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LM5045
SNVS699H – FEBRUARY 2011 – REVISED JANUARY 2015
LM5045 Full-Bridge PWM Controller With Integrated MOSFET Drivers
1 Features
3 Description
•
The LM5045 PWM controller contains all of the
features necessary to implement full-bridge topology
power converters using either current mode or
voltage mode control. This device is intended to
operate on the primary side of an isolated DC-DC
converter with input voltage up to 100 V. This highly
integrated controller-driver provides dual 2-A highside and low-side gate drivers for the four external
bridge MOSFETs plus control signals for the
secondary-side synchronous rectifier MOSFETs.
External resistors program the leading and trailing
edge dead-time between the main and synchronous
rectifier control signals. Intelligent start-up of the
synchronous rectifiers allows monotonic turnon of the
power converter even with prebias load conditions.
Additional features include cycle-by-cycle current
limiting, hiccup mode restart, programmable softstart, synchronous rectifier soft-start, and a 2-MHz
capable oscillator with synchronization capability and
thermal shutdown.
1
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Highest Integration Controller for Small Form
Factor, High-Density Power Converters
High-Voltage Start-Up Regulator
Intelligent Sync Rectifier Start-Up Allows Linear
Turnon Into Prebiased Loads
Synchronous Rectifiers Disabled in UVLO mode
and Hiccup Mode
Two Independent, Programmable Synchronous
Rectifier Dead-Time Adjustments
Four High-Current 2-A Bridge Gate Drivers
Wide-Bandwidth Optocoupler Interface
Configurable for Either Current Mode or Voltage
Mode Control
Dual-Mode Overcurrent Protection
Resistor Programmed 2-MHz Oscillator
Programmable Line UVLO and OVP
2 Applications
•
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Device Information(1)
E-Bike
Military: Radar and Electronic Warfare
Power: Telecom DC-DC Module: Analog
Private Branch Exchange (PBX)
Solar Power Inverters
Vector Signal Generators
Microwave Ovens
Point-to-Point Microwave Backhaul
Power: Telecom/Server AC/DC Supply: Dual
Controller: Analog
Solar Micro-Inverters
TETRA Base Stations
Washing Machine: Low-End
PART NUMBER
LM5045
PACKAGE
BODY SIZE (NOM)
HTSSOP (28)
9.70 mm × 4.40 mm
WQFN (28)
5.00 mm × 5.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Simplified Full-Bridge Power Converter
Vin
Vout
T1
T1
VCC
VCC
BST1 HS1 LO1
HO1
VIN
SLOPE RAMP CS
LO2
HS2 BST2 HO2
SR1
UVLO
SR2
LM5045 FULL-BRIDGE CONTROLLER
WITH INTEGRATED GATE DRIVERS
OVP
VCC
COMP
SSOFF
RT
RES
SS SSSR RD1
RD2
GATE
DRIVE
ISOLATION
ISOLATED
FEEDBACK
REF PGND AGND
ISOLATION
BOUNDARY
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.
LM5045
SNVS699H – FEBRUARY 2011 – REVISED JANUARY 2015
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
3
6
Absolute Maximum Ratings ..................................... 6
ESD Ratings ............................................................ 6
Recommended Operating Conditions....................... 6
Thermal Information .................................................. 6
Electrical Characteristics........................................... 7
Typical Characteristics ............................................ 10
7.3 Feature Description................................................. 14
7.4 Device Functional Modes........................................ 22
8
Application and Implementation ........................ 25
8.1 Application Information............................................ 25
8.2 Typical Application .................................................. 25
9 Power Supply Recommendations...................... 34
10 Layout................................................................... 34
10.1 Layout Guidelines ................................................. 34
10.2 Layout Example .................................................... 35
11 Device and Documentation Support ................. 37
Detailed Description ............................................ 12
11.1 Trademarks ........................................................... 37
11.2 Electrostatic Discharge Caution ............................ 37
11.3 Glossary ................................................................ 37
7.1 Overview ................................................................. 12
7.2 Functional Block Diagram ....................................... 13
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 G (March 2013) to Revision H
•
Added Pin Configuration and Functions section, ESD Ratings 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
Changes from Revision F (March 2013) to Revision G
•
2
Page
Page
Changed layout of National Data Sheet to TI format ........................................................................................................... 25
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SNVS699H – FEBRUARY 2011 – REVISED JANUARY 2015
5 Pin Configuration and Functions
PWP Package
28-Pin HTSSOP With PowerPAD™
Top View
UVLO
1
28
VIN
OVP
2
27
HS1
RAMP
3
26
HO1
CS
4
25
BST1
SLOPE
5
24
SR1
COMP
6
23
LO1
REF
7
22
PGND
RT
8
21
VCC
AGND
9
20
LO2
RD1
10
19
SR2
RD2
18
BST2
11
12
17
HO2
RES
13
16
HS2
SS
15
SS OFF
SSSR
PowerPAD™
14
RSG Package
28-Pin WQFN, 5 mm × 5 mm
Top View
CS
RAMP
OVP
UVLO
VIN
HS1
HO1
28
27
26
25
24
23
22
SLOPE
1
21
BST1
COMP
2
20
SR1
REF
3
19
LO1
RT
4
18
PGND
AGND
5
17
VCC
RD1
6
16
LO2
RD2
7
15
SR2
11
RES
SS
SSSR
SS OFF
12
13
14
BST2
10
HO2
9
HS2
8
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SNVS699H – FEBRUARY 2011 – REVISED JANUARY 2015
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Pin Functions
PIN
NAME
HTSSOP
WQFN
AGND
9
5
BST1
25
21
BST2
18
14
I/O
O
I
DESCRIPTION
APPLICATION INFORMATION
Analog Ground
Connect directly to the Power Ground.
Gate Drive Bootstrap
Bootstrap capacitors connected between BST1, 2 and SW1,
2 provide bias supply for the high side HO1, 2 gate drivers.
External diodes are required between VCC and BST1, 2 to
charge the bootstrap capacitors when SW1, 2 are low.
COMP
6
2
I
Input to the Pulse Width
Modulator
An external opto-coupler connected to the COMP pin
sources current into an internal NPN current mirror. The
PWM duty cycle is at maximum with zero input current, while
1 mA reduces the duty cycle to zero. The current mirror
improves the frequency response by reducing the AC voltage
across the opto-coupler.
