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LM22674, LM22674-Q1
SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
LM22674/-Q1 42 V, 500 mA SIMPLE SWITCHER® Step-Down Voltage Regulator
with Features
1 Features
3 Description
•
•
•
•
•
The LM22674 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
capable of providing up to 500 mA 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 response. A precision enable input
allows simplification of regulator control and system
power sequencing. In shutdown mode the regulator
draws only 25 µA (typ). Built in soft-start (500 µs, typ)
saves external components. The LM22674 also has
built in thermal shutdown, and current limiting to
protect against accidental overloads.
1
•
•
•
•
•
•
•
•
•
Wide Input Voltage Range: 4.5 V to 42 V
Internally Compensated Voltage Mode Control
Stable with Low ESR Ceramic Capacitors
200 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 Boot-Strap Diode
Integrated Soft-Start
Fully WEBENCH® Enabled
LM22674-Q1 is an Automotive Grade Product
that is AEC-Q100 Grade 1 Qualified (–40°C to
+125°C Operating Junction Temperature)
SO PowerPAD (Exposed Pad)
The LM22674 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
LM22674
LM22674-Q1
PACKAGE
HSOP (8)
BODY SIZE (NOM)
4.89 mm x 3.90 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Simplified Application Schematic
VIN
VIN
FB
LM22674-ADJ BOOT
VOUT
EN
GND
SW
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.
LM22674, LM22674-Q1
SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
6.7
4
4
4
4
4
5
6
Absolute Maximum Ratings ......................................
Handling Ratings: LM22674......................................
Handling Ratings: LM22674-Q1................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Detailed Description .............................................. 8
7.1 Overview ................................................................... 8
7.2 Functional Block Diagram ......................................... 8
7.3 Feature Description................................................... 8
7.4 Device Functional Modes........................................ 11
8
Applications and Implementation ...................... 13
8.1 Application Information............................................ 13
8.2 Typical Application .................................................. 14
9 Power Supply Recommendations...................... 17
10 Layout................................................................... 17
10.1 Layout Guidelines ................................................. 17
10.2 Layout Example .................................................... 18
10.3 Thermal Considerations ........................................ 18
11 Device and Documentation Support ................. 20
11.1
11.2
11.3
11.4
Documentation Support ........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
20
20
20
20
12 Mechanical, Packaging, and Orderable
Information ........................................................... 20
4 Revision History
Changes from Revision L (April 2013) to Revision M
Page
•
Added Pin Configuration and Functions section, Handling Rating table, Thermal Information 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 .............................................................................................................................. 13
2
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Product Folder Links: LM22674 LM22674-Q1
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SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
5 Pin Configuration and Functions
8-Pin
HSOP Package
Top View
BOOT
1
8
SW
NC
2
7
VIN
NC
3
6
GND
FB
4
5
EN
Exposed Pad
Connect to GND
Pin Functions
PIN
TYPE
DESCRIPTION
APPLICATION INFORMATION
NAME
NO.
BOOT
1
I
Bootstrap input
Provides the gate voltage for the high side NFET.
EN
5
I
Precision enable pin
Used to control regulator start-up and shutdown. See Precision Enable
and UVLO section of data sheet.
EP
EP
—
Exposed Pad
Connect to ground. Provides thermal connection to PCB. See
applications information.
FB
4
I
Feedback pin
Feedback input to regulator.
GND
6
—
System ground
System ground.
NC
2, 3
—
Not Connected
Pins are not electrically connected to die. Pins do function as thermal
conductor.
VIN
7
I
Source input voltage
Input supply to regulator
SW
8
O
Switch pin
Switching output of regulator
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LM22674, LM22674-Q1
SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
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6 Specifications
6.1 Absolute Maximum Ratings (1) (2)
MIN
MAX
VIN to GND
EN Pin Voltage
SW to GND (3)
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
UNIT
43
Internally Limited
Junction Temperature
For soldering specifications, refer to Application Report Absolute Maximum Ratings for Soldering (SNOA549).
(1)
(2)
(3)
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.
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.
6.2 Handling Ratings: LM22674
Tstg
Storage temperature range
V(ESD)
(1)
Electrostatic discharge
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: LM22674-Q1
Tstg
Storage temperature range
V(ESD)
Electrostatic discharge
(1)
Human body model (HBM), per AEC Q100-002 (1)
MIN
MAX
–65
150
UNIT
°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
VIN
MIN
MAX
Supply Voltage
4.5
42
UNIT
V
Junction Temperature
–40
125
°C
6.5 Thermal Information
LM22674,
LM22674-Q1
THERMAL METRIC (1)
DDA
UNIT
8 PINS
RθJA
(1)
4
Junction-to-ambient thermal resistance
60
°C/W
For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report (SPRA953).
