LM22678, LM22678-Q1
SNVS585M – SEPTEMBER 2008 – REVISED OCTOBER 2020
LM22678/-Q1 42-V, 5-A SIMPLE SWITCHER® Step-Down Voltage Regulator
With Easy-to-Use Package
(> 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 response. A precision enable input
allows 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.
1 Features
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
New product available: LM61460 3-V to 36-V, 6-A
low EMI synchronous converter
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
Integrated bootstrap diode
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 new product, LM61460, offers higher efficiency,
lower stand-by quiescent current, and improved EMI
performance. See the device comparison table to
compare.Start WEBENCH design with LM61460
The LM22678 device is a member of Texas
Instruments' SIMPLE SWITCHER® family. The
SIMPLE SWITCHER concept provides for an easyto-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 distributed power applications
Test and measurement
Appliances
General-purpose wide VIN applications
3 Description
Device Information
The LM22678 switching regulator provides all of the
functions necessary to implement an efficient highvoltage 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
PART NUMBER
LM22678
LM22678-Q1
(1)
PACKAGE(1)
TO-263 (7)
BODY SIZE (NOM)
10.16 mm x 9.85 mm
For all available packages, see the orderable addendum at
the end of the data sheet.
VIN
VIN
FB
LM22678-ADJ BOOT
VOUT
SW
EN
GND
Simplified Application Schematic
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
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SNVS585M – SEPTEMBER 2008 – REVISED OCTOBER 2020
Table of Contents
1 Features............................................................................1
2 Applications..................................................................... 1
3 Description.......................................................................1
4 Revision History.............................................................. 2
5 Pin Configuration and Functions...................................3
6 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
7 Detailed Description........................................................8
7.1 Overview..................................................................... 8
7.2 Functional Block Diagram........................................... 8
7.3 Feature Description.....................................................9
7.4 Device Functional Modes..........................................11
8 Application and Implementation.................................. 14
8.1 Application Information............................................. 14
8.2 Typical Applications.................................................. 15
9 Layout.............................................................................19
9.1 Layout Guidelines..................................................... 19
9.2 Layout Example........................................................ 20
9.3 Thermal Considerations............................................20
10 Device and Documentation Support..........................21
10.1 Documentation Support.......................................... 21
10.2 Support Resources................................................. 21
10.3 Receiving Notification of Documentation Updates..21
10.4 Electrostatic Discharge Caution..............................21
10.5 Glossary..................................................................21
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision L (November 2014) to Revision M (October 2020)
Page
• Added LM61460 bullet to the Features ..............................................................................................................1
• Updated the numbering format for tables, figures and cross-references throughout the document...................1
• Changed title from LM22678/-Q1 42-V, 5-A SIMPLE SWITCHER® Step-Down Voltage Regulator With
Features to LM22678/-Q1 42-V, 5-A SIMPLE SWITCHER® Step-Down Voltage Regulator With Easy to Use
Package .............................................................................................................................................................1
Changes from Revision K (March 2013) to Revision L (November 2014)
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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5 Pin Configuration and Functions
7 EN
6 FB
5 NC
4 GND
3 BOOT
2 VIN
1 SW
Exposed Pad
Connect to GND
Figure 5-1. 7-Pin PFM Package (Top View)
Table 5-1. Pin Functions
PIN
TYPE
DESCRIPTION
APPLICATION INFORMATION
NAME
NO.
BOOT
3
I
Bootstrap input
Provides the gate voltage for the high-side NFET.
EN
7
I
Enable input
Used to control regulator start-up and shutdown. See Section 7.3.1.
EP
EP
—
Exposed pad
Connect to ground. Provides thermal connection to PCB. See Section 8.
FB
6
I
Feedback input
Feedback input to the regulator
GND
4
—
Ground input to regulator;
system common
System ground pin
NC
5
—
Not connected
Pin is not electrically connected inside chip. Pin does function as
thermal conductor.
SW
1
O
Switch pin
Switching output of regulator
VIN
2
I
Input voltage
Supply input to the regulator
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6 Specifications
6.1 Absolute Maximum Ratings
MAX UNIT(1) (2)
MIN
VIN to GND
EN Pin Voltage
SW to GND (3)
43
V
–0.5
6
V
-5
VIN
V
VSW + 7
V
7
V
150
°C
BOOT Pin Voltage
FB Pin Voltage
–0.5
Power Dissipation
Internally Limited
Junction Temperature(4)
(1)
(2)
(3)
(4)
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 Section 6.4 is not implied. The Recommended Operating Conditions indicate conditions
at which the device is functional and should not be operated beyond such conditions.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
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.