CS
4
28
I
Current Sense Input
If CS exceeds 750 mV the PWM output pulse will be
terminated, entering cycle-by-cycle current limit. An internal
switch holds CS low for 40 nS after either output switches
high to blank leading edge transients.
HO1
26
22
O
High Side Output Driver
High side PWM outputs capable of driving the upper
MOSFET of the bridge with 1.5-A peak source and 2-A peak
sink current.
I
Switch Node
Common connection of the high side FET source, low side
FET drain and transformer primary winding.
O
Low Side Output Driver
Alternating output of the PWM gate driver. Capable of 1.5-A
peak source and 2-A peak sink current.
HO2
17
13
HS1
27
23
HS2
16
12
LO1
23
19
LO2
20
16
OVP/OTP
2
26
I
Overvoltage Protection
An external voltage divider from the input power supply sets
the shutdown level during an over-voltage condition.
Alternatively, an external NTC thermistor voltage divider can
be used to set the shutdown temperature. The threshold is
1.25 V. Hysteresis is set by an internal current that sources
20 µA of current into the external resistor divider.
PGND
22
18
O
Power Ground
Connect directly to Analog Ground
RAMP
3
27
I
Input to PWM Comparator
Modulation ramp for the PWM comparator. This ramp can be
a signal representative of the primary current (current mode)
or proportional to the input voltage (feed-forward voltage
mode). This pin is reset to GND at the end of every cycle.
RD1
10
6
I
Synchronous Rectifier
Leading Edge Delay
The resistance connected between RD1 and AGND sets the
delay from the falling edge of SR1 or SR2 and the rising
edge of HO2/LO1 or HO1/LO2, respectively.
RD2
11
7
I
Synchronous Rectifier
Trailing Edge Delay
The resistance connected between RD2 and AGND sets the
delay from the falling edge of HO1/LO2 or HO2/LO1 and the
rising edge of SR2 or SR1, respectively.
REF
7
3
O
Output of a 5V reference
Maximum output current is 15 mA. Locally decouple with a
0.1-µF capacitor.
I
Restart Timer
Whenever the CS pin exceeds the 750-mV cycle-cycle
current limit threshold, 30-µA current is sourced into the RES
capacitor for the remainder of the PWM cycle. If the RES
capacitor voltage reaches 1.0 V, the SS capacitor is
discharged to disable the HO1, HO2, LO1, LO2 and SR1,
SR2 outputs. The SS pin is held low until the voltage on the
RES capacitor has been ramped between 2-V and 4-V eight
times by 10-µA charge and 5-µA discharge currents. After
the delay sequence, the SS capacitor is released to initiate a
normal start-up sequence.
O
Oscillator Frequency
Control and Frequency
Synchronization
The resistance connected between RT and AGND sets the
oscillator frequency. Synchronization is achieved by AC
coupling a pulse to the RT/SYNC pin that raises the voltage
at least 1.5 V above the 2-V nominal bias level.
O
Slope Compensation
Current
A ramping current source from 0 to 100 µA is provided for
slope compensation in current mode control. This pin can be
connected through an appropriate resistor to the CS pin to
provide slope compensation. If slope compensation is not
required, SLOPE must be tied to ground.
RES
12
RT/SYNC
SLOPE
4
8
5
8
4
1
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Pin Functions (continued)
PIN
NAME
I/O
DESCRIPTION
APPLICATION INFORMATION
HTSSOP
WQFN
SR1
24
20
O
Synchronous Rectifier
Driver
Control output for synchronous rectifier gate. Capable of
peak sourcing 100 mA and sinking 400 mA.
SR2
19
15
O
Synchronous Rectifier
Driver
Control output for synchronous rectifier gate. Capable of
peak sourcing 100 mA and sinking 400 mA.
Soft-Start Input
An internal 20-µA current source charges the SS pin during
start-up. The input to the PWM comparator gradually rises as
the SS capacitor charges to steadily increase the PWM duty
cycle. Pulling the SS pin to a voltage below 20 0mV stops
PWM pulses at HO1, 2 and LO1, 2 and turns off the
synchronous rectifier FETs to a low state.
SS
13
9
I
SSOFF
15
11
I
Soft-Stop Disable
When SS OFF pin is connected to the AGND, the LM5045
soft-stops in the event of a VIN UVLO and Hiccup mode
current limit condition. If the SSOFF pin is connected to REF
pin, the controller hard-stops on any fault condition. Refer to
Table 1 for more details.
SSSR
14
10
I
Secondary Side Soft-Start
An external capacitor and an internal 20-µA current source
set the soft-start ramp for the synchronous rectifiers. The
SSSR capacitor charge-up is enabled after the first output
pulse and SS > 2 V and Icomp < 800 µA
Line Undervoltage
Lockout
An external voltage divider from the power source sets the
shutdown and standby comparator levels. When UVLO
reaches the 0.4-V threshold the VCC and REF regulators are
enabled. At the 1.25-V threshold, the SS pin is released and
the controller enters the active mode. Hysteresis is set by an
internal current sink that pulls 20 µA from the external
resistor divider.
UVLO
1
25
I
VCC
21
17
I
Output of Start-Up
Regulator
The output voltage of the start-up regulator is initially
regulated to 9.5 V. Once the secondary side soft-start (SSSR
pin) reaches 1 V, the VCC output is reduced to 7.7 V. If an
auxiliary winding raises the voltage on this pin above the
regulation set-point, the internal start-up regulator will
shutdown, thus reducing the IC power dissipation.
VIN
28
24
I
Input Power Source
Input to the Start-up Regulator. Operating input range is 14 V
to 100 V. For power sources outside of this range, the
LM5045 can be biased directly at VCC by an external
regulator.
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SNVS699H – FEBRUARY 2011 – REVISED JANUARY 2015
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6 Specifications
6.1 Absolute Maximum Ratings
(1)
MIN
MAX
UNIT
–0.3
105
V
–5
105
V
BST1/BST2 to GND
–0.3
116
V
BST1/BST2 to HS1/HS2
–0.3
16
V
HO1/HO2 to HS1/HS2 (3)
–0.3
BST1/BST2 + 0.3
V
LO1/LO2/SR1/SR2 (3)
–0.3
VCC + 0.3
V
VCC to GND
–0.3
16
V
REF,SSOFF,RT,OVP,UVLO to GND
–0.3
7
V
RAMP
–0.3
7
V
VIN to GND
HS to GND (2)
COMP
COMP Input Current
All other inputs to GND (3)
–0.3
Junction Temperature
Storage temperature, Tstg
(1)
(2)
(3)
–55
–0.3
V
10
mA
REF + 0.3
V
150
°C
150
°C
Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which
operation of the device is intended to be functional. For ensured specifications and test conditions, see the Electrical Characteristics.