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SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
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 = 12V.
PARAMETER
CONDITIONS
MIN (1)
TYP (2)
MAX (1)
4.925
5.0
5.075
UNIT
LM22674-5.0
VIN = 8 V to 42 V
VFB
Feedback Voltage
VIN = 8 V to 42 V, –40°C ≤ TJ ≤
125°C
4.9
5.1
V
LM22674-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
VFB = 5 V
3.4
VFB = 5 V, –40°C ≤ TJ ≤ 125°C
6
EN Pin = 0 V
0.56
25
40
0.7
0.84
0.62
0.9
VIN = 42 V, EN Pin = 0 V, VSW = 0 V
0.2
VSW = –1 V
RDS(ON)
Switch On-Resistance
VIN = 4.7 V to 42 V, –40°C ≤ TJ ≤
125°C
fO
Oscillator Frequency
VIN = 4.7 V to 42 V, –40°C ≤ TJ ≤
125°C
TOFFMIN
Minimum Off-time
VIN = 4.7 V to 42 V, –40°C ≤ TJ ≤
125°C
TONMIN
Minimum On-time
IBIAS
Feedback Bias Current
VEN
Enable Threshold Voltage
VENHYST
Enable Voltage Hysteresis
IEN
Enable Input Current
TSD
Thermal Shutdown Threshold
mA
µA
A
2
µA
0.1
3
µA
0.2
0.24
0.32
Ω
500
400
600
kHz
200
(1)
(2)
100
VFB = 1.3 V (ADJ Version Only)
Falling
Falling, –40°C ≤ TJ ≤ 125°C
EN Input = 0 V
300
ns
100
ns
230
nA
1.6
1.3
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 guaranteed 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 guaranteed.
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6.7 Typical Characteristics
Vin = 12 V, TJ = 25°C (unless otherwise specified)
6
Figure 1. Efficiency vs IOUT and VIN, VOUT = 3.3 V
Figure 2. Normalized Switching Frequency vs Temperature
Figure 3. Current Limit vs Temperature
Figure 4. Normalized RDS(ON) vs Temperature
Figure 5. Feedback Bias Current vs Temperature
Figure 6. Normalized Enable Threshold Voltage vs
Temperature
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SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
Typical Characteristics (continued)
Vin = 12 V, TJ = 25°C (unless otherwise specified)
Figure 7. Standby Quiescent Current vs Input Voltage
Figure 8. Normalized Feedback Voltage vs Temperature
Figure 9. Normalized Feedback Voltage vs Input Voltage
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LM22674, LM22674-Q1
SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014
www.ti.com
7 Detailed Description
7.1 Overview
The LM22674 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,
produce a rectangular waveform at the switch pin, that 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 5V 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
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.
8
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Feature Description (continued)
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 LM22674 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. The following equations 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.
7.3.3 Boot-Strap Supply
The LM22674 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.
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Feature Description (continued)
7.3.4 Internal Compensation
The LM22674 device 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
LM22674 stability can be verified using the WEBENCH Designer online circuit simulation tool. 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 LM22674 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 feedforward employed in the LM22674, 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 given 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 the Typical Application section for more details.
COMPENSATOR GAIN (dB)
40
35
-ADJ
-5.0
30
25
20
15
10
5
0
100
1k
10k
100k
1M
FREQUENCY (Hz)
10M
Figure 11. 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.
10
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7.4 Device Functional Modes
7.4.1 Current Limit
The LM22674 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 LM22674 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.
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 gives 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.
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Device Functional Modes (continued)
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 12. SOA
7.4.2 Thermal Protection
Internal thermal shutdown circuitry protects the LM22674 should the maximum junction temperature be
exceeded. This protection is activated at about 150°C, with the result that the regulator will shutdown until the
temperature drops below about 135°C.
7.4.3 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 LM22674. 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 parameters are found in the Electrical Characteristics table.
The worst case occurs at the lowest output voltage. If the input voltage, found in the above equation, is
exceeded, the regulator will skip cycles, 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).
