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LM25118
SNVS726F – JULY 2011 – REVISED MARCH 2018
LM25118 Wide Voltage Range Buck-Boost Controller
1 Features
3 Description
•
•
•
The LM25118 wide voltage range Buck-Boost
switching regulator controller features all of the
functions necessary to implement a highperformance, cost-efficient Buck-Boost regulator
using a minimum of external components. The BuckBoost topology maintains output voltage regulation
when the input voltage is either less than or greater
than the output voltage making it especially suitable
for automotive applications. The LM25118 operates
as a buck regulator while the input voltage is
sufficiently greater than the regulated output voltage
and gradually transitions to the buck-boost mode as
the input voltage approaches the output. This dualmode approach maintains regulation over a wide
range of input voltages with optimal conversion
efficiency in the buck mode and a glitch-free output
during mode transitions. This easy-to-use controller
includes drivers for the high-side buck MOSFET and
the low-side boost MOSFET. The control method of
the regulator is based upon current mode control
using an emulated current ramp. Emulated current
mode control reduces noise sensitivity of the pulsewidth modulation circuit, allowing reliable control of
the very small duty cycles necessary in high input
voltage applications. Additional protection features
include current limit, thermal shutdown, and an
enable input. The device is available in a powerenhanced, 20-pin HTSSOP package featuring an
exposed die attach pad to aid thermal dissipation.
Input Voltage Operating Range From 3 V to 42 V
Emulated Peak Current Mode Control
Smooth Transition Between Step-Down and StepUp Modes
Switching Frequency Programmable to 500 KHz
Oscillator Synchronization Capability
Internal High Voltage Bias Regulator
Integrated High and Low-Side Gate Drivers
Programmable Soft-Start Time
Ultra-Low Shutdown Current
Enable Input
Wide Bandwidth Error Amplifier
1.5% Feedback Reference Accuracy
Thermal Shutdown
Package: 20-Pin HTSSOP (Exposed Pad)
Create a Custom Design Using the LM25118 With
the WEBENCH® Power Designer
1
•
•
•
•
•
•
•
•
•
•
•
•
2 Applications
Industrial Buck-Boost Supplies
Device Information(1)
PART NUMBER
PACKAGE
LM25118
BODY SIZE (NOM)
HTSSOP (20)
6.50 mm × 4.40 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Simplified Schematic
Efficiency vs VIN and IOUT, VOUT = 12 V
VIN
100
VIN (1)
UVLO (2)
(16) VCC
VEN(OFF)
0V
VOUT
ON
(20) HS
EN (4)
OFF
LM25118MH
(12) CS
SS (7)
x
90
x
x
1 AMP
x
x
2 AMP
85
x
80
(15) LO
(10) VOUT
75
0
AGND (6)
x
x
(13) CSG
RAMP (5)
RT (3)
3 AMP
95
(19) HO
EFFICIENCY (%)
VCCX (17)
VEN(ON)
xx
x
x
x
xx xx
xx
xxx
(18) HB
SYNC (11)
10
20
30
40
50
(8) FB
VIN (V)
PGND (14)
(9) COMP
DAP (EP)
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.
LM25118
SNVS726F – JULY 2011 – REVISED MARCH 2018
www.ti.com
Table of Contents
1
2
3
4
5
6
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
6.1
6.2
6.3
6.4
6.5
6.6
7
1
1
1
2
4
6
Absolute Maximum Ratings ...................................... 6
ESD Ratings.............................................................. 6
Recommended Operating Conditions....................... 6
Thermal Information .................................................. 6
Electrical Characteristics........................................... 7
Typical Characteristics ............................................ 10
Detailed Description ............................................ 12
7.1
7.2
7.3
7.4
Overview .................................................................
Functional Block Diagram .......................................
Feature Description.................................................
Device Functional Modes........................................
12
12
13
21
8
Application and Implementation ........................ 23
8.1 Application Information............................................ 23
8.2 Typical Application .................................................. 23
9
Power Supply Recommendations...................... 34
9.1 Bias Power Dissipation Reduction .......................... 34
9.2 Thermal Considerations .......................................... 35
10 Layout................................................................... 35
10.1 Layout Guidelines ................................................. 35
10.2 Layout Example .................................................... 35
11 Device and Documentation Support ................. 36
11.1
11.2
11.3
11.4
11.5
11.6
Device Support......................................................
Receiving Notification of Documentation Updates
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
36
36
36
36
36
36
12 Mechanical, Packaging, and Orderable
Information ........................................................... 37
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision E (July 2017) to Revision F
•
Page
Deleted roque text from ESD Ratings table. ......................................................................................................................... 6
Changes from Revision D (August 2014) to Revision E
Page
•
Moved the automotive device to a standalone data sheet (SNVSAX7) ................................................................................. 1
•
Changed language of WEBENCH list item; added additional content and links for WEBENCH further in data sheet ......... 1
•
Moved the storage temp parameters to the Absolute Maximum Ratings .............................................................................. 6
•
Changed junction temperature range from: ±150°C max to: –40°C to 150°C ...................................................................... 6
•
Changed Handling Ratings table to ESD Ratings .................................................................................................................. 6
•
Moved the junction temperature ranges in the Electrical Characteristics table to the test conditions statement .................. 7
•
Removed the VOLL minimum value ......................................................................................................................................... 8
•
Removed the VOLH minimum value......................................................................................................................................... 8
•
Added Community Resources section ................................................................................................................................ 36
Changes from Revision C (Novemeber 2013) to Revision D
Page
•
Changed data sheet flow and layout to conform with new TI standards. Added the following sections: Device
Information Table, Application and Implementation; Power Supply Recommendations; Layout; Device and
Documentation Support; Mechanical, Packaging, and Ordering Information ........................................................................ 1
•
Changed Removed footnote to "VCC, LO, VCCX, UVLO to GND" ....................................................................................... 6
Changes from Revision B (April, 2013) to Revision C
Page
•
Deleted VIN Range on Functional Block Diagram................................................................................................................. 12
•
Changed Figure 19............................................................................................................................................................... 24
•
Changed VIN Specification................................................................................................................................................... 24
2
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SNVS726F – JULY 2011 – REVISED MARCH 2018
•
Changed L1 Buck Mode Value............................................................................................................................................. 26
•
Changed IRIPPLE(BUCK) Value .................................................................................................................................................. 26
•
Changed Inductor Selection – L1 Text ................................................................................................................................. 26
•
Changed Equation 16........................................................................................................................................................... 26
•
Changed Equation 17........................................................................................................................................................... 26
•
Added Efficiency Parameter ................................................................................................................................................. 26
•
Changed I1(PEAK)Value........................................................................................................................................................... 27
•
Changed I2(PEAK) Value.......................................................................................................................................................... 27
•
Changed R13 = RSENSE Text ................................................................................................................................................ 27
•
Added Equation 20 .............................................................................................................................................................. 27
•
Added Equation 21 .............................................................................................................................................................. 27
•
Changed Equation 24 .......................................................................................................................................................... 27
•
Changed Equation 25 .......................................................................................................................................................... 27
•
Changed R13(BUCK) Value ..................................................................................................................................................... 27
•
Changed R13(BUCK-BOOST) Value ............................................................................................................................................ 27
•
Added Inductor Current Limit Calculation Section................................................................................................................ 28
•
Added Equation 29 .............................................................................................................................................................. 28
•
Added Equation 31 .............................................................................................................................................................. 28
•
Changed Equation 34........................................................................................................................................................... 28
•
Changed ESRMAX Value ....................................................................................................................................................... 28
•
Deleted Previous Equation 17 .............................................................................................................................................. 28
•
Deleted Previous Equation 18 .............................................................................................................................................. 28
•
Changed C1 – C5 = Input Capacitors Text .......................................................................................................................... 29
•
Changed R8, R9 Text........................................................................................................................................................... 30
•
Changed R1, R3, C21 Text .................................................................................................................................................. 30
•
Changed Equation 45 .......................................................................................................................................................... 30
•
Changed R4, C18, C17 Text ................................................................................................................................................ 31
•
Changed DMIN to DMAX in Equation 47.................................................................................................................................... 31
•
Changed DMAX Value ............................................................................................................................................................ 31
•
Changed DC Gain(MOD) Value............................................................................................................................................... 31
•
Changed ESRZERO Value...................................................................................................................................................... 31
•
Changed R4, C18, C17 Text ................................................................................................................................................ 32
•
Changed Figure 21............................................................................................................................................................... 33
•
Changed Figure 22............................................................................................................................................................... 33
•
Changed Figure 23............................................................................................................................................................... 33
•
Added Figure 24 ................................................................................................................................................................... 33
•
Added Figure 25 ................................................................................................................................................................... 33
•
Added Figure 26 ................................................................................................................................................................... 33
Changes from Revision A (March 2013) to Revision B
•
Page
Changed layout of National Data Sheet to TI format ........................................................................................................... 24
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SNVS726F – JULY 2011 – REVISED MARCH 2018
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5 Pin Configuration and Functions
PWP Package
20-Pin HTSSOP With Exposed Pad
Top View
VIN
1
20
HS
UVLO
2
19
HO
RT
3
18
HB
EN
4
17
VCCX
RAMP
5
16
VCC
AGND
6
15
LO
SS
7
14
PGND
FB
8
13
CSG
COMP
9
12
CS
VOUT
10
11
SYNC
Exposed Pad
on Bottom
(DAP)
Pin Functions
PIN
NO.
