
VIN
VIN
Vcc
BOOT
INT REG, EN,UVLO
EN
ILimit
PWM Cmp.
TYPE III
COMP
-
+
-
LOGIC
+
FB
Error Amp.
VOUT
SW
OSC
1.285V
&
Soft-start
GND
8
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7.3 Feature Description
7.3.1 Precision Enable and UVLO
The precision enable input (EN) is used to control the regulator. The precision feature allows simple sequencing
of multiple power supplies with a resistor divider from another supply. Connecting this pin to ground or to a
voltage less than 1.6 V (typ) will turn off the regulator. The current drain from the input supply, in this state, is 25
µA (typ) at an input voltage of 12 V. The EN input has an internal pullup of about 6 µA. Therefore, this pin can be
left floating or pulled to a voltage greater than 2.2 V (typ) to turn the regulator on. The hysteresis on this input is
about 0.6 V (typ) above the 1.6 V (typ) threshold. When driving the enable input, the voltage must never exceed
the 6 V absolute maximum specification for this pin.
Although an internal pullup is provided on the EN pin, it is good practice to pull the input high when this feature is
not used, especially in noisy environments. This can most easily be done by connecting a resistor between VIN
and the EN pin. The resistor is required because the internal zener diode at the EN pin will conduct for voltages
above about 6 V. The current in this zener must be limited to less than 100 µA. A resistor of 470 kΩ will limit the
current to a safe value for input voltages as high 42 V. Smaller values of resistor can be used at lower input
voltages.
The LM22678 device also incorporates an input undervoltage lock-out (UVLO) feature. This prevents the
regulator from turning on when the input voltage is not great enough to properly bias the internal circuitry. The
rising threshold is 4.3 V (typ) while the falling threshold is 3.9 V (typ). In some cases, these thresholds may be
too low to provide good system performance. The solution is to use the EN input as an external UVLO to disable
the part when the input voltage falls below a lower boundary. This is often used to prevent excessive battery
discharge or early turn-on during start up. This method is also recommended to prevent abnormal device
operation in applications where the input voltage falls below the minimum of 4.5 V. Figure 10 shows the
connections to implement this method of UVLO. Equation 1 and Equation 2 can be used to determine the correct
resistor values.
(1)
(2)
Where:
Voff is the input voltage where the regulator shuts off.
Von is the voltage where the regulator turns on.
Due to the 6 µA pullup, the current in the divider should be much larger than this. A value of 20 kΩ, for RENB is a
good first choice. Also, a zener diode may be needed between the EN pin and ground, in order to comply with
the absolute maximum ratings on this pin.
Vin
RENT
EN
RENB
Figure 10. External UVLO Connections
7.3.2 Soft-Start
The soft-start feature allows the regulator to gradually reach steady-state operation, thus reducing start-up
stresses. The internal soft-start feature brings the output voltage up in about 500 µs. This time is fixed and can
not be changed. Soft-start is reset any time the part is shut down or a thermal overload event occurs.
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Feature Description (continued)
7.3.3 Boot-Strap Supply
The LM22678 device incorporates a floating high-side gate driver to control the power MOSFET. The supply for
this driver is the external boot-strap capacitor connected between the BOOT pin and SW. A good quality 10 nF
ceramic capacitor must be connected to these pins with short, wide PCB traces. One reason the regulator
imposes a minimum off-time is to ensure that this capacitor recharges every switching cycle. A minimum load of
about 5 mA is required to fully recharge the boot-strap capacitor in the minimum off-time. Some of this load can
be provided by the output voltage divider, if used.
7.3.4 Internal Loop Compensation
The LM22678 has internal loop compensation designed to provide a stable regulator over a wide range of
external power stage components.
The internal compensation of the -ADJ option is optimized for output voltages below 5 V. If an output voltage of 5
V or greater is needed, the -5.0 option with an external resistor divider can be used.
Ensuring stability of a design with a specific power stage (inductor and output capacitor) can be tricky. The
LM22678 stability can be verified using the WEBENCH Designer online circuit simulation tool at WEBENCH
Designer. A quick start spreadsheet can also be downloaded from the online product folder.
