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LM22677, LM22677-Q1
SNVS582O – SEPTEMBER 2008 – REVISED AUGUST 2015
LM22677/-Q1 42-V, 5-A SIMPLE SWITCHER®, Step-Down Voltage Regulator
With Features
1 Features
3 Description
•
•
•
•
•
The LM22677 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 (> 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. The default switching frequency is set at
500 kHz, thus allowing for small external components
and good transient response. In addition, the
frequency can be adjusted over a range of 200 kHz
to 1 MHz with a single external resistor. The internal
oscillator can be synchronized to a system clock or to
the oscillator of another regulator. A precision enable
input allows simplification of regulator control and
system power sequencing. In shutdown mode the
regulator draws only 25 µA (typical). The LM22677
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
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
500-kHz Default Switching Frequency
Adjustable Switching Frequency and
Synchronization
–40°C to +125°C Operating Junction Temperature
Range
Precision Enable Input
Integrated Boot-Strap Diode
Integrated Soft-Start
Fully WEBENCH® Enabled
LM22677-Q1 is an Automotive-Grade Product
that is AEC-Q100 Grade 1 Qualified (–40°C to
+125°C Operating Junction Temperature)
NDR (Exposed Pad)
2 Applications
•
•
•
•
Industrial Control
Telecom and Datacom Systems
Embedded Systems
Conversions from Standard 24-V, 12-V and 5-V
Input Rails
The LM22677 device is a member of Texas
Instruments' SIMPLE SWITCHER® family. The
SIMPLE SWITCHER concept provides for an easy-touse 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.
Device Information(1)
Simplified Application Schematic
PART NUMBER
VIN
VIN
FB
LM22677
LM22677-Q1
LM22677-ADJ BOOT
VOUT
RT/SYNC EN
GND
SW
PACKAGE
TO-263 (7)
BODY SIZE (NOM)
10.16 mm x 9.85 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
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.
LM22677, LM22677-Q1
SNVS582O – SEPTEMBER 2008 – REVISED AUGUST 2015
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 ......................................
ESD Ratings: LM22677 ............................................
ESD Ratings: LM22677-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........................................ 12
8
Applications and Implementation ...................... 15
8.1 Application Information............................................ 15
8.2 Typical Application .................................................. 16
9 Power Supply Recommendations...................... 19
10 Layout................................................................... 19
10.1 Layout Guidelines ................................................. 19
10.2 Layout Example .................................................... 20
10.3 Thermal Considerations ........................................ 21
11 Device and Documentation Support ................. 22
11.1
11.2
11.3
11.4
11.5
Documentation Support ........................................
Related Links ........................................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
22
22
22
22
22
12 Mechanical, Packaging, and Orderable
Information ........................................................... 22
4 Revision History
Changes from Revision N (November 2014) to Revision O
Page
•
Changed the word Pin to Input............................................................................................................................................... 1
•
Changed the Q grade part number corrected to LM22677 from LM2677'; added icon for Reference Design to top navs ... 1
•
Changed "PFM" Package to "NDR" Package ....................................................................................................................... 3
Changes from Revision M (April 2013) to Revision N
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 ............................................................................................................................... 15
2
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SNVS582O – SEPTEMBER 2008 – REVISED AUGUST 2015
5 Pin Configuration and Functions
NDR Package
7-Pin TO-263
Top View
7 EN
6 FB
5 RT/SYNC
4 GND
3 BOOT
2 VIN
1 SW
Exposed Pad
Connect to GND
Pin Functions
PIN
NAME
NO.
TYPE (1)
DESCRIPTION
APPLICATION INFORMATION
SW
1
O
Switch output
Switching output of regulator
VIN
2
I
Input voltage
Supply input to the regulator
BOOT
3
I
Bootstrap input
Provides the gate voltage for the high side N-FET
GND
4
—
Ground input to
regulator; system
common
System ground pin
RT/SYNC
5
I
Oscillator mode control
input
Used to control oscillator mode of regulator. See Switching Frequency
Adjustment and Synchronization section of data sheet.