1
TYPE
DESCRIPTION
NAME
VIN
P/I
Input supply voltage.
2
UVLO
I
If the UVLO pin is below 1.23 V, the regulator will be in standby mode (VCC regulator running,
switching regulator disabled). When the UVLO pin exceeds 1.23 V, the regulator enters the
normal operating mode. An external voltage divider can be used to set an undervoltage
shutdown threshold. A fixed 5-µA current is sourced out of the UVLO pin. If a current limit
condition exists for 256 consecutive switching cycles, an internal switch pulls the UVLO pin to
ground and then releases.
3
RT
I
The internal oscillator frequency is set with a single resistor between this pin and the AGND pin.
The recommended frequency range is 50 kHz to 500 kHz.
4
EN
I
If the EN pin is below 0.5 V, the regulator will be in a low power state drawing less than 10 µA
from VIN. EN must be raised above 3 V for normal operation.
5
RAMP
I
Ramp control signal. An external capacitor connected between this pin and the AGND pin sets
the ramp slope used for emulated current mode control.
6
AGND
G
Analog ground.
7
SS
I
Soft-Start. An external capacitor and an internal 10-µA current source set the rise time of the
error amp reference. The SS pin is held low when VCC is less than the VCC undervoltage
threshold (< 3.7 V), when the UVLO pin is low (< 1.23 V), when EN is low (< 0.5 V) or when
thermal shutdown is active.
8
FB
I
Feedback signal from the regulated output. Connect to the inverting input of the internal error
amplifier.
9
COMP
O
Output of the internal error amplifier. The loop compensation network should be connected
between COMP and the FB pin.
10
VOUT
I
Output voltage monitor for emulated current mode control. Connect this pin directly to the
regulated output.
11
SYNC
I
Sync input for switching regulator synchronization to an external clock.
12
CS
I
Current sense input. Connect to the diode side of the current sense resistor.
13
CSG
I
Current sense ground input. Connect to the ground side of the current sense resistor.
14
PGND
G
Power Ground.
15
LO
O
Boost MOSFET gate drive output. Connect to the gate of the external boost MOSFET.
16
VCC
P/I/O
4
Output of the bias regulator. Locally decouple to PGND using a low ESR/ESL capacitor located
as close to the controller as possible.
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Pin Functions (continued)
PIN
TYPE
DESCRIPTION
NO.
NAME
17
VCCX
P/I
Optional input for an externally supplied bias supply. If the voltage at the VCCX pin is greater
than 3.9 V, the internal VCC regulator is disabled and the VCC pin is internally connected to
VCCX pin supply. If VCCX is not used, connect to AGND.
18
HB
I
High-side gate driver supply used in bootstrap operation. The bootstrap capacitor supplies
current to charge the high-side MOSFET gate. This capacitor should be placed as close to the
controller as possible and connected between HB and HS.
19
HO
O
Buck MOSFET gate drive output. Connect to the gate of the high side buck MOSFET through a
short, low inductance path.
20
HS
I
Buck MOSFET source pin. Connect to the source terminal of the high-side buck MOSFET and
the bootstrap capacitor.
—
EP
—
Exposed thermal pad. Solder to the ground plane under the IC to aid in heat dissipation.
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1)
MIN
MAX
UNIT
VIN, EN, VOUT to GND
–0.3
45
V
VCC, LO, VCCX, UVLO to GND
–0.3
16
V
HB to HS
–0.3
16
V
HO to HS
–0.3
HB + 0.3
V
HS to GND
–4
45
V
CSG, CS to GND
–0.3
0.3
V
RAMP, SS, COMP, FB, SYNC, RT to GND
–0.3
7
V
Junction temperature
–40
150
°C
Storage temperature, Tstg
–55
150
°C
(1)
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
6.2 ESD Ratings
VALUE
V(ESD)
(1)
(2)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1)
±2000
Charged-device model (CDM), per JEDEC specification JESD22C101 (2)
±1000
UNIT
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted) (1)
VIN
MIN
MAX
(2)
UNIT
3
42
VCC, VCCX
4.75
14
V
Junction temperature
–40
+125
°C
(1)
(2)
V
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. Operating Range conditions indicate the conditions at which the device is
functional and the device should not be operated beyond such conditions. For ensured specifications and conditions, see Electrical
Characteristics.
VIN ≥ 5.0 V is required to initially start the controller.
6.4 Thermal Information
LM25118
THERMAL METRIC (1)
PWP (HTSSOP)
UNIT
20 PINS
110 (2)
RθJA
Junction-to-ambient thermal resistance
40 (3)
°C/W
35 (4)
RθJC(bot)
(1)
(2)
(3)
(4)
6
Junction-to-case (bottom) thermal resistance
4
°C/W
For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report.
JEDEC 2-Layer test board (JESD 51-3)
JEDEC 4-Layer test board (JESD 51-7) with 4 thermal vias under the Exposed Pad
JEDEC 4-Layer test board (JESD 51-7) with 12 thermal vias under the Exposed Pad
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SNVS726F – JULY 2011 – REVISED MARCH 2018
6.5 Electrical Characteristics
Unless otherwise specified, the following conditions apply: VIN = 24 V, VCCX = 0 V, EN = 5 V, RT = 29.11 kΩ, no load on LO
and HO. Typical values apply for TJ = 25°C; minimum and maximum values apply over the full junction temperature range for
operation: −40°C to +125°C. (1) (1)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
VIN SUPPLY
IBIAS
VIN operating current
VCCX = 0 V
4.5
5.5
mA
IBIASX
VIN operating current
VCCX = 5 V
1
1.85
mA
ISTDBY
VIN shutdown current
EN = 0 V
1
10
µA
6.8
7
7.2
V
5
5.25
5.5
21
35
3.68
3.85
VCC REGULATOR
VCC(REG)
VCC regulation
VCCX = 0 V
VCC(REG)
VCC regulation
VCCX = 0 V, VIN = 6 V
VCC sourcing current limit
VCC = 0
VCCX switch threshold
VCCX rising
VCCX switch hysterisis
4.02
V
5
12
Ω
1
µA
0.2
VCCX switch RDS(ON)
ICCX = 10 mA
VCCX switch leakage
VCCX = 0 V
0.5
VCCCX pulldown
resistance
VCCX = 3 V
70
VCC undervoltage lockout
voltage
VCC rising
3.52
VCC undervoltage
hysterisis
HB DC bias current
3.7
V
kΩ
3.86
0.21
HB-HS = 15 V
205
VC LDO mode turnoff
V
mA
V
V
260
10
µA
V
EN INPUT
VEN(OFF)
EN input low threshold
VEN falling
VEN(ON)
EN input high threshold
VEN rising
3
0.5
V
EN input bias current
VEN = 3 V
–1
1
µA
EN input bias current
VEN = 0.5 V
–1
1
µA
EN input bias current
VEN = 42 V
V
50
µA
UVLO THRESHOLDS
UVLO
ΔUVLO
UVLO standby threshold
UVLO Rising
UVLO threshold hysteresis
UVLO pullup current
source
UVLO = 0 V
UVLO pulldown RDS(ON)
(1)
1.191
1.231
1.271
V
0.105
V
5
µA
100
200
Ω
Minimum and Maximum limits are ensured through test, design, or statistical correlation. Typical values represent the most likely
parametric norm at TJ = 25°C, and are provided for reference purposes only.