The complete transfer function for the regulator loop is found by combining the compensation and power stage
transfer functions. The LM22678 has internal type III loop compensation, as detailed in Figure 11. This is the
approximate "straight line" function from the FB pin to the input of the PWM modulator. The power stage transfer
function consists of a dc gain and a second order pole created by the inductor and output capacitor(s). Due to
the input voltage feed forward employed in the LM22678, the power stage dc gain is fixed at 20 dB. The second
order pole is characterized by its resonant frequency and its quality factor (Q). For a first pass design, the
product of inductance and output capacitance should conform to Equation 3.
(3)
Alternatively, this pole should be placed between 1.5 kHz and 15 kHz and is determined by Equation 4.
(4)
The Q factor depends on the parasitic resistance of the power stage components and is not typically in the
control of the designer. Of course, loop compensation is only one consideration when selecting power stage
components (see Application and Implementation for more details).
40
-ADJ
-5.0
35
30
25
20
15
10
5
0
100
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 11. Compensator Gain
10
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Feature Description (continued)
In general, hand calculations or simulations can only aid in selecting good power stage components. Good
design practice dictates that load and line transient testing should be done to verify the stability of the application.
Also, Bode plot measurements should be made to determine stability margins. AN-1889 How to Measure the
Loop Transfer Function of Power Supplies (SNVA364) shows how to perform a loop transfer function
measurement with only an oscilloscope and function generator.
7.4 Device Functional Modes
7.4.1 Shutdown Mode
The EN pin provides electrical ON and OFF control for the LM22678 device. When VEN is below 1.6 V, the
device is in shutdown mode. The current drain from the input supply, in this state, is 25 µA (typ) at an input
voltage of 12 V. The EN input has an internal pullup of about 6 µA. The LM22678 also incorporates an input
undervoltage lock-out (UVLO) feature. This prevents the regulator from turning on when the input voltage is not
great enough to properly bias the internal circuitry. The rising threshold is 4.3 V (typ) while the falling threshold is
3.9 V (typ).
7.4.2 Active Mode
The LM22678 device is in active mode when VEN is above the precision enable threshold and its input voltage is
above its UVLO level. The simplest way to enable the LM22678 is to connect the EN pin to VIN through a
resistor. A resistor of 470 kΩ will limit the current to a safe value for input voltages as high 42 V.
7.4.3 Current Limit
The LM22678 device has current limiting to prevent the switch current from exceeding safe values during an
accidental overload on the output. This peak current limit is found in the Electrical Characteristics table under the
heading of ICL. The maximum load current that can be provided, before current limit is reached, is determined
from Equation 5.
(5)
Where:
L is the value of the power inductor.
When the LM22678 device enters current limit, the output voltage will drop and the peak inductor current will be
fixed at ICL at the end of each cycle. The switching frequency will remain constant while the duty cycle drops. The
load current will not remain constant, but will depend on the severity of the overload and the output voltage.
For very severe overloads "short-circuit", the regulator changes to a low frequency current foldback mode of
operation. The frequency foldback is about 1/5 of the nominal switching frequency. This will occur when the
current limit trips before the minimum on-time has elapsed. This mode of operation is used to prevent inductor
current "run-away", and is associated with very low output voltages when in overload. Equation 6 can be used to
determine what level of output voltage will cause the part to change to low frequency current foldback.
(6)
Where:
Fsw is the normal switching frequency.
Vin is the maximum for the application.
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Device Functional Modes (continued)
If the overload drives the output voltage to less than or equal to Vx, the part will enter current foldback mode. If a
given application can drive the output voltage to ≤ Vx, during an overload, then a second criterion must be
checked. Equation 7 determines the maximum input voltage, when in this mode, before damage occurs.
(7)
Where:
Vsc is the value of output voltage during the overload.
Fsw is the normal switching frequency.
NOTE
If the input voltage should exceed this value while in foldback mode, the regulator and/or
the diode may be damaged.
It is important to note that the voltages in these equations are measured at the inductor. Normal trace and wiring
resistance will cause the voltage at the inductor to be higher than that at a remote load. Therefore, even if the
load is shorted with zero volts across its terminals, the inductor will still see a finite voltage. It is this value that
should be used for Vx and Vsc in the calculations. In order to return from foldback mode, the load must be
reduced to a value much lower than that required to initiate foldback. This load "hysteresis" is a normal aspect of
any type of current limit foldback associated with voltage regulators.