For soldering specifications, refer to the application report Absolute Maximum Ratings for Soldering (SNOA549).
6.2 Handling Ratings: LM22678
Tstg
Storage temperature range
V(ESD)
Electrostatic discharge
(1)
Human body model (HBM), per ANSI/ESDA/JEDEC JS-001, all
pins(1)
MIN
MAX
UNIT
–65
150
°C
–2
2
kV
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
6.3 Handling Ratings: LM22678-Q1
Tstg
Storage temperature range
V(ESD)
(1)
Electrostatic discharge
Human body model (HBM), per AEC
Q100-002(1)
MIN
MAX
UNIT
–65
150
°C
–2
2
kV
AEC Q100-002 indicates HBM stressing is done in accordance with the ANSI/ESDA/JEDEC JS-001 specification.
6.4 Recommended Operating Conditions
MIN
VIN
MAX
UNIT
Supply Voltage
4.5
42
V
Junction Temperature Range
–40
125
°C
6.5 Thermal Information
THERMAL METRIC(1) (2)
LM22678,
LM22678-Q1
NDR
UNIT
7 PINS
RθJA
(1)
(2)
4
Junction-to-ambient thermal resistance
22
°C/W
For more information about traditional and new thermal metrics, see the application report IC Package Thermal Metrics (SPRA953).
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.
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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)
4.925
5.0
5.075
UNIT
LM22678-5.0
VFB
Feedback Voltage
VIN = 8 V to 42 V
VIN = 8 V to 42 V, –40°C ≤ TJ ≤ 125°C
4.9
5.1
V
LM22678-ADJ
VFB
Feedback Voltage
VIN = 4.7 V to 42 V
1.266
VIN = 4.7 V to 42 V, –40°C ≤ TJ ≤
125°C
1.259
1.285
1.304
1.311
V
ALL OUTPUT VOLTAGE VERSIONS
IQ
Quiescent Current
ISTDBY
Standby Quiescent Current
ICL
Current Limit
IL
Output Leakage Current
RDS(ON)
Switch On-Resistance
fO
Oscillator Frequency
TOFFMIN
Minimum Off-time
TONMIN
Minimum On-time
IBIAS
Feedback Bias Current
VEN
Enable Threshold Voltage
VENHYST
Enable Threshold Hysteresis
IEN
Enable Input Current
TSD
Thermal Shutdown Threshold
(1)
(2)
VFB = 5 V
3.4
VFB = 5 V, –40°C ≤ TJ ≤ 125°C
6
EN Pin = 0 V
25
6.0
–40°C ≤ TJ ≤ 125°C
7.1
5.75
40
8.4
8.75
mA
µA
A
VIN = 42 V, EN Pin = 0 V, VSW = 0 V
0.2
2
µA
VSW = –1 V
0.1
3
µA
0.1
0.14
–40°C ≤ TJ ≤ 125°C
0.2
500
–40°C ≤ TJ ≤ 125°C
400
600
200
–40°C ≤ TJ ≤ 125°C
100
300
Ω
kHz
ns
100
ns
VFB = 1.3 V (ADJ Version Only)
230
nA
Falling
1.6
Falling, –40°C ≤ TJ ≤ 125°C
1.3
EN Input = 0 V
1.9
V
0.6
V
6
µA
150
°C
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).
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)
6
Figure 6-1. Efficiency vs IOUT and VIN, VOUT = 3.3 V
Figure 6-2. Normalized Switching Frequency vs
Temperature
Figure 6-3. Current Limit vs Temperature
Figure 6-4. Normalized RDS(ON) vs Temperature
Figure 6-5. Feedback Bias Current vs Temperature
Figure 6-6. Normalized Enable Threshold Voltage
vs Temperature
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Figure 6-7. Standby Quiescent Current vs Input
Voltage
Figure 6-8. Normalized Feedback Voltage vs
Temperature
Figure 6-9. 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
feedforward 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.
FB
+
TYPE III
COMP
+
-
LOGIC
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 lockout (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
can 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 turnon 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 7-1 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 7-1. External UVLO Connections
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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
cannot be changed. Soft start is reset any time the part is shut down or a thermal overload event occurs.