The negative HS voltage must never be more negative than VCC–16 V. For example, if VCC = 12 V, the negative transients at HS must
not exceed –4 V.
These pins are output pins and as such should not be connected to an external voltage source. The voltage range listed is the limits the
internal circuitry is designed to reliably tolerate in the application circuit.
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)
±750
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)
MIN
VIN Voltage
NOM
14
External Voltage Applied to VCC
MAX
UNIT
100
V
10
14
V
Junction Temperature
–40
125
°C
SLOPE
–0.3
2
V
6.4 Thermal Information
LM5045
THERMAL METRIC
(1)
PWP
RSG
UNIT
28 PINS
RθJA
Junction-to-ambient thermal resistance
40
40
RθJC(top)
Junction-to-case (top) thermal resistance
4
4
(1)
6
°C/W
For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report, SPRA953.
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6.5 Electrical Characteristics
Limits in standard typeface are for TJ = 25°C only; MIN and MAX limits apply the junction temperature range of –40°C to
125°C. Unless otherwise specified, the following conditions apply: VIN = 48 V, RT = 25 kΩ, RD1 = RD2 = 20 kΩ. No load on
HO1, HO2, LO1, LO2, SR1, SR2, COMP = 0 V, UVLO = 2.5 V, OVP = 0 V, SSOFF = 0 V.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
START-UP REGULATOR (VCC PIN)
VCC1
VCC voltage
ICC= 10 mA (SSSR < 1 V)
9.3
9.6
9.9
VCC2
VCC voltage
ICC= 10 mA (SSSR > 1 V)
7.5
7.8
8.1
ICC(Lim)
VCC current limit
VCC= 6 V
60
80
mA
ICC(ext)
VCC supply current
Supply current into VCC from an
externally applied source. VCC = 10
V
4.6
mA
35
mV
VCC load regulation
ICC from 0 to 50 mA
VCC(UV)
VCC undervoltage threshold
Positive going VCC
IIN
VIN operating current
Negative going VCC
VIN shutdown current
VIN start-up regulator leakage
VCC1–0.2
VCC1–0.1
5.9
6.3
V
V
V
6.7
4
V
mA
VIN = 20 V, VUVLO = 0 V
300
520
µA
VIN = 100 V, VUVLO = 0 V
350
550
µA
VCC=10 V
160
µA
VOLTAGE REFERENCE REGULATOR (REF PIN)
VREF
REF voltage
IREF = 0 mA
4.85
5
5.15
25
50
REF voltage regulation
IREF = 0 to 10 mA
IREF(Lim)
REF current limit
VREF = 4.5 V
15
20
VREFUV
VREF undervoltage threshold
Positive going VREF
4.3
4.5
Hysteresis
V
mV
mA
4.7
0.25
V
V
UNDERVOLTAGE LOCK OUT AND SHUTDOWN (UVLO PIN)
VUVLO
Under-voltage threshold
1.18
1.25
1.32
V
IUVLO
Hysteresis current
UVLO pin sinking current when
VUVLO < 1.25 V
16
20
24
µA
Undervoltage standby enable
threshold
UVLO voltage rising
0.32
0.4
0.48
V
OVP shutdown threshold
OVP rising
1.18
1.25
1.32
V
OVP hysteresis current
OVP sources current when OVP >
1.25 V
16
20
24
µA
16
20
24
µA
1.93
2
2.2
V
Hysteresis
VOVP
0.05
V
SOFT-START (SS PIN)
ISS
SS charge current
VSS = 0 V
SS threshold for SSSR charge current ICOMP < 800 µA
enable
SS output low voltage
Sinking 100 µA
SS threshold to disable switching
40
mV
200
mV
ISSSR
SSSR charge current
VSS > 2 V, ICOMP < 800 µA
16
20
24
µA
ISSSR-DIS1
SSSR discharge current 1
VUVLO < 1.25 V
54
65
75
µA
ISSSR-DIS2
SSSR discharge current 2
VRES > 1 V
109
125
147
SSSR output low voltage
Sinking 100 µA
SSSR threshold to enable SR1/SR2
µA
50
mV
1.2
V
CURRENT SENSE INPUT (CS PIN)
VCS
RCS
Current limit threshold
0.71
0.75
CS delay to output
65
CS leading edge blanking
50
CS sink impedance (clocked)
Internal FET sink impedance
18
0.785
V
ns
ns
45
Ω
SOFT-STOP DISABLE (SS OFF PIN)
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Electrical Characteristics (continued)
Limits in standard typeface are for TJ = 25°C only; MIN and MAX limits apply the junction temperature range of –40°C to
125°C. Unless otherwise specified, the following conditions apply: VIN = 48 V, RT = 25 kΩ, RD1 = RD2 = 20 kΩ. No load on
HO1, HO2, LO1, LO2, SR1, SR2, COMP = 0 V, UVLO = 2.5 V, OVP = 0 V, SSOFF = 0 V.