The safe operating area, when in short circuit mode, is shown in Figure 12. Operating points below and to the
right of the curve represent safe operation.
45
40
35
30
25
20
15
10
5
SAFE OPERATING AREA
0.0
0.2
0.4
0.6
0.8
1.0
1.2
SHORT CIRCUIT VOLTAGE (v)
Figure 12. SOA
7.4.4 Thermal Protection
Internal thermal-shutdown circuitry protects the LM22678 should the maximum junction temperature be
exceeded. This protection is activated at about 150°C, with the result that the regulator will shut down until the
temperature drops below about 135°C.
12
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Device Functional Modes (continued)
7.4.5 Duty-Cycle Limits
Ideally the regulator would control the duty cycle over the full range of zero to one. However due to inherent
delays in the circuitry, there are limits on both the maximum and minimum duty cycles that can be reliably
controlled. This in turn places limits on the maximum and minimum input and output voltages that can be
converted by the LM22678. A minimum on-time is imposed by the regulator in order to correctly measure the
switch current during a current limit event. A minimum off-time is imposed in order the re-charge the bootstrap
capacitor. Equation 8 can be used to determine the approximate maximum input voltage for a given output
voltage.
(8)
Where:
Fsw is the switching frequency.
TON is the minimum on-time.
Both values can be found in the Electrical Characteristics table.
The worst case occurs at the lowest output voltage. If the input voltage found in Equation 8 is exceeded, the
regulator will skip cycles; thus, effectively lowering the switching frequency. The consequences of this are higher
output voltage ripple and a degradation of the output voltage accuracy.
The second limitation is the maximum duty cycle before the output voltage will "dropout" of regulation. Equation 9
can be used to approximate the minimum input voltage before dropout occurs.
(9)
Where:
The values of TOFF and RDS(ON) are found in the Electrical Characteristics table.
The worst case here occurs at the highest load. In this equation, RL is the dc inductor resistance. Of course, the
lowest input voltage to the regulator must not be less than 4.5 V (typ).
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The LM22678 device is a step down dc-to-dc regulator. It is typically used to convert a higher dc voltage to a
lower dc voltage with a maximum output current of 5 A. Detailed Design Procedure can be used to select
components for the LM22678. Alternately, the WEBENCH software may be used to generate complete designs.
When generating a design, the WEBENCH software utilizes iterative design procedure and accesses
comprehensive databases of components. Go to WEBENCH Designer for more details. This section presents a
simplified discussion of the design process.
8.1.1 Output Voltage Divider Selection
For output voltages between about 1.285 V and 5 V, the -ADJ option should be used, with an appropriate voltage
divider as shown in Figure 13. Equation 10 can be used to calculate the resistor values of this divider.
(10)
A good value for RFBB is 1 kΩ. This will help to provide some of the minimum load current requirement and
reduce susceptibility to noise pick-up. The top of RFBT should be connected directly to the output capacitor or to
the load for remote sensing. If the divider is connected to the load, a local high-frequency bypass should be
provided at that location.
For output voltages of 5 V, the -5.0 option should be used. In this case no divider is needed and the FB pin is
connected to the output. The approximate values of the internal voltage divider are as follows: 7.38 kΩ from the
FB pin to the input of the error amplifier and 2.55 kΩ from there to ground.
Both the -ADJ and -5.0 options can be used for output voltages greater than 5 V, by using the correct output
divider. As mentioned in Internal Loop Compensation, the -5.0 option is optimized for output voltages of 5 V.
However, for output voltages greater than 5 V, this option may provide better loop bandwidth than the -ADJ
option, in some applications. If the -5.0 option is to be used at output voltages greater than 5 V, Equation 11
should be used to determine the resistor values in the output divider.
(11)
Again, a value of RFBB of about 1 kΩ is a good first choice.
Vout
RFBT
FB
RFBB
Figure 13. Resistive Feedback Divider
A maximum value of 10 kΩ is recommended for the sum of RFBB and RFBT to maintain good output voltage
accuracy for the -ADJ option. A maximum of 2 kΩ is recommended for the -5.0 option. For the -5.0 option, the
total internal divider resistance is typically 9.93 kΩ.
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Application Information (continued)
In all cases the output voltage divider should be placed as close as possible to the FB pin of the LM22678
device; because this is a high impedance input and is susceptible to noise pick-up.