12
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8 Applications 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 LM22674 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 500 mA. Detailed Design Procedure can be used to select
components for the LM22674 device. 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: 7.38kΩ 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 the Internal Compensation section, 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)
A value of RFBB of about 1kΩ 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 LM22674,
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 LM22674 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 LM22674. 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 Application
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 0.5 A.
RFBB
976:
VIN 4.5V to 42V
FB
VIN
EN
EN
C2
22 PF
+
C1
2.2 PF
C3
10 nF
LM22674-ADJ
BOOT
GND
RFBT
1.54 k:
L1
39 PH
SW
D1
60V, 1A
VOUT 3.3V
C4
22 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
Calculated based on RFBB and VREF of 1.285 V.
RFBB
1 kΩ to 10 kΩ
IOUT
500 mA
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 LM22674 device.
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8.2.1.2.2 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-topeak 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.
(12)
Where:
Fsw is the switching frequency.
Vin should be taken at its maximum value, for the given application.
The above formula 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 found from 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 LM22674 and/or the power diode.
8.2.1.2.3 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 gives an approximation to this
current.
(15)
The capacitor must be rated for at least this level of RMS current at the switching frequency.
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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 LM22674.
It is good practice to include a high frequency bypass capacitor as close as possible to the LM22674. 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.
8.2.1.2.4 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
gives 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.
8.2.1.2.5 Boot-Strap Capacitor
The bootstrap capacitor between the BOOT pin and the SW pin supplies the gate current to turn on the Nchannel 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 Curve
Figure 15. Efficiency vs IOUT and VIN, VOUT = 3.3 V
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9 Power Supply Recommendations
The LM22674 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 LM22674 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 LM22674, 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 16 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 LM22674, the bypass capacitor, the Schottky diode, RFFB, RFFT, and the inductor are placed as shown in the
example. Note that, in the layout shown, R1 = RFBB and R2 = RFBT. It is also recommended to use 2oz copper
boards or heavier to help thermal dissipation and to reduce the parasitic inductances of board traces. See AN1229 SIMPLE SWITCHER® PCB Layout Guidelines (SNVA054) for more information.
Figure 16. Current Flow in a Buck Application
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10.2 Layout Example
10.3 Thermal Considerations
The components with the highest power dissipation are the power diode and the power MOSFET internal to the
LM22674 regulator. The easiest method to determine the power dissipation within the LM22674 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:
(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 LM22674 will vary with the application. The most significant variables are the area of copper in
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Thermal Considerations (continued)
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 LM22674 SO
PowerPAD 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-1895 LM22671 Evaluation Board (SNVA368)
• AN-1889 How to Measure the Loop Transfer Function of Power Supplies (SNVA364)
11.1.2 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
PARTS
PRODUCT FOLDER
SAMPLE & BUY
TECHNICAL
DOCUMENTS
TOOLS &
SOFTWARE
SUPPORT &
COMMUNITY
LM22674
Click here
Click here
Click here
Click here
Click here
LM22674-Q1
Click here
Click here
Click here
Click here
Click here
11.2 Trademarks
SIMPLE SWITCHER, WEBENCH are registered trademarks of Texas Instruments.
All other trademarks are the property of their respective owners.
11.3 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.
11.4 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.
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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)
LM22674MR-5.0/NOPB
ACTIVE SO PowerPAD
DDA
8
95
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
5.0
LM22674MR-ADJ/NOPB
ACTIVE SO PowerPAD
DDA
8
95
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
ADJ
LM22674MRE-5.0/NOPB
ACTIVE SO PowerPAD
DDA
8
250
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
5.0
LM22674MRE-ADJ/NOPB
ACTIVE SO PowerPAD
DDA
8
250
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
ADJ
LM22674MRX-5.0/NOPB
ACTIVE SO PowerPAD
DDA
8
2500
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
5.0
LM22674MRX-ADJ/NOPB
ACTIVE SO PowerPAD
DDA
8
2500
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
ADJ
LM22674QMR-5.0/NOPB
ACTIVE SO PowerPAD
DDA
8
95
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
Q5.0
LM22674QMR-ADJ/NOPB
ACTIVE SO PowerPAD
DDA
8
95
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
QADJ
LM22674QMRE-5.0/NOPB
ACTIVE SO PowerPAD
DDA
8
250
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
Q5.0
LM22674QMRE-ADJ/NOPB
ACTIVE SO PowerPAD
DDA
8
250
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
L22674
QADJ
(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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