PARAMETER
VIH(min)
TEST CONDITIONS
MIN
TYP
MAX
UNIT
SSOFF Input-threshold
2.8
V
SSOFF pulldown resistance
200
kΩ
CURRENT LIMIT RESTART (RES Pin)
RRES
RES pulldown resistance
37
Ω
VRES
RES hiccup threshold
1
V
RES upper counter threshold
4
V
RES lower counter threshold
2
V
Termination of hiccup timer
IRES-SRC1
Charge current source 1
VRES < 1 V, VCS> 750 mV
30
µA
IRES-SRC2
Charge current source 2
1 V < VRES < 4 V
10
µA
IRES-DIS2
Discharge current source 1
VCS < 750 mV
5
µA
IRES-DIS2
Discharge current source 2
2 V < VRES < 4 V
5
µA
Ratio of time in hiccup mode to time
in current limit
VRES > 1 V, Hiccup counter
147
VOLTAGE FEED-FORWARD (RAMP PIN)
RAMP sink impedance (Clocked)
5.5
20
Ω
OSCILLATOR (RT PIN)
FSW1
Frequency (LO1, half oscillator
frequency)
RT = 25 kΩ
185
200
215
kHz
FSW2
Frequency (LO1, half oscillator
frequency)
RT = 10 kΩ
420
480
540
kHz
DC level
2
RT sync threshold
8
2.8
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3
V
3.3
V
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Electrical Characteristics (continued)
Limits in standard typeface are for TJ = 25°C only; MIN and MAX limits apply the junction temperature range of –40°C to
125°C. Unless otherwise specified, the following conditions apply: VIN = 48 V, RT = 25 kΩ, RD1 = RD2 = 20 kΩ. No load on
HO1, HO2, LO1, LO2, SR1, SR2, COMP = 0 V, UVLO = 2.5 V, OVP = 0 V, SSOFF = 0 V.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
RD1 = 20 kΩ
45
65
90
ns
RD1 = 100 kΩ
232
300
388
ns
RD2 = 20 kΩ
43
65
90
ns
RD2 = 100 kΩ
227
300
384
ns
800
940
µA
1.0
1.22
SYNCHRONOUS RECTIFIER TIMING CONTROL (RD1 and RD2 PINS)
T1
SR trailing edge delay
SR turnoff to HO&LO both on
T2
SR leading edge
HO or LO turnoff to SR turnon
COMP PIN
VPWM-OS
COMP current to RAMP offset
VRAMP = 0 V
680
VSS-OS
SS to RAMP offset
VRAMP = 0 V
0.78
COMP current to RAMP gain
ΔRAMP/ΔICOMP
SS to RAMP gain
ΔSS/ΔRAMP
COMP current for SSSR charge
current enable
VSS > 2 V
0.5
690
COMP to output delay
Minimum duty cycle
V
Ω
2400
800
915
120
ICOMP = 1 mA
µA
ns
0%
SLOPE COMPENSATION (SLOPE PIN)
ISLOPE
Slope compensation current ramp
Peak of RAMP current
100
µA
BOOST (BST PIN)
VBst
uv
BST under-voltage threshold
VBST – VHS rising
Hysteresis
3.8
4.7
5.6
0.5
V
V
HO1, HO2, LO1, LO2 GATE DRIVERS
VOL
Low-state output voltage
IHO/LO = 100 mA
0.16
0.32
VOH
High-state output voltage
IHO/LO = 100 mA
VOHL = VCC – VLO
VOHH = VBST – VHO
0.27
0.495
V
V
Rise Time
C-load = 1000 pF
16
ns
Fall Time
C-load = 1000 pF
11
ns
IOHL
Peak Source Current
VHO/LO = 0 V
1.5
A
IOLL
Peak Sink Current
VHO/LO = VCC
2
A
SR1, SR2 GATE DRIVERS
VOL
Low-state output voltage
ISR1/SR2 = 10 mA
0.05
0.1
V
VOH
High-state output voltage
ISR1/SR2 = 10 mA,
VOH = VREF – VSR
0.17
0.28
V
Rise Time
C-load = 1000 pF
60
ns
Fall Time
C-load = 1000 pF
20
ns
IOHL
Peak Source Current
VSR = 0 V
0.1
A
IOLL
Peak Sink Current
VSR = VREF
0.4
A
160
°C
25
°C
THERMAL SHUTDOWN
TSD
Thermal Shutdown Temp
Thermal Shutdown Hysteresis
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6.6 Typical Characteristics
100
36V
EFFICIENCY (%)
90
48V
80
72V
VOUT= 3.3V
70
60
50
5
10
15
20
25
LOAD CURRENT (A)
30
Figure 2. VCC vs ICC
Figure 1. Application Board Efficiency
6
5
VUVLO=3V
IIN(V)
4
3
VUVLO=1V
2
1
VUVLO=0V
0
0
10
20
40
60
VIN(V)
80
100
Figure 3. VCC and VREF vs VIN
Figure 4. IIN vs VIN
Figure 5. VREF vs IREF
Figure 6. Oscillator Frequency vs RT
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Typical Characteristics (continued)
DEAD-TIME T1,T2 (ns)
75
70
65
T2
T1
60
55
-50
Figure 7. Dead-Time T1, T2 vs RD1, RD2
0
50
100
TEMPERATURE(°C)
150
Figure 8. Dead-Time T1, T2 vs. Temperature
Figure 9. CS Threshold vs Temperature
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7 Detailed Description
7.1 Overview
The LM5045 PWM controller contains all of the features necessary to implement a full-bridge topology power
converter using either current mode or voltage mode control. This device is intended to operate on the primary
side of an isolated DC-DC converter with input voltage up to 100 V. This highly integrated controller-driver
provides dual 2-A high-side and low-side gate drivers for the four external bridge MOSFETs plus control signals
for secondary side synchronous rectifiers. External resistors program the leading and trailing edge dead-time
between the main and synchronous rectifier control signals. Intelligent start-up of synchronous rectifier allows
turnon of the power converter into the prebias loads. Cycle-by-cycle current limit protects the power components
from load transients while hiccup mode protection limits average power dissipation during extended overload
conditions. Additional features include programmable soft-start, soft-start of the synchronous rectifiers, and a 2MHz capable oscillator with synchronization capability and thermal shutdown.
12
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7.2 Functional Block Diagram
VOLTAGE
REGULATOR
VIN
1.25V
VCC
+
OVP
UVLO
0.4V
VCC
UVLO
20 PA
-
5V
HYSTERESIS
+
SHUTDOWN
+
1.25V
REF
5V
REFERENCE
BST1
LOGIC
HO1
THERMAL
LIMIT
(160°C)
STANDBY
HS1
-
VCC
UVLO
HYSTERESIS
LO1
20 PA
BST2
CLK
HO2
RT
DELAY
OSCILLATOR
Q
S
100 PA
HS2
TIMERS
T
VCC
Q
AND
Q
DRIVER
LO2
LOGIC
R
REF
0 PA
SR1
SLOPE
REF
SLOPECOMP
RAMP GENERATOR
SR2
RD1
RD2
SSOFF
RAMP
20 PA
SSSR
5V
5k
COMP
1V
+
PWM
-
R
DRIVER
LOGIC
SS
20 PA
SS
R
1:1
SS
SS
Buffer
0.75V
CS
-
10 PA
CS
+
HICCUP
MODE
TIMER and
LOGIC
CLK + LEB
PGND
30 PA
RES
5 PA
+
AGND
-
1.0V
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7.3 Feature Description
7.3.1 High-Voltage Start-Up Regulator
The LM5045 contains an internal high-voltage start-up regulator that allows the input pin (VIN) to be connected
directly to the supply voltage over a wide range from 14 V to 100 V. The input can withstand transients up to
105 V. When the UVLO pin potential is greater than 0.4 V, the VCC regulator is enabled to charge an external
capacitor connected to the VCC pin. The VCC regulator provides power to the voltage reference (REF) and the
gate drivers (HO1/HO2 and LO1/LO2). When the voltage on the VCC pin exceeds its undervoltage (UV)
threshold, the internal voltage reference (REF) reaches its regulation set point of 5 V and the UVLO voltage is
greater than 1.25 V, the soft-start capacitor is released and normal operation begins. The regulator output at
VCC is internally current limited. The value of the VCC capacitor depends on the total system design, and its
start-up characteristics. The recommended range of values for the VCC capacitor is 0.47 μF to 10 µF.