8.1.2 Power Diode
A Schottky-type power diode is required for all LM22678 applications. Ultra-fast diodes are not recommended
and may result in damage to the IC due to reverse recovery current transients. The near ideal reverse recovery
characteristics and low forward voltage drop of Schottky diodes are particularly important for high input voltage
and low output voltage applications common to the LM22678 device. The reverse breakdown rating of the diode
should be selected for the maximum VIN, plus some safety margin. A good rule of thumb is to select a diode with
a reverse voltage rating of 1.3 times the maximum input voltage.
Select a diode with an average current rating at least equal to the maximum load current that will be seen in the
application.
8.2 Typical Applications
8.2.1 Typical Buck Regulator Application
Figure 14 shows an example of converting an input voltage range of 5.5 V to 42 V, to an output of 3.3 V at 5 A.
R
FBB
976:
VIN 4.5V to 42V
FB
LM22678-ADJ
VIN
EN
C3
10 nF
R
FBT
1.54 k:
EN
L1
4.7 PH
BOOT
+
C2
C1
6.8 PF
C7
6.8 PF
SW
VOUT 3.3V
22 PF
GND
+
D1
60V, 5A
C4
180 PF
GND
GND
Figure 14. Typical Buck Regulator Application
8.2.1.1 Design Requirements
DESIGN PARAMETERS
EXAMPLE VALUE
Driver Supply Voltage (VIN)
4.5 to 42 V
Output Voltage (VOUT)
3.3 V
RFBT
RFBB
IOUT
Calculated based on RFBB and VREF of 1.285 V.
1 kΩ to 10 kΩ
5 A
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 External Components
The following guidelines should be used when designing a step-down (buck) converter with the LM22678 device.
8.2.1.2.1.1 Inductor
The inductor value is determined based on the load current, ripple current, and the minimum and maximum input
voltages. To keep the application in continuous conduction mode (CCM), the maximum ripple current, IRIPPLE
,
should be less than twice the minimum load current. The general rule of keeping the inductor current peak-to-
peak ripple around 30% of the nominal output current is a good compromise between excessive output voltage
ripple and excessive component size and cost. Using this value of ripple current, the value of inductor (L) is
calculated using Equation 12.
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(12)
Where:
Fsw is the switching frequency.
Vin should be taken at its maximum value, for the given application.
Equation 12 provides a guide to select the value of the inductor L; the nearest standard value will then be used in
the circuit.
Once the inductor is selected, the actual ripple current can be determined by Equation 13.
(13)
Increasing the inductance will generally slow down the transient response but reduce the output voltage ripple.
Reducing the inductance will generally improve the transient response but increase the output voltage ripple.
The inductor must be rated for the peak current, IPK, in a given application, to prevent saturation. During normal
loading conditions, the peak current is equal to the load current plus 1/2 of the inductor ripple current.
During an overload condition, as well as during certain load transients, the controller may trip current limit. In this
case the peak inductor current is given by ICL, found in the Electrical Characteristics table. Good design practice
requires that the inductor rating be adequate for this overload condition.
NOTE
If the inductor is not rated for the maximum expected current, it can saturate resulting in
damage to the LM22678 and/or the power diode.
8.2.1.2.2 Input Capacitor
The input capacitor selection is based on both input voltage ripple and RMS current. Good quality input
capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the regulator current
during switch on-time. Low ESR ceramic capacitors are preferred. Larger values of input capacitance are
desirable to reduce voltage ripple and noise on the input supply. This noise may find its way into other circuitry,
sharing the same input supply, unless adequate bypassing is provided. A very approximate formula for
determining the input voltage ripple is shown in Equation 14.
(14)
Where:
Vri is the peak-to-peak ripple voltage at the switching frequency.
Another concern is the RMS current passing through this capacitor. Equation 15 determines an approximation to
this current.
(15)
The capacitor must be rated for at least this level of RMS current at the switching frequency.
All ceramic capacitors have large voltage coefficients, in addition to normal tolerances and temperature
coefficients. To help mitigate these effects, multiple capacitors can be used in parallel to bring the minimum
capacitance up to the desired value. This may also help with RMS current constraints by sharing the current
among several capacitors. Many times it is desirable to use an electrolytic capacitor on the input, in parallel with
the ceramics. The moderate ESR of this capacitor can help to damp any ringing on the input supply caused by
long power leads. This method can also help to reduce voltage spikes that may exceed the maximum input
voltage rating of the LM22678 device.