FB
6
I
Feedback input
Feedback input to regulator
EN
7
I
Enable input
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 and Implementation.
(1)
O = Output, I = Input, G = Ground, P = Power
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SNVS582O – SEPTEMBER 2008 – REVISED AUGUST 2015
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6 Specifications
6.1 Absolute Maximum Ratings (1) (2)
MIN
MAX
VIN to GND
UNIT
43
V
EN Pin Voltage
–0.5
6
V
RT/SYNC Pin Voltage
–0.5
7
V
SW to GND
(3)
–5
BOOT Pin Voltage
FB Pin Voltage
VIN
V
VSW + 7
V
7
V
–0.5
Power Dissipation
Internally Limited
Junction Temperature
150
°C
For soldering specifications, refer to Application Report Absolute Maximum Ratings for Soldering (SNOA549).
Storage temperature, Tstg
(1)
(2)
(3)
–65
150
°C
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, 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 ESD Ratings: LM22677
V(ESD)
(1)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001
VALUE
UNIT
±2000
V
(1)
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
6.3 ESD Ratings: LM22677-Q1
V(ESD)
(1)
Human-body model (HBM), per AEC Q100-002 (1)
Electrostatic discharge
VALUE
UNIT
±2000
V
AEC Q100-002 indicates that HBM stressing shall be in accordance with the ANSI/ESDA/JEDEC JS-001 specification.
6.4 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
VIN
MIN
MAX
Supply Voltage
4.5
42
UNIT
V
Junction Temperature
–40
125
°C
6.5 Thermal Information
LM22677
THERMAL METRIC (1) (2)
NDR (TO-263)
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 Semiconductor and IC Package Thermal Metrics application
report (SPRA953).
The value of RθJA for the NDR (TJ) 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 TO-263
THIN Package (SNVA328) for more information.
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SNVS582O – SEPTEMBER 2008 – REVISED AUGUST 2015
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
LM22677-5.0
VFB
VIN = 8 V to 42 V
Feedback Voltage
VIN = 8 V to 42 V, –40°C ≤ TJ ≤ 125°C
4.9
5.1
V
LM22677-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
VFB = 5 V
3.4
VFB = 5 V, –40°C ≤ TJ ≤ 125°C
6
EN Pin = 0 V
6.0
–40°C ≤ TJ ≤ 125°C
25
40
7.1
8.4
5.75
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
–40°C ≤ TJ ≤ 125°C
100
600
200
300
Ω
kHz
ns
100
ns
VFB = 1.3 V (ADJ Version Only)
230
nA
Falling
1.6
VEN
Enable Threshold Voltage
VENHYST
Enable Voltage Hysteresis
IEN
Enable Input Current
EN Input = 0 V
6
µA
FSYNC
Maximum Synchronization Frequency
VSYNC = 3.5 V, 50% duty-cycle
1
MHz
VSYNC
Synchronization Threshold Voltage
1.75
V
TSD
Thermal Shutdown Threshold
150
°C
(1)
(2)
Falling, –40°C ≤ TJ ≤ 125°C
1.3
1.9
0.6
V
V
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 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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SNVS582O – SEPTEMBER 2008 – REVISED AUGUST 2015
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
Figure 10. Switching Frequency vs RT/SYNC Resistor
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SNVS582O – SEPTEMBER 2008 – REVISED AUGUST 2015
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7 Detailed Description
7.1 Overview
The LM22677 device 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 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 (typical).
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
RT/SYNC
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 (typical) will turn off the regulator. The current drain from the input supply, in this state, is
25 µA (typical) 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 (typical) to turn the regulator on. The hysteresis on
this input is about 0.6 V (typical) above the 1.6 V (typical) 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 LM22677 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 (typical) while the falling threshold is 3.9 V (typical). 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 11 shows the connections to
implement this method of UVLO. Equation 1 and Equation 2 can be used to determine the correct resistor
values.
(1)
where
•
•
Voff is the input voltage where the regulator shuts off.
Von is the voltage where the regulator turns on.