The internal power dissipation of the LM5045 can be reduced by powering VCC from an external supply. The
output voltage of the VCC regulator is initially regulated to 9.5 V. After the synchronous rectifiers are engaged
(which is approximately when the output voltage in within regulation), the VCC voltage is reduced to 7.7 V. In
typical applications, an auxiliary transformer winding is connected through a diode to the VCC pin. This winding
must raise the VCC voltage above 8V to shut off the internal start-up regulator. Powering VCC from an auxiliary
winding improves efficiency while reducing the power dissipation of the controller. The VCC UV circuit will still
function in this mode, requiring that VCC never falls below its UV threshold during the start-up sequence. The
VCC regulator series pass transistor includes a diode between VCC and VIN that should not be forward biased
in normal operation. Therefore, the auxiliary VCC voltage should never exceed the VIN voltage.
An external DC bias voltage can be used instead of the internal regulator by connecting the external bias voltage
to both the VCC and the VIN pins. This implementation is shown in the Application and Implementation section.
The external bias must be greater than 10 V and less than the VCC maximum voltage rating of 14 V.
7.3.2 Line Undervoltage Detector
The LM5045 contains a dual level undervoltage lockout (UVLO) circuit. When the UVLO pin voltage is below 0.4
V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.4 V but less
than 1.25 V, the controller is in standby mode. In standby mode the VCC and REF bias regulators are active
while the controller outputs are disabled. When the VCC and REF outputs exceed their respective under-voltage
thresholds and the UVLO pin voltage is greater than 1.25 V, the soft-start capacitor is released and the normal
operation begins. An external set-point voltage divider from VIN to GND can be used to set the minimum
operating voltage of the converter. The divider must be designed such that the voltage at the UVLO pin will be
greater than 1.25 V when VIN enters the desired operating range. UVLO hysteresis is accomplished with an
internal 20 μA current sink that is switched on or off into the impedance of the set-point divider. When the UVLO
threshold is exceeded, the current sink is deactivated to quickly raise the voltage at the UVLO pin. When the
UVLO pin voltage falls below the 1.25 V threshold, the current sink is enabled causing the voltage at the UVLO
pin to quickly fall. The hysteresis of the 0.4 V shutdown comparator is internally fixed at 50 mV.
The UVLO pin can also be used to implement various remote enable / disable functions. Turning off the
converter by forcing the UVLO pin to standby condition (0.4 V < UVLO < 1.25 V) provides a controlled soft-stop.
Refer to the Soft-Stop section for more details.
7.3.3 Overvoltage Protection
An external voltage divider can be used to set either an overvoltage or an overtemperature protection. During an
OVP condition, the SS and SSSR capacitors are discharged and all the outputs are disabled. The divider must
be designed such that the voltage at the OVP pin is greater than 1.25 V when overvoltage/temperature condition
exists. Hysteresis is accomplished with an internal 20 μA current source. When the OVP pin voltage exceeds
1.25 V, the 20 μA current source is activated to quickly raise the voltage at the OVP pin. When the OVP pin
voltage falls below the 1.25 V threshold, the current source is deactivated causing the voltage at the OVP to
quickly fall. Refer to the Application and Implementation section for more details.
7.3.4 Reference
The REF pin is the output of a 5-V linear regulator that can be used to bias an optocoupler transistor and
external housekeeping circuits. The regulator output is internally current limited to 15 mA. The REF pin must be
locally decoupled with a ceramic capacitor, the recommended range of values are from 0.1 μF to 10 μF
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Feature Description (continued)
7.3.5 Oscillator, Sync Input
The LM5045 oscillator frequency is set by a resistor connected between the RT pin and AGND. The RT resistor
should be located very close to the device. To set a desired oscillator frequency (FOSC), the necessary value of
RT resistor can be calculated from the following equation:
RT =
1
FOSC x 1 x 10-10
(1)
For example, if the desired oscillator frequency is 400 kHz, that is, each phase (LO1 or LO2) at 200 kHz, the
value of RT will be 25 kΩ. If the LM5045 is to be synchronized to an external clock, that signal must be coupled
into the RT pin through a 100 pF capacitor. The RT pin voltage is nominally regulated at 2.0 V and the external
pulse amplitude should lift the pin to between 3.5 V and 5.0 V on the low-to-high transition. The synchronization
pulse width should be between 15 and 200 ns. The RT resistor is always required, whether the oscillator is free
running or externally synchronized and the SYNC frequency must be equal to, or greater than the frequency set
by the RT resistor. When syncing to an external clock, it is recommended to add slope compensation by
connecting an appropriate resistor from the VCC pin to the CS pin. Also disable the SLOPE pin by grounding it.
7.3.6 Cycle-by-Cycle Current Limit
The CS pin is to be driven by a signal representative of the primary current of the transformer. If the voltage on
the CS pin exceeds 0.75 V, the current sense comparator immediately terminates the PWM cycle. A small RC
filter connected to the CS pin and located near the controller is recommended to suppress noise. An internal 18Ω MOSFET discharges the external current sense filter capacitor at the conclusion of every cycle. The discharge
MOSFET remains on for an additional 40 ns after the start of a new PWM cycle to blank leading edge spikes.
The current sense comparator is very fast and may respond to short duration noise pulses. Layout is critical for
the current sense filter and the sense resistor. The capacitor associated with CS filter must be placed very close
to the device and connected directly to the CS and AGND pins. If a current sense transformer is used, both the
leads of the transformer secondary should be routed to the filter network, which should be located close to the
IC. When designing with a current sense resistor, all of the noise sensitive low power ground connections should
be connected together near the AGND pin, and a single connection should be made to the power ground (sense
resistor ground point).