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Electrical Characteristics (continued)
Unless otherwise specified, the following conditions apply: VIN = 24 V, VCCX = 0 V, EN = 5 V, RT = 29.11 kΩ, no load on LO
and HO. Typical values apply for TJ = 25°C; minimum and maximum values apply over the full junction temperature range for
operation: −40°C to +125°C.(1)(1)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
7.5
10.5
13.5
µA
SOFT START
SS current source
SS = 0 V
SS to FB offset
FB = 1.23 V
SS output low voltage
Sinking 100 µA, UVLO = 0 V
150
mV
7
mV
ERROR AMPLIFIER
VREF
FB reference voltage
Measured at FB pin,
FB = COMP
FB input bias current
FB = 2 V
COMP sink/source current
AOL
DC gain
fBW
Unity bain bandwidth
1.212
1.23
1.248
20
200
3
V
nA
mA
80
dB
3
MHz
PWM COMPARATORS
tHO(OFF)
Forced HO off-time
TON(MIN)
Minimum HO on-time
305
COMP to comparator offset
400
495
ns
70
ns
200
mV
OSCILLATOR (RT PIN)
fSW1
Frequency 1
RT = 29.11 kΩ
178
200
224
kHz
fSW2
Frequency 2
RT = 9.525 kΩ
450
515
575
kHz
SYNC
Sync threshold falling
1.3
V
CURRENT LIMIT
VCS(TH)
Cycle-by-cycle sense
voltage threshold (CSCSG)
RAMP = 0 buck mode
–103
–125
–147
mV
VCS(THX)
Cycle-by-cycle sense
voltage threshold (CSCSG)
RAMP = 0 buck-boost mode
–218
–255
–300
mV
CS bias current
CS = 0 V
45
60
µA
CSG bias current
CSG = 0 V
45
60
µA
Current limit fault timer
256
cycles
RAMP GENERATOR
IR2
RAMP current 2
VIN = 12 V, VOUT = 12 V
95
115
135
µA
IR3
RAMP current 3
VIN = 5 V, VOUT = 12 V
65
80
95
µA
VOUT bias current
VOUT = 42 V
245
VOLL
LO low-state output voltage ILO = 100 mA
0.14
VOHL
LO high-state output
voltage
ILO = –100 mA
VOHL = VCC-VLO
0.25
V
LO rise time
C-load = 1 nF, VCC = 8 V
16
ns
µA
LOW-SIDE (LO) GATE DRIVER
0.23
V
LO fall time
C-load = 1 nF, VCC = 8 V
14
ns
IOHL
Peak LO source current
VLO = 0 V, VCC = 8 V
2.2
A
IOLL
Peak LO sink current
VLO = VCC = 8 V
2.7
A
HIGH-SIDE (HO) GATE DRIVER
8
VOLH
HO low-state output
voltage
IHO = 100 mA
VOHH
HO high-state output
voltage
IHO = –100 mA,
VOHH = VHB-VOH
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0.135
0.25
0.21
V
V
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SNVS726F – JULY 2011 – REVISED MARCH 2018
Electrical Characteristics (continued)
Unless otherwise specified, the following conditions apply: VIN = 24 V, VCCX = 0 V, EN = 5 V, RT = 29.11 kΩ, no load on LO
and HO. Typical values apply for TJ = 25°C; minimum and maximum values apply over the full junction temperature range for
operation: −40°C to +125°C.(1)(1)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
HO rise time
C-load = 1 nF, VCC = 8 V
14
ns
HO fall time
C-load = 1 nF, VCC = 8 V
12
ns
IOHH
Peak HO source current
VHO = 0 V, VCC = 8 V
2.2
A
IOLH
Peak HO sink current
VHO = VCC = 8 V
3.5
A
3
V
HB-HS undervoltage
lockout
BUCK-BOOST CHARACTERISTICS (2)
Buck-boost mode
Buck duty cycle
69%
75%
80%
THERMAL
TSD
ΔTSD
(2)
Thermal shutdown junction
temperature
Thermal shutdown
hysterisis
165
°C
25
°C
When the duty cycle exceeds 75%, the LM25118 controller gradually phases into the Buck-Boost mode.
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6.6 Typical Characteristics
-100
100
EFFICIENCY (%)
x
90
x
x
x
1 AMP
x
x
x
2 AMP
85
-125
CURRENT LIMIT THRESHOLD (mV)
xx
x
x
x
xx xx
xx
xxx
3 AMP
95
x
80
-150
-175
-200
-225
-250
-275
65
75
0
10
20
40
30
50
70
75
VIN (V)
85
90
95
100
105
110
VOUT/VIN DC (%)
Figure 1. Efficiency vs VIN and IOUT VOUT = 12 V
Figure 2. Current Limit Threshold vs VOUT/VIN VOUT = 12 V
10
10
8
8
6
6
VCC (V)
VCC (V)
80
4
4
2
2
0
0
0
2
4
6
8
10
0
12
10
20
VIN (V)
30
40
50
IVCC (mA)
Figure 3. VCC vs VIN
Figure 4. VCC vs IVCC
50
150
40
120
30
90
4
3.5
60
10
30
CURRENT (A)
20
HO Sink
PHASE (°)
GAIN (dB)
3
2.5
LO Sink
2
1.5
LO Source
1
0
0
-10
10k
HO Source
0.5
100k
1M
-30
10M
0
0
FREQUENCY (Hz)
2
3
4
5
6
7
8
OUTPUT VOLTAGE (V)
Figure 5. Error Amplifier Gain/Phase
10
1
Figure 6. LO and HO Peak Gate Current vs Output Voltage
VCC = 8 V
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Typical Characteristics (continued)
600
500
FOSC (kHz)
400
300
200
100
0
0
20
40
60
80
100
120
140
160
RT (k:)
Figure 7. Oscillator Frequency vs RT
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7 Detailed Description
7.1 Overview
The LM25118 high voltage switching regulator features all of the functions necessary to implement an efficient
high voltage buck or buck-boost regulator using a minimum of external components. The regulator switches
smoothly from buck to buck-boost operation as the input voltage approaches the output voltage, allowing
operation with the input greater than or less than the output voltage. This easy-to-use regulator integrates highside and low-side MOSFET drivers capable of supplying peak currents of 2 A. The regulator control method is
based on current mode control using an emulated current ramp. Peak current mode control provides inherent line
feedforward, cycle-by-cycle current limiting and ease-of-loop compensation. The use of an emulated control ramp
reduces noise sensitivity of the pulse-width modulation circuit, allowing reliable processing of very small duty
cycles necessary in high input voltage applications. The operating frequency is user programmable from 50 kHz
to 500 kHz. An oscillator synchronization pin allows multiple LM25118 regulators to self synchronize or be
synchronized to an external clock. Fault protection features include current limiting, thermal shutdown, and
remote shutdown capability. An undervoltage lockout input allows regulator shutdown when the input voltage is
below a user selected threshold, and a low state at the enable pin will put the regulator into an extremely low
current shutdown state. The device is available in the 20-pin HTSSOP package featuring an exposed pad to aid
in thermal dissipation.
7.2 Functional Block Diagram
17
VIN
1
VCCX
LM25118
Vin
7V
VIN
VCC
16
REGULATOR
VCC UVLO
R8
C8
C2
C1
4
EN
3.9V
3V
5 PA
THERMAL
SHUTDOWN
SHUTDOWN
AND STANDBY
MODE
CONTROL
R1
HB
18
C13
HB UVLO
DISABLE
2
UVLO
R2
C12
1.23V
Vin
HICCUP MODE
FAULT TIMER
10 PA
7
SS
Q1
CLK
5V
PWM
C4
S
Q
R
Q
HO
19
HS
20
L1
LEVEL
SHIFT
D2
VOUT
1.23V
I-LIMIT
8 FB
C5
C6
C11
ERROR
AMP
Vth ( buck) = 1.25V
Vth ( buck - boost ) = 2.50V
10
+
R4
9
10 Rs V/A
C9
R7
CS 12
TRACK
and
HOLD
C10
D1
CLK
A=10
Rs
CSG 13
COMP
R5
VOUT 10
Vin
11 SYNC
3 RT
RAMP GENERATOR
OSCILLATOR
12
Q2
IRAMP (buck-boost) = 5 PA x Vin + 50 PA
RAMP
AGND
IRAMP
C3
15
IRAMP (buck) = ( 5 PA x ( Vin - Vout )) + 50 PA
CLK
5
R3
LO
R6
BUCK-BOOST
MODE CONTROL
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7.3 Feature Description
A buck-boost regulator can maintain regulation for input voltages either higher or lower than the output voltage.
The challenge is that buck-boost power converters are not as efficient as buck regulators. The LM25118 has
been designed as a dual-mode controller whereby the power converter acts as a buck regulator while the input
voltage is above the output. As the input voltage approaches the output voltage, a gradual transition to the buckboost mode occurs. The dual mode approach maintains regulation over a wide range of input voltages, while
maintaining the optimal conversion efficiency in the normal buck mode. The gradual transition between modes
eliminates disturbances at the output during transitions. Figure 8 shows the basic operation of the LM25118
regulator in the buck mode. In buck mode, transistor Q1 is active and Q2 is disabled. The inductor current ramps
in proportion to the Vin – Vout voltage difference when Q1 is active and ramps down through the recirculating
diode D1 when Q1 is off. The first order buck mode transfer function is VOUT/VIN = D, where D is the duty cycle
of the buck switch, Q1.
VIN
Buck Switch
Current
HB
Q1
HO
D2
VOUT
HS
D1
LM25118
CS
Diode
Current
CSG
Q2 (OFF)
LO
Figure 8. Buck Mode Operation
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Feature Description (continued)
Figure 9 shows the basic operation of buck-boost mode. In buck-boost mode, both Q1 and Q2 are active for the
same time interval each cycle. The inductor current ramps up (proportional to VIN) when Q1 and Q2 are active
and ramps down through the recirculating diode during the off-time. The first order buck-boost transfer function is
VOUT/VIN = D/(1 – D), where D is the duty cycle of Q1 and Q2.