(2)
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 11. 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 Switching Frequency Adjustment and Synchronization
The LM22677 will operate in three different modes, depending on the condition of the RT/SYNC pin. With the
RT/SYNC pin floating, the regulator will switch at the internally set frequency of 500 kHz (typ). With a resistor in
the range of 25 kΩ to 200 kΩ, connected from RT/SYNC to ground, the internal switching frequency can be
adjusted from 1 MHz to 200 kHz. Figure 12 shows the typical curve for switching frequency versus the external
resistance connected to the RT/SYNC pin. The accuracy of the switching frequency, in this mode, is slightly
worse than that of the internal oscillator; about ±25% is to be expected. Finally, an external clock can be applied
to the RT/SYNC pin to allow the regulator to synchronize to a system clock or another LM22677. The mode is
set during start-up of the regulator. When the LM22677 is enabled, or after VIN is applied, a weak pullup is
connected to the RT/SYNC pin and, after approximately 100 µs, the voltage on the pin is checked against a
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Feature Description (continued)
threshold of about 0.8 V. With the RT/SYNC pin open, the voltage floats above this threshold, and the mode is
set to run with the internal clock. With a frequency set resistor present, an internal reference holds the pin
voltage at 0.8 V; the resulting current sets the mode to allow the resistor to control the clock frequency. If the
external circuit forces the RT/SYNC pin to a voltage much greater or less than 0.8 V, the mode is set to allow
external synchronization. The mode is latched until either the EN or the input supply is cycled.
The choice of switching frequency is governed by several considerations. As an example, lower frequencies may
be desirable to reduce switching losses or improve duty cycle limits. Higher frequencies, or a specific frequency,
may be desirable to avoid problems with EMI or reduce the physical size of external components. The flexibility
of increasing the switching frequency above 500 kHz can also be used to operate outside a critical signal
frequency band for a given application. Keep in mind that the values of inductor and output capacitor cannot be
reduced dramatically, by operating above 500 kHz. This is true because the design of the internal loop
compensation restricts the range of these components.
Frequency synchronization requires some care. First the external clock frequency must be greater than the
internal clock frequency, and less than 1 MHz. The maximum internal switching frequency is specified in the
Electrical Characteristics table.
NOTE
The frequency adjust feature and the synchronization feature can not be used
simultaneously.
The synchronizing frequency must always be greater than the internal clock frequency. Secondly, the RT/SYNC
pin must see a valid high or low voltage, during start-up, in order for the regulator to go into the synchronizing
mode (see above). Also, the amplitude of the synchronizing pulses must comport with VSYNC levels found in the
Electrical Characteristics table. The regulator will synchronize on the rising edge of the external clock. If the
external clock is lost during normal operation, the regulator will revert to the 500 kHz (typical) internal clock.
If the frequency synchronization feature is used, current limit foldback is not operational (see the Current Limit
section for details).
Figure 12. Switching Frequency vs RT/SYNC Resistor
10
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Feature Description (continued)
7.3.4 Self Synchronization
It is possible to synchronize multiple LM22677 regulators together to share the same switching frequency. This
can be done by tying the RT/SYNC pins together through a MOSFET and connecting a 1 kΩ resistor to ground
at each pin. Figure 13 shows this connection. The gate of the MOSFET should be connected to the regulator
with the highest output voltage. Also, the EN pins of both regulators should be tied to the common system
enable, in order to properly initialize both regulators. The operation is as follows: When the regulators are
enabled, the outputs are low and the MOSFET is off. The 1 kΩ resistors pull the RT/SYNC pins low, thus
enabling the synchronization mode. These resistors are small enough to pull the RT/SYNC pin low, rather than
activate the frequency adjust mode. Once the output voltage of one of the regulators is sufficient to turn on the
MOSFET, the two RT/SYNC pins are tied together and the regulators will run in synchronization mode. The two
regulators will be clocked at the same frequency but slightly phase shifted according to the minimum off-time of
the regulator with the fastest internal oscillator. The slight phase shift helps to reduce stress on the input
capacitors of the regulator. It is important to choose a MOSFET with a low gate threshold voltage so that the
MOSFET will be fully enhanced. Also, a MOSFET with low inter-electrode capacitance is required. The 2N7002
is a good choice.