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It is good practice to include a high frequency bypass capacitor as close as possible to the LM22678 device. This
small case size, low ESR ceramic capacitor should be connected directly to the VIN and GND pins with the
shortest possible PCB traces. Values in the range of 0.47 µF to 1 µF are appropriate. This capacitor helps to
provide a low impedance supply to sensitive internal circuitry. It also helps to suppress any fast noise spikes on
the input supply that may lead to increased EMI.
8.2.1.2.3 Output Capacitor
The output capacitor is responsible for filtering the output voltage and supplying load current during transients.
Capacitor selection depends on application conditions as well as ripple and transient requirements. Best
performance is achieved with a parallel combination of ceramic capacitors and a low ESR SP™ or POSCAP™
type. Very low ESR capacitors such as ceramics reduce the output ripple and noise spikes, while higher value
electrolytics or polymer provide large bulk capacitance to supply transients. Assuming very low ESR, Equation 16
determines an approximation to the output voltage ripple.
(16)
Typically, a total value of 100 µF or greater is recommended for output capacitance.
In applications with Vout less than 3.3 V, it is critical that low ESR output capacitors are selected. This will limit
potential output voltage overshoots as the input voltage falls below the device normal operating range.
8.2.1.2.4 Boot-Strap Capacitor
The bootstrap capacitor between the BOOT pin and the SW pin supplies the gate current to turn on the N-
channel MOSFET. The recommended value of this capacitor is 10 nF and should be a good quality, low ESR
ceramic capacitor. In some cases it may be desirable to slow down the turn-on of the internal power MOSFET, in
order to reduce EMI. This can be done by placing a small resistor in series with the Cboot capacitor. Resistors in
the range of 10 Ω to 50 Ω can be used. This technique should only be used when absolutely necessary, because
it will increase switching losses and thereby reduce efficiency.
8.2.1.3 Application Curves
Figure 16. Normalized Feedback Voltage vs Input Voltage
Figure 15. Efficiency vs IOUT and VIN, VOUT = 3.3 V
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9 Power Supply Recommendations
The LM22678 device is designed to operate from an input voltage supply range between 4.5 V and 42 V. This
input supply should be well regulated and able to withstand maximum input current and maintain a stable
voltage. The resistance of the input supply rail should be low enough that an input current transient does not
cause a high enough drop at the LM22678 supply voltage that can cause a false UVLO fault triggering and
system reset. If the input supply is located more than a few inches from the LM22678 device, additional bulk
capacitance may be required in addition to the ceramic bypass capacitors. The amount of bulk capacitance is not
critical, but a 47 µF or 100 µF electrolytic capacitor is a typical choice.
10 Layout
10.1 Layout Guidelines
Board layout is critical for the proper operation of switching power supplies. First, the ground plane area must be
sufficient for thermal dissipation purposes. Second, appropriate guidelines must be followed to reduce the effects
of switching noise. Switch mode converters are very fast switching devices. In such cases, the rapid increase of
input current combined with the parasitic trace inductance generates unwanted L di/dt noise spikes. The
magnitude of this noise tends to increase as the output current increases. This noise may turn into
electromagnetic interference (EMI) and can also cause problems in device performance. Therefore, care must be
taken in layout to minimize the effect of this switching noise.
The most important layout rule is to keep the ac current loops as small as possible. Figure 17 shows the current
flow in a buck converter. The top schematic shows a dotted line which represents the current flow during the FET
switch on-state. The middle schematic shows the current flow during the FET switch off-state.
The bottom schematic shows the currents referred to as ac currents. These ac currents are the most critical
because they are changing in a very short time period. The dotted lines of the bottom schematic are the traces to
keep as short and wide as possible. This will also yield a small loop area reducing the loop inductance. To avoid
functional problems due to layout, review the PCB layout example. Best results are achieved if the placement of
the LM22678 device, the bypass capacitor, the Schottky diode, RFBB, RFBT, and the inductor are placed as shown
in Figure 17. In the layout shown, R1 = RFBB and R2 = RFBT. It is also recommended to use 2 oz copper boards
or heavier to help thermal dissipation and to reduce the parasitic inductances of board traces. See AN-1229
SIMPLE SWITCHER® PCB Layout Guidelines (SNVA054) for more information.