VIN
Buck Switch
Current
HB
Q1
HO
D2
VOUT
HS
D1
LM25118
Diode
Current
CS
CSG
Q2
LO
Figure 9. Buck-Boost Mode Operation
100
90
70
60
50
40
30
DUTY CYCLE (%)
80
20
10
18
17
16
15
14
13
12
0
VIN (V)
Figure 10. Mode Dependence on Duty Cycle (VOUT =12 V)
14
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Feature Description (continued)
7.3.1 UVLO
An undervoltage lockout pin is provided to disable the regulator when the input is below the desired operating
range. If the UVLO pin is below 1.13 V, the regulator enters a standby mode with the outputs disabled, but with
VCC regulator operating. If the UVLO input exceeds 1.23 V, the regulator will resume normal operation. A
voltage divider from the input to ground can be used to set a VIN threshold to disable the regulator in brownout
conditions or for low input faults.
If a current limit fault exists for more than 256 clock cycles, the regulator will enter a hiccup mode of current
limiting and the UVLO pin will be pulled low by an internal switch. This switch turns off when the UVLO pin
approaches ground potential allowing the UVLO pin to rise. A capacitor connected to the UVLO pin will delay the
return to a normal operating level and thereby set the off-time of the hiccup mode fault protection. An internal 5µA pullup current pulls the UVLO pin to a high state to ensure normal operation when the VIN UVLO function is
not required and the pin is left floating.
7.3.2 Oscillator and Sync Capability
The LM25118 oscillator frequency is set by a single external resistor connected between the RT pin and the
AGND pin. The RT resistor should be located very close to the device and connected directly to the pins of the
IC. To set a desired oscillator frequency (f), the necessary value for the RT resistor can be calculated from
Equation 1:
9
RT =
6.4 x 10
3
- 3.02 x 10
f
(1)
The SYNC pin can be used to synchronize the internal oscillator to an external clock. The external clock must be
of higher frequency than the free-running frequency set by the RT resistor. A clock circuit with an open-drain
output is the recommended interface from the external clock to the SYNC pin. The clock pulse duration should
be greater than 15 ns.
LM25118
SYNC
LM25118
SYNC
UP TO FIVE
LM25118 DEVICES
Figure 11. Sync From Multiple Devices
Multiple LM25118 devices can be synchronized together simply by connecting the SYNC pins together. In this
configuration, all of the devices will be synchronized to the highest frequency device. The diagram in Figure 11
shows the SYNC input and output features of the LM25118. The internal oscillator circuit drives the SYNC pin
with a strong pulldown and weak pullup inverter. When the SYNC pin is pulled low, either by the internal
oscillator or an external clock, the ramp cycle of the oscillator is terminated and forced 400 ns off-time is initiated
before a new oscillator cycle begins. If the SYNC pins of several LM25118 ICs are connected together, the IC
with the highest internal clock frequency will pull all the connected SYNC pins low and terminate the oscillator
ramp cycles of the other ICs. The LM25118 with the highest programmed clock frequency will serve as the
master and control the switching frequency of all the devices with lower oscillator frequencies.
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Feature Description (continued)
SYNC
100 PA
I
1/RT
1.23V
Q
S
Q
R
DEADTIME
ONE - SHOT
Figure 12. Simplified Oscillator and Block Diagram With Sync I/O Circuit
7.3.3 Error Amplifier and PWM Comparator
The internal high gain error amplifier generates an error signal proportional to the difference between the
regulated output voltage and an internal precision reference (1.23 V). The output of the error amplifier is
connected to the COMP pin. Loop compensation components, typically a type II network shown in Figure 18 are
connected between the COMP and FB pins. This network creates a low-frequency pole, a zero, and a noise
reducing high-frequency pole. The PWM comparator compares the emulated current sense signal from the
RAMP generator to the error amplifier output voltage at the COMP pin. The same error amplifier is used for
operation in buck and buck-boost mode.
v
Buck: ( 5 PA x ( VIN - VOUT) + 50 PA ) x
Emulated
Ramp
Buck - Boost: ( 5 PA x VIN + 50 PA ) x
tON
CRAMP
tON
CRAMP
Pedestal Level = 10 x Rs (volts/amp)
t
Ton
Figure 13. Composition of Emulated Current Signal
16
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Feature Description (continued)
7.3.4 Ramp Generator
The ramp signal of a pulse-width modulator with current mode control is typically derived directly from the buck
switch drain current. This switch current corresponds to the positive slope portion of the inductor current signal.
Using this signal for the PWM ramp simplifies the control loop transfer function to a single pole response and
provides inherent input voltage feed-forward compensation. The disadvantage of using the buck switch current
signal for PWM control is the large leading edge spike due to circuit parasitics. The leading edge spike must be
filtered or blanked to avoid early termination of the PWM pulse. Also, the current measurement may introduce
significant propagation delays. The filtering, blanking time, and propagation delay limit the minimal achievable
pulse width. In applications where the input voltage may be relatively large in comparison to the output voltage,
controlling a small pulse width is necessary for regulation. The LM25118 uses a unique ramp generator which
does not actually measure the buck switch current but instead creates a signal representing or emulating the
inductor current. The emulated ramp provides signal to the PWM comparator that is free of leading edge spikes
and measurement or filtering delays. The current reconstruction is comprised of two elements: a sample-andhold pedestal level and a ramp capacitor that is charged by a controlled current source. See Figure 13 for details.
The sample-and-hold pedestal level is derived from a measurement of the recirculating current through a current
sense resistor in series with the recirculating diode of the buck regulator stage. A small value current-sensing
resistor is required between the recirculating diode anode and ground. The CS and CSG pins should be Kelvin
connected directly to the sense resistor. The voltage level across the sense resistor is sampled and held just
prior to the onset of the next conduction interval of the buck switch. The current sensing and sample-and-hold
provide the DC level of the reconstructed current signal. The sample and hold of the recirculating diode current is
valid for both buck and buck-boost modes. The positive slope inductor current ramp is emulated by an external
capacitor connected from the RAMP pin to the AGND and an internal voltage controlled current source. In buck
mode, the ramp current source that emulates the inductor current is a function of the VIN and VOUT voltages per
Equation 2.
IRAMP (buck) =
5 PA
x (VIN - VOUT) + 50 PA
V
(2)
In buck-boost mode, the ramp current source is a function of the input voltage VIN, per Equation 3.
IRAMP (buck - boost) =
5 PA
x VIN + 50 PA
V
(3)
Proper selection of the RAMP capacitor (CRAMP) depends upon the value of the output inductor (L) and the
current sense resistor (RS). For proper current emulation, the sample and hold pedestal value and the ramp
amplitude must have the same relative relationship to the actual inductor current. That is:
RS x A =
CRAMP =
gm x L
CRAMP
gm x L
A x RS
where
•
•
gm is the ramp generator transconductance (5 µA/V)
A is the current sense amplifier gain (10 V/V)
(4)
The ramp capacitor should be located very close to the device and connected directly to the RAMP and AGND
pins.
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Feature Description (continued)
The relationship between the average inductor current and the pedestal value of the sampled inductor current
can cause instability in certain operating conditions. This instability is known as sub-harmonic oscillation, which
occurs when the inductor ripple current does not return to its initial value by the start of the next switching cycle.
Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow pulses at the switch
node. Adding a fixed slope voltage ramp (slope compensation) to the current sense signal prevents this
oscillation. The 50 µA of offset current provided from the emulated current source adds enough slope
compensation to the ramp signal for output voltages less than or equal to 12 V. For higher output voltages,
additional slope compensation may be required. In such applications, the ramp capacitor can be decreased from
the nominal calculated value to increase the ramp slope compensation.
The pedestal current sample is obtained from the current sense resistor (Rs) connected to the CS and CSG pins.
It is sometimes helpful to adjust the internal current sense amplifier gain (A) to a lower value to obtain the higher
current limit threshold. Adding a pair of external resistors RG in a series with CS and CSG as shown in Figure 14
reduces the current sense amplifier gain A according to Equation 5.
10k
A=
1k + RG
(5)
7.3.5 Current Limit
In the buck mode the average inductor current is equal to the output current (Iout). In buck-boost mode the
average inductor current is approximately equal to:
Iout x 1 +
VOUT
VIN
(6)
Consequently, the inductor current in buck-boost mode is much larger especially when VOUT is large relative to
VIN. The LM25118 provides a current monitoring scheme to protect the circuit from possible overcurrent
conditions. When set correctly, the emulated current sense signal is proportional to the buck switch current with a
scale factor determined by the current sense resistor. The emulated ramp signal is applied to the current limit
comparator. If the peak of the emulated ramp signal exceeds 1.25 V when operating in the buck mode, the PWM
cycle is immediately terminated (cycle-by-cycle current limiting). In buck-boost mode the current limit threshold is
increased to 2.50 V to allow higher peak inductor current. To further protect the external switches during
prolonged overload conditions, an internal counter detects consecutive cycles of current limiting. If the counter
detects 256 consecutive current limited PWM cycles, the LM25118 enters a low power dissipation hiccup mode.