ENABLE
EN
EN
LM22677
LM22677
RT/SYNC
RT/SYNC
2N7002
1k
1k
Vout
Figure 13. Self Synchronizing Setup
7.3.5 Boot-Strap Supply
The LM22677 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.
7.3.6 Internal Compensation
The LM22677 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
LM22677 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 LM22677 has internal type III loop compensation, as detailed in Figure 14. 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 LM22677, 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)
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Feature Description (continued)
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 Applications and Implementation 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 14. 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 Current Limit
The LM22677 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.
where
•
L is the value of the power inductor.
(5)
When the LM22677 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.
12
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Device Functional Modes (continued)
where
•
•
Fsw is the normal switching frequency.
Vin is the maximum for the application.
(6)
If the overload drives the output voltage to less than or equal to Vx, the device 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:
where
•
•
Vsc is the value of output voltage during the overload.
Fsw is the normal switching frequency.
(7)
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.
If the frequency synchronization feature is used, the current limit frequency fold-back is not operational, and the
system may not survive a hard short-circuit at the output.
The safe operating areas, when in short circuit mode, are shown in Figure 15 through Figure 17, for different
switching frequencies. Operating points below and to the right of the curve represent safe operation.
45
45
40
40
35
35
INPUT VOLTAGE (v)
INPUT VOLTAGE (v)
NOTE
Figure 15, Figure 16, and Figure 17 curves are not valid when the LM22677 is in
frequency synchronization mode.
30
25
SAFE OPERATING AREA
20
15
10
30
25
SAFE OPERATING AREA
20
15
10
5
5
0.0
0.2
0.4
0.6
0.8
1.0
SHORT CIRCUIT VOLTAGE (v)
1.2
0.0
Figure 15. SOA at 300 kHz
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0.2
0.4
0.6
0.8
1.0
SHORT CIRCUIT VOLTAGE (v)
1.2
Figure 16. SOA at 500 kHz
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Device Functional Modes (continued)
45
INPUT VOLTAGE (v)
40
35
30
25
20
SAFE OPERATING AREA
15
10
5
0.0
0.2
0.4
0.6
0.8
1.0
SHORT CIRCUIT VOLTAGE (v)
1.2
Figure 17. SOA at 800 kHz
7.4.2 Thermal Protection
Internal thermal shutdown circuitry protects the LM22677 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.
7.4.3 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 LM22677. 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.
where
•
•
Fsw is the switching frequency.
TON is the minimum on-time.
(8)
If the frequency adjust feature is used, that value should be used for Fsw. Nominal values should be used. The
worst case is lowest output voltage, and highest switching frequency. If this input voltage 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.
where
•
The values of TOFF and RDS(ON) are found in the Electrical Characteristics table.
(9)
The worst case here is highest switching frequency and highest load. In Equation 9, RL is the dc inductor
resistance. Of course, the lowest input voltage to the regulator must not be less than 4.5 V (typical).
14
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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 LM22677 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. The Detailed Design Procedure can be used to select
components for the LM22677. Alternately, the WEBENCH software may be used to generate complete designs.
When generating a design, the WEBENCH software utilizes iterative design procedures and accesses
comprehensive databases of components. Go to WEBENCH Designer for more details.
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 18. 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 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 1 kΩ is a good first choice.
Vout
RFBT
FB
RFBB
Figure 18. 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.
In all cases the output voltage divider should be placed as close as possible to the FB pin of the LM22677,
because this is a high impedance input and is susceptible to noise pick-up.
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Application Information (continued)
8.1.2 Power Diode
A Schottky-type power diode is required for all LM22677 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 LM22677. 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
Figure 19 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
EN
C2
22 PF
+
C3
10 nF
LM22677-ADJ
EN
C1
6.8 PF
C7
6.8 PF
RT/SYNC
R3
GND
RFBT
1.54 k:
L1
4.7 PH
BOOT
SYNC
SW
VOUT 3.3V
D1
60V, 5A
C4
180 PF
+
GND
GND
Figure 19. Typical Buck Regulator Application
8.2.1 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
8.2.2 Detailed Design Procedure
8.2.2.1 External Components
The following guidelines should be used when designing a step-down (buck) converter with the LM22677.