Figure 17. Current Flow in a Buck Application
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10.2 Layout Example
Figure 18. LM22678 Layout Example
10.3 Thermal Considerations
The components with the highest power dissipation are the power diode and the power MOSFET internal to the
LM22678 regulator. The easiest method to determine the power dissipation within the LM22678 is to measure
the total conversion losses then subtract the power losses in the diode and inductor. The total conversion loss is
the difference between the input power and the output power. An approximation for the power diode loss is
shown in Equation 17.
(17)
Where:
VD is the diode voltage drop.
An approximation for the inductor power is determined by Equation 18.
(18)
Where:
RL is the dc resistance of the inductor.
The 1.1 factor is an approximation for the ac losses.
The regulator has an exposed thermal pad to aid power dissipation. Adding multiple vias under the device to the
ground plane will greatly reduce the regulator junction temperature. Selecting a diode with an exposed pad will
also aid the power dissipation of the diode. The most significant variables that affect the power dissipation of the
regulator are output current, input voltage and operating frequency. The power dissipated while operating near
the maximum output current and maximum input voltage can be appreciable. The junction-to-ambient thermal
resistance of the LM22678 will vary with the application. The most significant variables are the area of copper in
the PC board, the number of vias under the IC exposed pad and the amount of forced air cooling provided. A
large continuous ground plane on the top or bottom PCB layer will provide the most effective heat dissipation.
The integrity of the solder connection from the IC exposed pad to the PC board is critical. Excessive voids will
greatly diminish the thermal dissipation capacity. The junction-to-ambient thermal resistance of the LM22678
PFM package is specified in the Electrical Characteristics table. See AN-2020 Thermal Design By Insight, Not
Hindsight (SNVA419) for more information.
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11 Device and Documentation Support
11.1 Documentation Support
11.1.1 Related Documentation
•
•
•
•
AN-2020 Thermal Design By Insight, Not Hindsight (SNVA419)
AN-1229 SIMPLE SWITCHER® PCB Layout Guidelines (SNVA054)
AN-1892 LM22677 Evaluation Board (SNVA366)
AN-1889 How to Measure the Loop Transfer Function of Power Supplies (SNVA364)
11.2 Related Links
The table below lists quick access links. Categories include technical documents, support and community
resources, tools and software, and quick access to sample or buy.
Table 1. Related Links
TECHNICAL
DOCUMENTS
TOOLS &
SOFTWARE
SUPPORT &
COMMUNITY
PARTS
PRODUCT FOLDER
SAMPLE & BUY
LM22678
Click here
Click here
Click here
Click here
Click here
Click here
Click here
Click here
Click here
Click here
LM22678-Q1
11.3 Trademarks
WEBENCH, SIMPLE SWITCHER are registered trademarks of Texas Instruments.
All other trademarks are the property of their respective owners.
11.4 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
11.5 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
20
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PACKAGE OPTION ADDENDUM
www.ti.com
23-Sep-2014
PACKAGING INFORMATION
Orderable Device
LM22678QTJ-5.0/NOPB
LM22678QTJ-ADJ/NOPB
LM22678QTJE-5.0/NOPB
LM22678QTJE-ADJ/NOPB
LM22678TJ-5.0/NOPB
LM22678TJ-ADJ/NOPB
LM22678TJE-5.0/NOPB
LM22678TJE-ADJ/NOPB
Status Package Type Package Pins Package
Eco Plan
Lead/Ball Finish
MSL Peak Temp
Op Temp (°C)
-40 to 125
-40 to 125
-40 to 125
-40 to 125
-40 to 125
-40 to 125
-40 to 125
-40 to 125
Device Marking
Samples
Drawing
Qty
(1)
(2)
(6)
(3)
(4/5)
ACTIVE
TO-263
TO-263
TO-263
TO-263
TO-263
TO-263
TO-263
TO-263
NDR
7
7
7
7
7
7
7
7
1000
Green (RoHS
& no Sb/Br)
CU SN
CU SN
CU SN
CU SN
CU SN
CU SN
CU SN
CU SN
Level-1-260C-UNLIM
Level-1-260C-UNLIM
Level-1-260C-UNLIM
Level-1-260C-UNLIM
Level-1-260C-UNLIM
Level-1-260C-UNLIM
Level-1-260C-UNLIM
Level-1-260C-UNLIM
LM22678
QTJ-5.0
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
NDR
NDR
NDR
NDR
NDR
NDR
NDR
1000
250
Green (RoHS
& no Sb/Br)
LM22678
QTJ-ADJ
Green (RoHS
& no Sb/Br)
LM22678
QTJ-5.0
250
Green (RoHS
& no Sb/Br)
LM22678
QTJ-ADJ
1000
1000
250
Green (RoHS
& no Sb/Br)
LM22678
TJ-5.0
Green (RoHS
& no Sb/Br)
LM22678
TJ-ADJ
Green (RoHS
& no Sb/Br)
LM22678
TJ-5.0
250
Green (RoHS
& no Sb/Br)
LM22678
TJ-ADJ
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
Addendum-Page 1
PACKAGE OPTION ADDENDUM
www.ti.com
23-Sep-2014
(4) There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.