In the hiccup mode, the output drivers are disabled, the UVLO pin is momentarily pulled low, and the soft-start
capacitor is discharged. The regulator is restarted with a normal soft-start sequence once the UVLO pin charges
back to 1.23 V. The hiccup mode off-time can be programmed by an external capacitor connected from UVLO
pin to ground. This hiccup cycle will repeat until the output overload condition is removed.
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Feature Description (continued)
In applications with low output inductance and high input voltage, the switch current may overshoot due to the
propagation delay of the current limit comparator and control circuitry. If an overshoot should occur, the sampleand-hold circuit will detect the excess recirculating diode current. If the sample-and-hold pedestal level exceeds
the internal current limit threshold, the buck switch will be disabled and will skip PWM cycles until the inductor
current has decayed below the current limit threshold. This approach prevents current runaway conditions due to
propagation delays or inductor saturation since the inductor current is forced to decay before the buck switch is
turned on again.
CURRENT LIMIT
COMPARATOR
CURRENT SENSE
AMPLIFIER
Vth
TRACK
and HOLD
-
10k
1k
CS
RG
1k
CSG
RG
+
IRAMP
10k
IL
RS
+
0.2V
RAMP
RESET
RAMP
CRAMP
Figure 14. Current Limit and Ramp Circuit
7.3.6 Maximum Duty Cycle
Each conduction cycle of the buck switch is followed by a forced minimum off-time of 400 ns to allow sufficient
time for the recirculating diode current to be sampled. This forced off-time limits the maximum duty cycle of the
controller. The actual maximum duty cycle will vary with the operating frequency of Equation 7.
DMAX = 1 - f × 400 × 10–9
where
•
f is the oscillator frequency in Hz
(7)
1
MAX DUTY CYCLE
0.95
0.9
0.85
0.8
0.75
0
100
200
300
400
500
600
FREQUENCY (kHz)
Figure 15. Maximum Duty Cycle vs Frequency
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Feature Description (continued)
Limiting the maximum duty cycle will limit the maximum boost ratio (VOUT/VIN) while operating in buck-boost
mode. For example, from Figure 15, at an operating frequency of 500 kHz, DMAX is 80%. Using the buck-boost
transfer function.
Vout
D=
Vin + Vout
(8)
with
• D = 80%, solving for VOUT results in
• VOUT = 4 × VIN
With a minimum input voltage of 5 V, the maximum possible output voltage is 20 V at f = 500 kHz. The buckboost step-up ratio can be increased by reducing the operating frequency which increases the maximum duty
cycle.
7.3.7 Soft Start
The soft-start feature allows the regulator to gradually reach the initial steady-state operating point, thus reducing
start-up stresses and surges. The internal 10-µA soft-start current source gradually charges an external soft-start
capacitor connected to the SS pin. The SS pin is connected to the positive input of the internal error amplifier.
The error amplifier controls the pulse-width modulator such that the FB pin approximately equals the SS pin as
the SS capacitor is charged. Once the SS pin voltage exceeds the internal 1.23-V reference voltage, the error
amp is controlled by the reference instead of the SS pin. The SS pin voltage is clamped by an internal amplifier
at a level of 150 mV above the FB pin voltage. This feature provides a soft-start controlled recovery in the event
a severe overload pulls the output voltage (and FB pin) well below normal regulation but doesn’t persist for 256
clock cycles.
Various sequencing and tracking schemes can be implemented using external circuits that limit or clamp the
voltage level of the SS pin. The SS pin acts as a noninverting input to the error amplifier anytime SS voltage is
less than the 1.23-V reference. In the event a fault is detected (overtemperature, VCC undervoltage, hiccup
current limit), the soft-start capacitor will be discharged. When the fault condition is no longer present, a new softstart sequence will begin.
7.3.8 HO Output
The LM25118 contains a high-side, high-current gate driver and associated high voltage level shift. This gate
driver circuit works in conjunction with an internal diode and an external bootstrap capacitor. A 0.1-µF ceramic
capacitor, connected with short traces between the HB pin and HS pin is recommended for most circuit
configurations. The size of the bootstrap capacitor depends on the gate charge of the external FET. During the
off-time of the buck switch, the HS pin voltage is approximately –0.5 V and the bootstrap capacitor is charged
from VCC through the internal bootstrap diode. When operating with a high PWM duty cycle, the buck switch will
be forced off each cycle for 400 ns to ensure that the bootstrap capacitor is recharged.
7.3.9 Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum junction
temperature is exceeded. When activated, typically at 165°C, the controller is forced into a low power reset state,
disabling the output driver and the bias regulator. This protection is provided to prevent catastrophic failures from
accidental device overheating.
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7.4 Device Functional Modes
Figure 10 shows how duty cycle effects the operational mode and is useful for reference in the following
discussions. Initially, only the buck switch is active and the buck duty cycle increases to maintain output
regulation as VIN decreases. When VIN is approximately equal to 15.5 V, the boost switch begins to operate with
a low duty cycle. If VIN continues to fall, the boost switch duty cycle increases and the buck switch duty cycle
decreases until they become equal at VIN = 13.2 V.
7.4.1 Buck Mode Operation: VIN > VOUT
The LM25118 buck-boost regulator operates as a conventional buck regulator with emulated current mode
control while VIN is greater than VOUT and the buck mode duty cycle is less than 75%. In buck mode, the LO
gate drive output to the boost switch remains low.
7.4.2 Buck-Boost Mode Operation: VIN ≊ VOUT
When VIN decreases relative to VOUT, the duty cycle of the buck switch will increase to maintain regulation.
Once the duty cycle reaches 75%, the boost switch starts to operate with a very small duty cycle. As VIN is
further decreased, the boost switch duty cycle increases until it is the same as the buck switch. As VIN is further
decreased below VOUT, the buck and boost switch operate together with the same duty cycle and the regulator
is in full buck-boost mode. This feature allows the regulator to transition smoothly from buck to buck-boost mode.
Note that the regulator can be designed to operate with VIN less than 4 V, but VIN must be at least 5 V during
start-up. Figure 16 shows a timing illustration of the gradual transition from buck to buck-boost mode when the
input voltage ramps downward over a few switching cycles.
Figure 16. Buck (HO) and Boost (LO) Switch Duty Cycle vs. Time,
Illustrating Gradual Mode Change With Decreasing Input Voltage
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Device Functional Modes (continued)
7.4.3 High Voltage Start-Up Regulator
The LM25118 contains a dual-mode, high voltage linear regulator that provides the VCC bias supply for the
PWM controller and the MOSFET gate driver. The VIN input pin can be connected directly to input voltages as
high as 42 V. For input voltages below 10 V, an internal low dropout switch connects VCC directly to VIN. In this
supply range, VCC is approximately equal to VIN. For VIN voltages greater than 10 V, the low dropout switch is
disabled and the VCC regulator is enabled to maintain VCC at approximately 7 V. A wide operating range of 4 V
to 42 V (with a startup requirement of at least 5 V) is achieved through the use of this dual-mode regulator.
The output of the VCC regulator is current limited to 35 mA, typical. Upon power up, the regulator sources
current into the capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds the VCC undervoltage threshold of 3.7 V and the UVLO input pin voltage is greater than 1.23 V, the gate driver outputs are
enabled and a soft-start sequence begins. The gate driver outputs remain enabled until VCC falls below 3.5 V or
the voltage at the UVLO pin falls below 1.13 V.
In many applications, the regulated output voltage or an auxiliary supply voltage can be applied to the VCCX pin
to reduce the IC power dissipation. For output voltages between 4 V and 15 V, VOUT can be connected directly
to VCCX. When the voltage at the VCCX pin is greater than 3.85 V, the internal VCC regulator is disabled and
an internal switch connects VCCX to VCC, reducing the internal power dissipation.
In high voltage applications, take extra care to ensure the VIN pin voltage does not exceed the absolute
maximum voltage rating of 45 V. During line or load transients, voltage ringing on the VIN line that exceeds the
absolute maximum rating can damage the IC. Both careful PCB layout and the use of quality bypass capacitors
located close to the VIN and GND pins are essential.