7.3.7 Hiccup Mode
The LM5045 provides a current limit restart timer to disable the controller outputs and force a delayed restart
(such as Hiccup mode) if a current limit condition is repeatedly sensed. The number of cycle-by-cycle current
limit events required to trigger the restart is programmed by the external capacitor at the RES pin. During each
PWM cycle, the LM5045 either sources or sinks current from the RES capacitor. If current limit is detected, the 5
μA current sink is disabled and a 30μA current source is enabled. If the RES voltage reaches the 1.0 V
threshold, the following restart sequence occurs, as shown in Figure 10:
• The SS and SSSR capacitors are fully discharged
• The 30 μA current source is turned-off and the 10 μA current source is turned-on.
• Once the voltage at the RES pin reaches 4.0 V the 10 μA current source is turned-off and a 5μA current sink
is turned-on, ramping the voltage on the RES capacitor down to 2.0 V.
• Once RES capacitor reaches 2.0 V, threshold, the 10 μA current source is turned-on again. The RES
capacitor voltage is ramped between 4.0 V and 2.0 V eight times.
• When the counter reaches eight, the RES pin voltage is pulled low and the soft-start capacitor is released to
begin a soft-start sequence. The SS capacitor voltage slowly increases. When the SS voltage reaches 1.0 V,
the PWM comparator will produce the first narrow pulse.
• If the overload condition persists after restart, cycle-by-cycle current limiting will begin to increase the voltage
on the RES capacitor again, repeating the hiccup mode sequence.
• If the overload condition no longer exists after restart, the RES pin will be held at ground by the 5 μA current
sink and the normal operation resumes.
The hiccup mode function can be completely disabled by connecting the RES pin to the AGND pin. In this
configuration the cycle-by-cycle protection will limit the maximum output current indefinitely, no hiccup restart
sequences will occur.
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Feature Description (continued)
4V
2V
1V
Count to Eight
Restart delay
Soft-Start
1V
Hiccup Mode off-time
Figure 10. Hiccup Mode Delay and Soft-Start Timing Diagram
7.3.8 PWM Comparator
The LM5045 pulse width modulator (PWM) comparator is a three input device, it compares the signal at the
RAMP pin to the loop error signal or the soft-start, whichever is lower, to control the duty cycle. This comparator
is optimized for speed to achieve minimum controllable duty cycles. The loop error signal is received from the
external feedback and isolation circuit in the form of a control current into the COMP pin. The COMP pin current
is internally mirrored by a matching pair of NPN transistors which sink current through a 5-kΩ resistor connected
to the 5-V reference. The resulting control voltage passes through a 1-V offset, followed by a 2:1 resistor divider
before being applied to the PWM comparator.
An optocoupler detector can be connected between the REF pin and the COMP pin. Because the COMP pin is
controlled by a current input, the potential difference across the optocoupler detector is nearly constant. The
bandwidth limiting phase delay which is normally introduced by the significant capacitance of the optocoupler is
thereby greatly reduced. Higher loop bandwidths can be realized because the bandwidth limiting pole associated
with the optocoupler is now at a much higher frequency. The PWM comparator polarity is configured such that
with no current flowing into the COMP pin, the controller produces maximum duty cycle.
7.3.9 Ramp Pin
The voltage at the RAMP pin provides the modulation ramp for the PWM comparator. The PWM comparator
compares the modulation ramp signal at the RAMP pin to the loop error signal to control the duty cycle. The
modulation ramp signal can be implemented either as a ramp proportional to the input voltage, known as feedforward voltage mode control, or as a ramp proportional to the primary current, known as current mode control.
The RAMP pin is reset by an internal MOSFET with an RDS(ON) of 5.5 Ω at the conclusion of each PWM cycle.
The ability to configure the RAMP pin for either voltage mode or current mode allows the controller to be
implemented for the optimum control method depending upon the design constraints. Refer to the Application
and Implementation section for more details on configuring the RAMP pin for feed-forward voltage mode control
and peak current mode control.
7.3.10 Slope Pin
For duty cycles greater than 50% (25% for each phase), peak current mode control is subject to sub-harmonic
oscillation. Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow duty
cycles. This can be eliminated by adding an artificial ramp, known as slope compensation, to the modulating
signal at the RAMP pin. The SLOPE pin provides a current source ramping from 0 to 100μA, at the frequency set
by the RT resistor, for slope compensation. The ramping current source at the SLOPE pin can be used in several
different ways to add slope compensation to the RAMP signal:
• As shown in Figure 11(a), the SLOPE and RAMP pins can be connected together through an appropriate
resistor to the CS pin. This configuration will inject current sense signal plus slope compensation to the RAMP
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Feature Description (continued)
•
pin but CS pin will not see any slope compensation. Therefore, in this scheme slope compensation will not
affect the current limit.
In a second configuration, as shown in Figure 11(b), the SLOPE, RAMP and CS pins can be tied together. In
this configuration the ramping current source from the SLOPE pin will flow through the filter resistor and filter
capacitor, therefore both the CS pin and the RAMP pin will see the current sense signal plus the slope
compensation ramp. In this scheme, the current limit is compensated by the slope compensation and the
current limit onset point will vary.
If the slope compensation is not required for example, in feed-forward voltage mode control, the SLOPE pin must
be connected to the AGND pin. When the RT pin is synched to an external clock, it is recommended to disable
the SLOPE pin and add slope compensation externally by connecting an appropriate resistor from the VCC pin to
the CS pin. Refer to the Application and Implementation section for more details.
LM5045
LM5045
100 PA
SLOPE
100 PA
0
SLOPE
RAMP
RAMP
CLK
Current
Sense
RSLOPE
RFILTER
CLK
Current
Sense
RFILTER
CS
CS
CLK + LEB
RCS
0
CLK + LEB
RCS
CFILTER
(a)
CFILTER
(b)
A.
Slope Compensation Configured for PWM Only (No Current Limit Slope)
B.