(5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation
of the previous line and the two combined represent the entire Device Marking for that device.
(6) Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish
value exceeds the maximum column width.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
OTHER QUALIFIED VERSIONS OF LM22678, LM22678-Q1 :
Catalog: LM22678
•
Automotive: LM22678-Q1
•
NOTE: Qualified Version Definitions:
Catalog - TI's standard catalog product
•
Automotive - Q100 devices qualified for high-reliability automotive applications targeting zero defects
•
Addendum-Page 2
PACKAGE MATERIALS INFORMATION
www.ti.com
23-Sep-2014
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device
Package Package Pins
Type Drawing
SPQ
Reel
Reel
A0
B0
K0
P1
W
Pin1
Diameter Width (mm) (mm) (mm) (mm) (mm) Quadrant
(mm) W1 (mm)
LM22678QTJ-5.0/NOPB TO-263
LM22678QTJ-ADJ/NOPB TO-263
LM22678QTJE-5.0/NOPB TO-263
NDR
NDR
NDR
NDR
7
7
7
7
1000
1000
250
330.0
330.0
178.0
178.0
24.4
24.4
24.4
24.4
10.6
10.6
10.6
10.6
15.4
15.4
15.4
15.4
2.45
2.45
2.45
2.45
12.0
12.0
12.0
12.0
24.0
24.0
24.0
24.0
Q2
Q2
Q2
Q2
LM22678QTJE-ADJ/NOP TO-263
B
250
LM22678TJ-5.0/NOPB
TO-263
NDR
NDR
NDR
NDR
7
7
7
7
1000
1000
250
330.0
330.0
178.0
178.0
24.4
24.4
24.4
24.4
10.6
10.6
10.6
10.6
15.4
15.4
15.4
15.4
2.45
2.45
2.45
2.45
12.0
12.0
12.0
12.0
24.0
24.0
24.0
24.0
Q2
Q2
Q2
Q2
LM22678TJ-ADJ/NOPB TO-263
LM22678TJE-5.0/NOPB TO-263
LM22678TJE-ADJ/NOPB TO-263
250
Pack Materials-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
23-Sep-2014
*All dimensions are nominal
Device
Package Type Package Drawing Pins
SPQ
Length (mm) Width (mm) Height (mm)
LM22678QTJ-5.0/NOPB
LM22678QTJ-ADJ/NOPB
LM22678QTJE-5.0/NOPB
LM22678QTJE-ADJ/NOPB
LM22678TJ-5.0/NOPB
TO-263
TO-263
TO-263
TO-263
TO-263
TO-263
TO-263
TO-263
NDR
NDR
NDR
NDR
NDR
NDR
NDR
NDR
7
7
7
7
7
7
7
7
1000
1000
250
367.0
367.0
210.0
210.0
367.0
367.0
210.0
210.0
367.0
367.0
185.0
185.0
367.0
367.0
185.0
185.0
35.0
35.0
35.0
35.0
35.0
35.0
35.0
35.0
250
1000
1000
250
LM22678TJ-ADJ/NOPB
LM22678TJE-5.0/NOPB
LM22678TJE-ADJ/NOPB
250
Pack Materials-Page 2
MECHANICAL DATA
NDR0007A
BOTTOM SIDE OF PACKAGE
TOP SIDE OF PACKAGE
TJ7A (Rev D)
www.ti.com
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