Vin
10V
7V
3.7V
Vcc
3.5V
VIN and VCC
Internal Enable Signal
Figure 17. VIN and VCC Sequencing
7.4.4 Enable
The LM25118 contains an enable function which provides a very low input current shutdown mode. If the EN pin
is pulled below 0.5 V, the regulator enters shutdown mode, drawing less than 10 µA from the VIN pin. Raising
the EN input above 3 V returns the regulator to normal operation. The EN pin can be tied directly to the VIN pin if
this function is not needed. It must not be left floating. A 1-MΩ pullup resistor to VIN can be used to interface with
an open-collector or open-drain control signal.
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The LM25118 high voltage switching regulator features all of the functions necessary to implement an efficient
high voltage buck or buck-boost regulator using a minimum of external components. A buckboost regulator can
maintain regulation for input voltages either higher or lower than the output voltage.
8.2 Typical Application
VIN
VIN (1)
UVLO (2)
(16) VCC
(18) HB
SYNC (11)
VCCX (17)
VEN(ON)
VEN(OFF)
0V
(19) HO
VOUT
ON
(20) HS
EN (4)
OFF
LM25118MH
(12) CS
SS (7)
(13) CSG
RAMP (5)
RT (3)
(15) LO
(10) VOUT
AGND (6)
(8) FB
PGND (14)
(9) COMP
DAP (EP)
Figure 18. Typical Application Circuit
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Typical Application (continued)
Figure 19. 12-V, 3-A Typical Application Schematic
8.2.1 Design Requirements
The procedure for calculating the external components is illustrated with the following design example. The
designations used in the design example correlate to the Figure 19. The design specifications are:
• VOUT = 12 V
• VIN = 5 V to 42 V
• f = 300 kHz
• Minimum load current (CCM operation) = 600 mA
• Maximum load current = 3 A
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Typical Application (continued)
8.2.2 Detailed Design Procedure
8.2.2.1 Custom Design With WEBENCH® Tools
Click here to create a custom design using the LM25118 device with the WEBENCH® Power Designer.
1. Start by entering the input voltage (VIN), output voltage (VOUT), and output current (IOUT) requirements.
2. Optimize the design for key parameters such as efficiency, footprint, and cost using the optimizer dial.
3. Compare the generated design with other possible solutions from Texas Instruments.
The WEBENCH Power Designer provides a customized schematic along with a list of materials with real-time
pricing and component availability.
In most cases, these actions are available:
• Run electrical simulations to see important waveforms and circuit performance
• Run thermal simulations to understand board thermal performance
• Export customized schematic and layout into popular CAD formats
• Print PDF reports for the design, and share the design with colleagues
Get more information about WEBENCH tools at www.ti.com/WEBENCH.
8.2.2.2 R7 = RT
RT sets the oscillator switching frequency. Generally speaking, higher operating frequency applications will use
smaller components, but have higher switching losses. An operating frequency of 300 kHz was selected for this
example as a reasonable compromise for both component size and efficiency. The value of RT can be calculated
as:
9
RT =
6.4 x 10
3
- 3.02 x 10
f
(9)
therefore, R7 = 18.3 kΩ
Ipk+
L1 Current
Io Buck
Iripple
Io/(1-D) B-B
Ipk-
Iripple
1/Fs
Figure 20. Inductor Current Waveform
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Typical Application (continued)
8.2.2.3
Inductor Selection – L1
The inductor value is determined based upon the operating frequency, load current, ripple current, and the input
and output voltages. See Figure 20 for details.
To keep the circuit in continuous conduction mode (CCM), the maximum ripple current IRIPPLE should be less
than twice the minimum load current. For the specified minimum load of 0.6 A, the maximum ripple current is 1.2
Ap-p. Also, the minimum value of L must be calculated both for a buck and buck-boost configurations. The final
value of inductance will generally be a compromise between the two modes. It is desirable to have a larger value
inductor for buck mode, but the saturation current rating for the inductor must be large for buck-boost mode,
resulting in a physically large inductor. Additionally, large value inductors present buck-boost mode loop
compensation challenges which will be discussed in Error Amplifier Configuration . For the design example, the
inductor values in both modes are calculated as:
VOUT (VIN(MAX) VOUT )
L1
Buck Mode
VIN(MAX) u f u IRIPPLE
(10)
L1
VIN(MIN) (VOUT )
(VOUT
VIN(MIN) ) u f u IRIPPLE
Buck-Boost Mode
where
•
•
•
•
•
•
VOUT is the output voltage
VIN(MAX) is the maximum input voltage
f is the switching frequency
IRIPPLE is the selected inductor peak to peak ripple current (1.2 A selected for this example)
VIN(MIN) is the minimum input voltage
(11)
The resulting inductor values are:
L1 = 23.8 µH, Buck Mode
L1 = 9.8 µH, Buck-Boost mode
(12)
(13)
A 10-µH inductor was selected which is a compromise between these values, while favoring the buck-boost
mode. As illustrated in the compensation section, the inductor value should be as low as possible to move the
buck-boost right-half-plane zero to a higher frequency. The ripple current is then rechecked with the selected
inductor value using Equation 10 and Equation 11.
IRIPPLE(BUCK) = 2.86 A
IRIPPLE(BUCK-BOOST) = 1.17 A
(14)
(15)
Because the inductor selected is lower than calculated for the Buck mode, the minimum load current for CCM in
buck mode is 1.42 A at maximum VIN.
With a 10-µH inductor, the worst case peak inductor currents can be estimated for each case, assuming a 20%
inductor value tolerance and 80% efficiency of the converter.
IRIPPLE(BUCK)
I
I1(PEAK) OUT
K
2(1 LTOL )
(16)
IOUT (VOUT VIN(MIN) ) IRIPPLE(BUCK-BOOST)
I2(PEAK)
Ku VIN(MIN)
2(1 LTOL )
where
•
•
26
η is efficiency
LTOL is the inductor tolerance
(17)
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Typical Application (continued)
For this example, Equation 16 and Equation 17 yield:
I1(PEAK) = 5.33 A
I2(PEAK) = 13.4 A
(18)
(19)
An acceptable current limit setting would be 6.7 A for buck mode because the LM25118 automatically doubles
the current limit threshold in buck-boost mode. The selected inductor must have a saturation current rating at
least as high as the buck-boost mode cycle-by-cycle current limit threshold, in this case at least 13.5 A. A 10-µH,
15-A inductor was chosen for this application.
8.2.2.4 R13 = RSENSE
To select the current sense resistor, begin by calculating the minimum K values for each mode using
Equation 20 and Equation 21. K represents the slope compensation of the controller and is different for each
mode, KBUCK and KBUCK-BOOST. KBUCK and KBUCK-BOOST were selected to be 1.33 and 3, respectively.
10
KBUCK t 1
VIN(MAX) VOUT
(20)
KBUCK
BOOST
t1
10
VIN(MIN)
(21)
(22)
(23)
KBUCK = 1.33
KBUCK-BOOST = 3
Use Equation 24 and Equation 25 to calculate RSENSE for each mode of operation. A design margin, M, should be
selected between 10%-30% to allow for component tolerances. For this design M was selected to be 10%.
1.25(1 M)
R13(BUCK)
§ IOUT IRIPPLE(BUCK)
·
10 ˜ ¨
˜ K BUCK ¸
2
© K
¹
(24)
R13(BUCK-BOOST)
2.5 ˜ (1 M)
§ VIN(MIN) VOUT IOUT
10 ˜ ¨
˜
¨
VIN(MIN)
K
©
IRIPPLE(BUCK-BOOST)
2
·
˜ K BUCK-BOOST ¸
¸
¹
(25)
(26)
(27)
R13(BUCK) = 19.89 mΩ
R13(BUCK-BOOST) = 15.5 mΩ
An RSENSE value of no more than 15.5 mΩ must be used to ensure the required maximum output current in the
buck-boost mode. A standard value of 15 mΩ was selected for this design.
8.2.2.5 C15 = CRAMP
With the inductor value selected, the value of C3 necessary for the emulation ramp circuit is:
-6
C15 = CRAMP =
L x 10
2 x RSENSE
(28)
With the inductance value (L1) selected as 10 µH, the calculated value for CRAMP is 333 pF. A standard value of
330 pF was selected.
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Typical Application (continued)
8.2.2.6 Inductor Current Limit Calculation
The current limit for each mode can be calculated using Equation 29 and Equation 31. If the peak current limit is
less than the calculated inductor peak current, the R13 and C15 need to be recalculated. This can be done by
increasing the previous K values or M and reiterating the calculations.