Slope Compensation Configured for PWM and Current Limit
Figure 11. Slope Compensation Configuration
7.3.11 Soft-Start
The soft-start circuit allows the power converter to gradually reach a steady state operating point, thereby
reducing the start-up stresses and current surges. When bias is supplied to the LM5045, the SS capacitor is
discharged by an internal MOSFET. When the UVLO, VCC and REF pins reach their operating thresholds, the
SS capacitor is released and is charged with a 20-µA current source. Once the SS pin voltage crosses the 1-V
offset, SS controls the duty cycle. The PWM comparator is a three input device; it compares the RAMP signal
against the lower of the signals between the soft-start and the loop error signal. In a typical isolated application,
as the secondary bias is established, the error amplifier on the secondary side soft-starts and establishes closedloop control, steering the control away from the SS pin.
One method to shutdown the regulator is to ground the SS pin. This forces the internal PWM control signal to
ground, reducing the output duty cycle quickly to zero. Releasing the SS pin begins a soft-start cycle and normal
operation resumes. A second shutdown method is presented in the UVLO and OVP Voltage Divider Selection
section.
7.3.12 Gate Driver Outputs
The LM5045 provides four gate drivers: two floating high-side gate drivers HO1 and HO2 and two ground
referenced low-side gate drivers LO1 and LO2. Each internal driver is capable of source 1.5-A peak and sinking
2-A peak. Initially, the diagonal HO1 and LO2 are turned-on together, followed by an off-time when all the four
gate driver outputs are off. In the subsequent phase the diagonal HO2 and LO1 are turned on together followed
by an off-time. The low-side gate drivers are powered directly by the VCC regulator. The HO1 and HO2 gate
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Feature Description (continued)
drivers are powered from a bootstrap capacitor connected between BST1/BST2 and HS1/HS2, respectively. An
external diode connected between VCC (anode pin) and BST (cathode pin) provides the high-side gate driver
power by charging the bootstrap capacitor from VCC when the corresponding switch node (HS1/HS2 pin) is low.
When the high side MOSFET is turned on, BST1 rises to a peak voltage equal to VCC + VHS1 where VHS1 is the
switch node voltage.
The BST and VCC capacitors should be placed close to the pins of the LM5045 to minimize voltage transients
due to parasitic inductances because the peak current sourced to the MOSFET gates can exceed 1.5 A. The
recommended value of the BST capacitor is 0.1 μF or greater. A low ESR / ESL capacitor, such as a surface
mount ceramic, should be used to prevent voltage droop during the HO transitions.
If the COMP pin is open circuit, the outputs will operate at maximum duty cycle. The maximum duty cycle for
each phase is limited by the dead-time set by the RD1 resistor. If the RD1 resistor is set to zero then the
maximum duty cycle is slightly less than 50% due to the internally fixed dead-time. The internally fixed dead-time
is 30ns which does not vary with the operating frequency. The maximum duty cycle for each output can be
calculated from the following equation:
( F 1 ) - (T1)
DMAX =
OSC
(F 2 )
OSC
where
•
•
T1 is the time set by the RD1 resistor
FOSC is the frequency of the oscillator
(2)
For example, if the oscillator frequency is set at 400 kHz and the T1 time set by the RD1 resistor is 60 ns, the
resulting DMAX will be equal to 0.488.
CLK
HO1,LO2
Tonmax
Tosc
HO2,LO1
T1
T1
TOSC =
1
FOSC
T1 DRD1
Figure 12. Timing Diagram Illustrating the Maximum Duty Cycle and Dead-Time Set by RD1
7.3.13 Synchronous Rectifier Control Outputs (SR1 and SR2)
Synchronous rectification (SR) of the transformer secondary provides higher efficiency, especially for low output
voltage converters, compared to the diode rectification. The reduction of rectifier forward voltage drop (0.5 V to
1.5 V) to 10 mV to 200 mV VDS voltage for a MOSFET significantly reduces rectification losses. In a typical
application, the transformer secondary winding is center tapped, with the output power inductor in series with the
center tap. The SR MOSFETs provide the ground path for the energized secondary winding and the inductor
current. From Figure 13 it can be seen that when the HO1/LO2 diagonal is turned ON, power transfer is enabled
from the primary. During this period, the SR1 MOSFET is enabled and the SR2 MOSFET is turned-off. The
secondary winding connected to the SR2 MOSFET drain is twice the voltage of the center tap at this time. At the
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Feature Description (continued)
conclusion of the HO1/LO2 pulse, the inductor current continues to flow through the SR2 MOSFET body diode.
Because the body diode causes more loss than the SR MOSFET, efficiency can be improved by minimizing the
T2 period while maintaining sufficient timing margin over all conditions (component tolerances, etc.) to prevent
the shoot-through current. When HO2/LO1 enables power transfer from the primary, the SR2 MOSFET is
enabled and the SR1 MOSFET is off.
During the freewheeling period, the inductor current is almost equally shared between both the SR1 and SR2
MOSFETs which effectively shorts the transformer secondary. The SR2 MOSFET is disabled before HO1/LO2 is
turned-on. The SR2 MOSFET body diode continues to carry about the half inductor current until the primary
power raises the SR2 MOSFET drain voltage and reverse biases the body diode. Ideally, dead-time T1 would be
set to the minimum time that allows the SR MOSFET to turn off before the SR MOSFET body diode starts
conducting.
The SR drivers are powered by the REF regulator and each SR output is capable of sourcing 0.1 A and sinking
0.4-A peak. The amplitude of the SR drivers is limited to 5 V. The 5-V SR signals enable the LM5045 to transfer
SR control across the isolation barrier either through a solid-state isolator or a pulse transformer. The actual gate
sourcing and sinking currents for the synchronous MOSFETs are provided by the secondary-side bias and gate
drivers.
T1 and T2 can be programmed by connecting a resistor between RD1 and RD2 pins and AGND. It should be
noted that while RD1 effects the maximum duty cycle, RD2 does not. The RD1 and RD2 resistors should be
located very close to the device. The formula for RD1 and RD2 resistors are given below:
RD(1,2) =
T(1,2)
; For 20k < (1,2) < 100k
3 pF
(3)
If the desired dead-time for T1 is 60ns, then the RD1 will be 20 kΩ.