8.2.2.2 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-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.
where
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•
•
Fsw is the switching frequency.
Vin should be taken at its maximum value, for the given application.
(12)
The formula in 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 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 LM22677 and/or the power diode.
8.2.2.3 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.
where
•
Vri is the peak-to-peak ripple voltage at the switching frequency.
(14)
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.
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 LM22677.
It is good practice to include a high frequency bypass capacitor as close as possible to the LM22677. 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.
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8.2.2.4 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 polymers 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.
If the switching frequency is set higher than 500 kHz, the capacitance value may not be reduced proportionally
due to stability requirements. The internal compensation is optimized for circuits with a 500 kHz switching
frequency. See the Internal Compensation section for more details.
8.2.2.5 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.
8.2.3 Application Curves
Figure 20. Efficiency vs IOUT and VIN, VOUT = 3.3 V
18
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Figure 21. Switching Frequency vs RT/SYNC Resistor
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9 Power Supply Recommendations
The LM22677 is designed to operate from an input voltage supply range between 4.5 V and 42 V. This input
supply should be able to withstand the maximum input current and maintain a voltage above 4.5 V. 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 LM22677 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 LM22677 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 22 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 LM22677, the bypass capacitor, the Schottky diode, RFBB, RFBT 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 2-oz copper
boards or heavier to help thermal dissipation and to reduce the parasitic inductances of board traces. See
application note AN-1229 SIMPLE SWITCHER ® PCB Layout Guidelines (SNVA054) for more information.
Figure 22. Current Flow in a Buck Application
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10.2 Layout Example
Figure 23. PCB Layout Example
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10.3 Thermal Considerations
The components with the highest power dissipation are the power diode and the power MOSFET internal to the
LM22677 regulator. The easiest method to determine the power dissipation within the LM22677 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 in
Equation 17.
where
•
VD is the diode voltage drop.
(17)
An approximation for the inductor power is shown in Equation 18.
where
•
•
RL is the dc resistance of the inductor.
The 1.1 factor is an approximation for the ac losses.
(18)
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 LM22677 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 continuos ground plane on the top of 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 LM22677 NDR
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-1889 How to Measure the Loop Transfer Function of Power Supplies (SNVA364)
• AN-1892 LM22677 Evaluation Board (SNVA366)
• AN-1229 SIMPLE SWITCHER ® PCB Layout Guidelines (SNVA054)
• AN-2020 Thermal Design By Insight, Not Hindsight (SNVA419)
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
PARTS
PRODUCT FOLDER
SAMPLE & BUY
TECHNICAL
DOCUMENTS
TOOLS &
SOFTWARE
SUPPORT &
COMMUNITY
LM22677
Click here
Click here
Click here
Click here
Click here
LM22677-Q1
Click here
Click here
Click here
Click here
Click here
11.3 Trademarks
SIMPLE SWITCHER, WEBENCH 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.
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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)
LM22677QTJ-ADJ/NOPB
ACTIVE
TO-263
NDR
7
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22677
QTJ-ADJ
LM22677QTJE-5.0/NOPB
ACTIVE
TO-263
NDR
7
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22677
QTJ-5.0
LM22677QTJE-ADJ/NOPB
ACTIVE
TO-263
NDR
7
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22677
QTJ-ADJ
LM22677TJ-5.0/NOPB
ACTIVE
TO-263
NDR
7
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22677
TJ-5.0
LM22677TJ-ADJ/NOPB
ACTIVE
TO-263
NDR
7
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22677
TJ-ADJ
LM22677TJE-5.0/NOPB
ACTIVE
TO-263
NDR
7
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22677
TJ-5.0
LM22677TJE-ADJ/NOPB
ACTIVE
TO-263
NDR
7
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM22677
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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