50 u 10 6 u VOUT
1.25
C15 u f u VIN(MAX)
ILIMIT(BUCK)
10 u R13
ILIMIT
(BUCK)
(29)
(30)
= 7.37 A
2.5
ILIMIT(BUCK-BOOST)
50 u 10 6 u VOUT
C15 u f u (VIN(MIN) VOUT )
10 u R13
(31)
(32)
ILIMIT (BUCK-BOOST) = 14.29 A
8.2.2.7 C9 - C12 = Output Capacitors
In buck-boost mode, the output capacitors C9 – C12 must supply the entire output current during the switch ontime. For this reason, the output capacitors are chosen for operation in buck-boost mode, the demands being
much less in buck operation. Both bulk capacitance and ESR must be considered to ensure a given output ripple
voltage. Buck-boost mode capacitance can be estimated from:
I
VOUT
u DMAX
CMIN OUT
With DMAX
f u 'VOUT
VIN(MIN) VOUT
(33)
ESR requirements can be estimated from:
ESRMAX
VOUT
VIN(MIN)
VIN(MIN)
'VOUT
IRIPPLE(BUCK-BOOST)
˜ IOUT
2
(34)
For this example, with a ΔVOUT (output ripple) of 50 mV:
CMIN = 141 µF
ESRMAX = 4.6 mΩ
(35)
(36)
If hold-up times are a consideration, the values of input and output capacitors must be increased appropriately.
Note that it is usually advantageous to use multiple capacitors in parallel to achieve the ESR value required.
Also, it is good practice to put a 0.1-µF to 0.47-µF ceramic capacitor directly on the output pins of the supply to
reduce high-frequency noise. Ceramic capacitors have good ESR characteristics, and are a good choice for input
and output capacitors. Note that the effective capacitance of ceramic capacitors decreases with DC bias. For
larger bulk values of capacitance, a low-ESR electrolytic is usually used. However, electrolytic capacitors have
poor tolerance, especially over temperature, and the selected value should be selected larger than the calculated
value to allow for temperature variation. Allowing for component tolerances, the following values of Cout were
chosen for this design example:
Two 180-µF Oscon electrolytic capacitors for bulk capacitance
Two 47-µF ceramic capacitors to reduce ESR
Two 0.47-µF ceramic capacitors to reduce spikes at the output
8.2.2.8 D1
Reverse recovery currents degrade performance and decrease efficiency. For these reasons, a Schottky diode of
appropriate ratings should be used for D1. The voltage rating of the boost diode should be equal to VOUT plus
some margin.
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Typical Application (continued)
8.2.2.9 D4
A Schottky type recirculating diode is required for all LM25118 applications. The near ideal reverse recovery
characteristics and low forward voltage drop are particularly important diode characteristics for high input voltage
and low output voltage applications. The reverse recovery characteristic determines how long the current surge
lasts each cycle when the buck switch is turned on. The reverse recovery characteristics of Schottky diodes
minimize the peak instantaneous power in the buck switch during the turn-on transition. The reverse breakdown
rating of the diode should be selected for the maximum VIN plus some safety margin.
The forward voltage drop has a significant impact on the conversion efficiency, especially for applications with a
low output voltage. Rated current for diodes vary widely from various manufacturers. For the LM25118 this
current is user selectable through the current sense resistor value. Assuming a worst-case, 0.6-V drop across the
diode, the maximum diode power dissipation can be high. The diode should have a voltage rating of VIN and a
current rating of IOUT. A conservative design would at least double the advertised diode rating because
specifications between manufacturers vary. For the reference design, a 100-V, 10-A Schottky in a D2PAK
package was selected.
8.2.2.10 C1 – C5 = Input Capacitors
A typical regulator supply voltage has a large source impedance at the switching frequency. Good-quality input
capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current
during the buck switch on-time. When the buck switch turns on, the current into the buck switch steps from zero
to the lower peak of the inductor current waveform, then ramps up to the peak value, and then drops to the zero
at turnoff. The RMS current rating of the input capacitors depends on which mode of operation is most critical.
IRMS(BUCK) = IOUT D(1 -D)
(37)
The RMS current demand on the input capacitor(s) is at the maximum value when the duty cycle is at 50%.
IOUT
D(1 -D)
IRMS(BUCK-BOOST) =
1-D
(38)
Checking both modes of operation we find:
IRMS(BUCK) = 1.5 A
IRMS(BUCK-BOOST) = 4.7 A
(39)
(40)
Therefore C1 — C5 should be sized to handle 4.7 A of ripple current. Quality ceramic capacitors with a low ESR
should be selected. To allow for capacitor tolerances, five 2.2-µF, 100-V ceramic capacitors will be used. If step
input voltage transients are expected near the maximum rating of the LM25118, a careful evaluation of the
ringing and possible spikes at the device VIN pin should be completed. An additional damping network or input
voltage clamp may be required in these cases.
8.2.2.11 C20
The capacitor at the VCC pin provides noise filtering and stability for the VCC regulator. The recommended value
of C20 should be no smaller than 0.1 µF, and should be a good-quality, low-ESR, ceramic capacitor. A value of 1
µF was selected for this design. C20 should be 10 x C8.
If operating without VCCX, then
fOSC x (QCBuck + Boost) + ILOAD(INTERNAL)
(41)
must be less than the VCC current limit.
8.2.2.12 C8
The bootstrap capacitor between the HB and HS pins supplies the gate current to charge the buck switch gate at
turnon. The recommended value of C8 is 0.1 µF to 0.47 µF, and should be a good-quality, low-ESR, ceramic
capacitor. A value of 0.1 µF was chosen for this design.
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Typical Application (continued)
8.2.2.13 C16 = CSS
The capacitor at the SS pin determines the soft-start time, that is, the time for the reference voltage and the
output voltage, to reach the final regulated value. The time is determined from:
C16 x 1.23V
tSS =
10 PA
(42)
and assumes a current limit>Iload + ICout
For this application, a C16 value of 0.1 µF was chosen which corresponds to a soft-start time of about 12 ms.
8.2.2.14 R8, R9
R8 and R9 set the output voltage level, the ratio of these resistors is calculated from:
R8 VOUT - 1
=
R9 1.23V
(43)
For a 12-V output, the R8/R9 ratio calculates to 8.76. The resistors should be chosen from standard value
resistors and a good starting point is to select resistors within power ratings appropriate for the output voltage.
Values of 309 Ω for R9 and 2.67 kΩ for R8 were selected.
8.2.2.15 R1, R3, C21
A voltage divider can be connected to the UVLO pin to set a minimum operating voltage VIN(UVLO) for the
regulator. If this feature is required, the easiest approach to select the divider resistor values is to choose a value
for R1 between 10 kΩ and 100 kΩ, while observing the minimum value of R1 necessary to allow the UVLO
switch to pull the UVLO pin low. This value is:
R1 ≥ 1000 × VIN(MAX)
R1 ≥ 75 k
R3 is then calculated from:
R3 = 1.23 x
R1
VIN(MIN) + 5 PA x R1 - 1.23
(44)
Because VIN(MIN) for our example is 5 V, set VIN(UVLO) to 4 V for some margin in component tolerances and input
ripple.
R1 = 75 k is chosen because it is a standard value
R3 = 29.332 k is calculated from Equation 44. 29.4 k was used because it is a standard value
Capacitor C21 provides filtering for the divider and the off time of the hiccup duty cycle during current limit. The
voltage at the UVLO pin should never exceed 15 V when using an external set-point divider. It may be necessary
to clamp the UVLO pin at high input voltages.
Knowing the desired off time during hiccup current limit, the value of C21 is given by:
t OFF
ª
C21˜ R1˜ R3
R1 R3 º
˜ Ln «1 .98 ˜
»
R1 R3
VIN ˜ R3 ¼
¬
(45)
Notice that tOFF varies with VIN
In this example, C21 was chosen to be 0.1 µF. This will set the tOFF time to 723 µs with VIN = 12 V.
8.2.2.16 R2
A 1-M pullup resistor connected from the EN pin to the VIN pin is sufficient to keep enable in a high state if on-off
control is not used.
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Typical Application (continued)
8.2.2.17 Snubber
A snubber network across the buck recirculating diode reduces ringing and spikes at the switching node.
Excessive ringing and spikes can cause erratic operation and increase noise at the regulator output. In the limit,
spikes beyond the maximum voltage rating of the LM25118 or the recirculating diode can damage these devices.
Selecting the values for the snubber is best accomplished through empirical methods. First, make sure the lead
lengths for the snubber connections are very short. Start with a resistor value between 5 and 20 Ω. Increasing
the value of the snubber capacitor results in more damping, however the snubber losses increase. Select a
minimum value of the capacitor that provides adequate clamping of the diode waveform at maximum load. A
snubber may be required for the boost diode as well. The same empirical procedure applies. Snubbers were not
necessary in this example.