7.3.3 Bootstrap Supply
The LM22678 device incorporates a floating high-side gate driver to control the power MOSFET. The supply
for this driver is the external bootstrap 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 bootstrap 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 7-2. 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 or
capacitors. Due to the input voltage feedforward 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 Section 8 for more details).
10
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COMPENSATOR GAIN (dB)
40
35
-ADJ
-5.0
30
25
20
15
10
5
0
100
1k
10k
100k
1M
FREQUENCY (Hz)
10M
Figure 7-2. Compensator Gain
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 lockout (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 Section 6.6 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
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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.
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, diode, or both can
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 7-3. Operating points below and to the
right of the curve represent safe operation.
45
INPUT VOLTAGE (v)
40
35
30
25
SAFE OPERATING AREA
20
15
10
5
0.0
0.2
0.4
0.6
0.8
1.0
SHORT CIRCUIT VOLTAGE (v)
1.2
Figure 7-3. SOA
12
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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.
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 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 Section 6.6.
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 Section 6.6.
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. Section 8.2.1.2 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 8-1. 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 Section 7.3.4, 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 8-1. Resistive Feedback Divider
14
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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.
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 8-2 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.
RFBB
976:
VIN 4.5V to 42V
FB
VIN
C3
10 nF
LM22678-ADJ
EN
C2
22 PF
+
C1
6.8 PF
C7
6.8 PF
EN
RFBT
1.54 k:
L1
4.7 PH
BOOT
SW
VOUT 3.3V
GND
D1
60V, 5A
C4
180 PF
+
GND
GND
Figure 8-2. 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
Calculated based on RFBB and VREF of 1.285 V.
RFBB
1 kΩ to 10 kΩ
IOUT
5A
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 can trip current limit. In
this case the peak inductor current is given by ICL, found in Section 6.6. 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, the power diode, or both.
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 can 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 can also help with RMS current constraints by sharing the current
16
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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.
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 Bootstrap 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 can be desirable to slow down the turnon 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 8-3. Efficiency vs IOUT and VIN, VOUT = 3.3 V
Figure 8-4. Normalized Feedback Voltage vs Input
Voltage
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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.
18
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9 Layout
9.1 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 9-1 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 9-1. 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 9-1. Current Flow in a Buck Application
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9.2 Layout Example
Figure 9-2. LM22678 Layout Example
9.3 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 Section 6.6. See AN-2020 Thermal Design By Insight, Not Hindsight (SNVA419) for
more information.
20
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10 Device and Documentation Support
10.1 Documentation Support
10.1.1 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)
10.2 Support Resources
TI E2E™ support forums are an engineer's go-to source for fast, verified answers and design help — straight
from the experts. Search existing answers or ask your own question to get the quick design help you need.
Linked content is provided "AS IS" by the respective contributors. They do not constitute TI specifications and do
not necessarily reflect TI's views; see TI's Terms of Use.
10.3 Receiving Notification of Documentation Updates
To receive notification of documentation updates, navigate to the device product folder on ti.com. Click on
Subscribe to updates to register and receive a weekly digest of any product information that has changed. For
change details, review the revision history included in any revised document.
10.4 Electrostatic Discharge Caution
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled
with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may
be more susceptible to damage because very small parametric changes could cause the device not to meet its published
specifications.
10.5 Glossary
TI Glossary
This glossary lists and explains terms, acronyms, and definitions.
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.
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM22678QTJ-5.0/NOPB
ACTIVE
TO-263
NDR
7
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22678
QTJ-5.0
LM22678QTJ-ADJ/NOPB
ACTIVE
TO-263
NDR
7
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22678
QTJ-ADJ
LM22678QTJE-5.0/NOPB
ACTIVE
TO-263
NDR
7
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22678
QTJ-5.0
LM22678QTJE-ADJ/NOPB
ACTIVE
TO-263
NDR
7
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22678
QTJ-ADJ
LM22678TJ-5.0/NOPB
ACTIVE
TO-263
NDR
7
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22678
TJ-5.0
LM22678TJ-ADJ/NOPB
ACTIVE
TO-263
NDR
7
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22678
TJ-ADJ
LM22678TJE-5.0/NOPB
ACTIVE
TO-263
NDR
7
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22678
TJ-5.0
LM22678TJE-ADJ/NOPB
ACTIVE
TO-263
NDR
7
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
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)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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