8.2.2.18 Error Amplifier Configuration
8.2.2.18.1 R4, C18, C17
These components configure the error amplifier gain characteristics to accomplish a stable overall loop gain. One
advantage of current mode control is the ability to close the loop with only three feedback components, R4, C18,
and C17. The overall loop gain is the product of the modulator gain and the error amplifier gain. The DC
modulator gain of the LM25118 is as follows:
RLOAD x VIN
DCGain(MOD) =
10RS(VIN + 2VOUT)
(46)
The dominant, low frequency pole of the modulator is determined by the load resistance (RLOAD) and output
capacitance (COUT). The corner frequency of this pole is:
fP(MOD)
1 DMAX
2S u RLOAD u COUT
(47)
For this example, RLOAD = 4 Ω, DMAX = 0.705, and COUT = 454 µF, therefore:
fP(MOD) = 149 Hz
DC Gain(MOD) = 4.59 = 13.25 dB
(48)
(49)
Additionally, there is a right-half plane (RHP) zero associated with the modulator. The frequency of the RHP zero
is:
fRHPzero =
RLOAD (1 - D)2
2S x L x D
(50)
(51)
fRHPzero = 7.8 kHz
The output capacitor ESR produces a zero given by:
ESRzero =
1
2S x ESR x COUT
(52)
(53)
ESRZERO = 76 kHz
The RHP zero complicates compensation. The best design approach is to reduce the loop gain to cross zero at
about 25% of the calculated RHP zero frequency. The Type ll error amplifier compensation provided by R4, C18,
and C17 places one pole at the origin for high DC gain. The second pole should be placed close to the RHP
zero. The error amplifier zero (Equation 54) should be placed near the dominate modulator pole. This is a good
starting point for compensation.
Components R4 and C18 configure the error amplifier as a Type II configuration which has a DC pole and a zero
at
1
fz =
2 x S x R4 x C18
(54)
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Typical Application (continued)
C17 introduces an additional pole used to cancel high frequency switching noise. The error amplifier zero
cancels the modulator pole leaving a single pose response at the crossover frequency of the loop gain if the
crossover frequency is much lower than the right half plane zero frequency. A single pole response at the
crossover frequency yields a very stable loop with 90 degrees of phase margin.
For the design example, a target loop bandwidth (crossover frequency) of 2.0 kHz was selected (about 25% of
the right-half-plane zero frequency). The error amplifier zero (fz) should be selected at a frequency near that of
the modulator pole and much less than the target crossover frequency. This constrains the product of R4 and
C18 for a desired compensation network zero to be less than 2 kHz. Increasing R4, while proportionally
decreasing C18 increases the error amp gain. Conversely, decreasing R4 while proportionally increasing C18
decreases the error amp gain. For the design example C18 was selected for 100 nF and R4 was selected to be
10 kΩ. These values set the compensation network zero at 159 Hz. The overall loop gain can be predicted as
the sum (in dB) of the modulator gain and the error amp gain.
If a network analyzer is available, the modulator gain can be measured and the error amplifier gain can be
configured for the desired loop transfer function. If a network analyzer is not available, the error amplifier
compensation components can be designed with the guidelines given. Step load transient tests can be
performed to verify acceptable performance. The step load goal is minimal overshoot with a damped response.
Please see the plots shown in Figure 21 through Figure 26 which illustrate the gain and phase diagrams of the
design example.
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Typical Application (continued)
8.2.3 Application Curves
The plots shown in Figure 21 through Figure 26 show the gain and phase diagrams of the design example. The
overall bandwidth is lower in a buck-boost application due the compensation challenges associated with the
right-half-plane zero. For a pure buck application, the bandwidth could be much higher. The LM5116 data sheet
is a good reference for compensation design of a pure buck mode regulator.
Figure 21. Modulator Gain and Phase - Buck Mode
Figure 22. Modulator Gain and Phase - Buck-Boost Mode
Figure 23. Error Amplifier Gain and Phase - Buck Mode
Figure 24. Error Amplifier Gain and Phase - Buck-Boost
Mode
Figure 25. Overall Loop Gain and Phase - Buck Mode
Figure 26. Overall Loop Gain and Phase - Buck-Boost
Mode
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9 Power Supply Recommendations
9.1 Bias Power Dissipation Reduction
Buck or Buck-boost regulators operating with high-input voltage can dissipate an appreciable amount of power
while supplying the required bias current of the IC. The VCC regulator must step-down the input voltage VIN to a
nominal VCC level of 7 V. The large voltage drop across the VCC regulator translates into high power dissipation
in the VCC regulator. There are several techniques that can significantly reduce this bias regulator power
dissipation. Figure 27 and Figure 28 depict two methods to bias the IC, one from the output voltage and one from
a separate bias supply. In the first case, the internal VCC regulator is used to initially bias the VCC pin. After the
output voltage is established, the VCC pin bias current is supplied through the VCCX pin, which effectively
disables the internal VCC regulator. Any voltage greater than 4 V can supply VCC bias through the VCCX pin.
However, the voltage applied to the VCCX pin should never exceed 15 V. The voltage supplied through VCCX
must be large enough to drive the switching MOSFETs into full saturation.
HB
VIN
HO
Q1
D2
VOUT
L1
HS
D1
LM25118
Cout
CS
CSG
LO
Q2
GND
VCC
VCCX
Figure 27. VCC Bias From VOUT 4 V < VOUT < 15 V
HB
VIN
Q1
D2
HO
VOUT
L1
HS
LM25118
D1
CS
Cout
Q2
LO
VOUT
CSG
VCC
VCCX
VBIAS
Figure 28. VCC Bias With Additional Bias Supply
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9.2 Thermal Considerations
The highest power dissipating components are the two power MOSFETs, the recirculating diode, and the output
diode. The easiest way to determine the power dissipated in the MOSFETs is to measure the total conversion
losses (PIN - POUT), then subtract the power losses in the Schottky diodes, output inductor and any snubber
resistors. An approximation for the recirculating Schottky diode loss is:
P = (1-D) × IOUT × VFWD
(55)
The boost diode loss is:
P = IOUT × VFWD
(56)
If a snubber is used, the power loss can be estimated with an oscilloscope by observation of the resistor voltage
drop at both turnon and turnoff transitions. The LM25118 package has an exposed thermal pad to aid power
dissipation. Selecting diodes with exposed pads will aid the power dissipation of the diodes as well. When
selecting the MOSFETs, pay careful attention to RDS(ON) at high temperature. Also, selecting MOSFETs with low
gate charge will result in lower switching losses.
See Application Notes AN-1520 A Guide to Board Layout for Best Thermal Resistance for Exposed Packages
(SNVA183) and AN-2020 Thermal Design By Insight, Not Hindsight (SNVA419) for thermal management
techniques for use with surface mount components.
10 Layout
10.1 Layout Guidelines
In a buck-boost regulator, there are two loops where currents are switched very fast. The first loop starts from the
input capacitors, and then to the buck switch, the inductor, the boost switch then back to the input capacitor. The
second loop starts from the inductor, and then to the output diode, the output capacitor, the recirculating diode,
and back to the inductor. Minimizing the PCB area of these two loops reduces the stray inductance and
minimizes noise and the possibility of erratic operation. A ground plane in the PCB is recommended as a means
to connect the input filter capacitors to the output filter capacitors and the PGND pins of the LM25118. Connect
all of the low current ground connections (CSS, RT, CRAMP) directly to the regulator AGND pin. Connect the AGND
and PGND pins together through topside copper area covering the entire underside of the device. Place several
vias in this underside copper area to the ground plane of the input capacitors.
10.2 Layout Example
Inductor
DBUCK
QBOOST
QBUCK
DBOOST
RSENSE
COUT
CIN
CIN
VIN
COUT
Controller
GND
GND
VOUT
Figure 29. LM25118 Layout Example
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11 Device and Documentation Support
11.1 Device Support
11.1.1 Development Support
11.1.1.1 Custom Design With WEBENCH® Tools
Click here to create a custom design using the LM25118 device with the WEBENCH® Power Designer.
1. Start by entering the input voltage (VIN), output voltage (VOUT), and output current (IOUT) requirements.
2. Optimize the design for key parameters such as efficiency, footprint, and cost using the optimizer dial.
3. Compare the generated design with other possible solutions from Texas Instruments.
The WEBENCH Power Designer provides a customized schematic along with a list of materials with real-time
pricing and component availability.
In most cases, these actions are available:
• Run electrical simulations to see important waveforms and circuit performance
• Run thermal simulations to understand board thermal performance
• Export customized schematic and layout into popular CAD formats
• Print PDF reports for the design, and share the design with colleagues
Get more information about WEBENCH tools at www.ti.com/WEBENCH.
11.2 Receiving Notification of Documentation Updates
To receive notification of documentation updates, navigate to the device product folder on ti.com. In the upper
right corner, click on Alert me to register and receive a weekly digest of any product information that has
changed. For change details, review the revision history included in any revised document.
11.3 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
11.4 Trademarks
E2E is a trademark of Texas Instruments.
WEBENCH is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.5 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.
11.6 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
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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
www.ti.com
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)
LM25118MH/NOPB
ACTIVE
HTSSOP
PWP
20
73
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM25118
MH
LM25118MHE/NOPB
ACTIVE
HTSSOP
PWP
20
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM25118
MH
LM25118MHX/NOPB
ACTIVE
HTSSOP
PWP
20
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM25118
MH
(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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