HO1, LO2
SR2
T1
T2
HO2, LO1
SR1
T1
T2
Figure 13. Synchronous Rectifier Timing Diagram
7.3.14 Soft-Start of the Synchronous Rectifiers
In addition to the basic soft-start already described, the LM5045 contains a second soft-start function that
gradually turns on the synchronous rectifiers to their steady-state duty cycle. This function keeps the
synchronous rectifiers off during the basic soft-start allowing a linear start-up of the output voltage even into prebiased loads. Then the SR output duty cycle is gradually increased to prevent output voltage disturbances due to
the difference in the voltage drop between the body diode and the channel resistance of the synchronous
MOSFETs. Initially, when bias is supplied to the LM5045, the SSSR capacitor is discharged by an internal
MOSFET. When the SS capacitor reaches a 2-V threshold and once it is established that COMP is in control of
the duty cycle such as ICOMP < 800 µA, the SSSR discharge is released and SSSR capacitor begins charging
with a 20-µA current source. Once the SSSR cap crosses the internal 1-V threshold, the LM5045 begins the softstart of the synchronous FETs. The SR soft-start follows a leading edge modulation technique such as the
leading edge of the SR pulse is soft-started as opposed trailing edge modulation of the primary FETs. As shown
in the Figure 14(a), SR1 and SR2 are turned-on simultaneously with a narrow pulse-width during the
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Feature Description (continued)
freewheeling cycle. At the end of the freewheel cycle, that is, at the rising edge of the internal CLK, the SR FET
in-phase with the next power transfer cycle is kept on while the SR FET out of phase with it is turned-off. The inphase SR FET is kept on throughout the power transfer cycle and at the end of it, both the primary FETs and the
in-phase SR FETs are turned-off together. The synchronous rectifier outputs can be disabled by grounding the
SSSR pin.
CLK
HO2/LO1
HO1/LO2
SR1
T1
T2
SR2
Power
Transfer
T1
Freewheel
Waveforms during soft-start
A.
Waveforms during Soft-Start
B.
Waveforms after Soft-Start
T2
Waveforms after soft-start
Figure 14. Waveforms
7.3.15 Prebias Startup
A common requirement for power converters is to have a monotonic output voltage start-up into a prebiased load
such as a precharged output capacitor. In a prebiased load condition, if the synchronous rectifiers are engaged
prematurely they will sink current from the precharged output capacitors resulting in an undesired output voltage
dip. This condition is undesirable and could potentially damage the power converter. The LM5045 uses unique
control circuitry to ensure intelligent turnon of the synchronous rectifiers such that the output has a monotonic
start-up. Initially, the SSSR capacitor is held at ground to disable the synchronous MOSFETs allowing the body
diode to conduct. The synchronous rectifier soft-start is initiated once it is established the duty cycle is controlled
by the COMP instead of the soft-start capacitor, that is, ICOMP < 800 µA and the voltage at the SS pin > 2 V. The
SSSR capacitor is then released and is charged by a 20-µA current source. Further, as shown in Figure 15, a 1V offset on the SSSR pin is used to provide additional delay. This delay ensures the output voltage is in
regulation avoiding any reverse current when the synchronous MOSFETs are engaged.
7.3.16 Soft-Stop
As shown in Figure 16, if the UVLO pin voltage falls below the 1.25-V standby threshold, but above the 0.4-V
shutdown threshold, the SSSR capacitor is soft-stopped with a 60-µA current source (3 times the charging
current). Once the SSSR pin reaches the 1.0-V threshold, both the SS and SSSR pins are immediately
discharged to GND. Soft-stopping the power converter gradually winds down the energy in the output capacitors
and results in a monotonic decay of the output voltage. During the hiccup mode, the same sequence is executed
except that the SSSR is discharged with a 120-µA current source (6 times the charging current). In case of an
OVP, VCC UV, thermal limit or a VREF UV condition, the power converter hard-stops, whereby all of the control
outputs are driven to a low state immediately.
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Feature Description (continued)
2.0V
SS
1.0V
Primary
Secondary
Bias
COMP
1.0V
SSSR
SR1, SR2
VOUT
Prebiased Load
Figure 15. Prebias Voltage Start-up Waveforms
7.3.17 Soft-Stop Off
The Soft-Start Off (SSOFF) pin gives additional flexibility by allowing the power converter to be configured for
hard-stop during line UVLO and hiccup mode condition. If the SS OFF pin is pulled up to the 5-V REF pin, the
power converter hard-stops in any fault condition. Hard-stop drives each control output to a low state
immediately. Refer to Table 1 for more details.
1.25V
VIN UVLO
1.25V
0.45V
SS
SSSR
1.0V
Figure 16. Stop-Stop Waveforms During a UVLO Event
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Feature Description (continued)
Table 1. Soft-Stop in Fault Conditions (1)
(1)
FAULT CONDITION
SSSR
UVLO
(UVLO1.25V)
Hard-Stop
Hiccup
(CS>0.75 and RES>1V)
Soft-Stop
6x the charging rate
VCC/VREF UV
Hard-Stop
Internal Thermal Limit
Hard-Stop
Note: All the above conditions are valid with SSOFF pin tied to GND. If SSOFF=5V, the LM5045 hard-stops in all the conditions. The SS
pin remains high in all the conditions until the SSSR pin reaches 1V.
7.3.18 Thermal Protection
Internal thermal shutdown circuitry is provided to protect the integrated circuit in the event the maximum rated
junction temperature is exceeded. When activated, typically at 160°C, the controller is forced into a shutdown
state with the output drivers, the bias regulators (VCC and REF) disabled. This helps to prevent catastrophic
failures from accidental device overheating. During thermal shutdown, the SS and SSSR capacitors are fully
discharged and the controller follows a normal start-up sequence after the junction temperature falls to the
operating level (140°C).
7.4 Device Functional Modes
7.4.1 Control Method Selection
The LM5045 is a versatile PWM control IC that can be configured for either current mode control or voltage
mode control. The choice of the control method usually depends upon the designer preference. The following
must be taken into consideration while selecting the control method. Current mode control can inherently balance
flux in both phases of the full-bridge topology. The full-bridge topology, like other double ended topologies, is
susceptible to the transformer core saturation. Any asymmetry in the volt-second product applied between the
two alternating phases results in flux imbalance that causes a dc buildup in the transformer. This continual dc
buildup may eventually push the transformer into saturation. The volt-second asymmetry can be corrected by
employing current mode control. In current mode control, a signal representative of the primary current is
compared against an error signal to control the duty cycle. In steady-state, this results in each phase being
terminated at the same peak current by adjusting the pulse-width and thus applying equal volt-seconds to both
the phases.
Current mode control can be susceptible to noise and sub-harmonic oscillation, while voltage mode control
employs a larger ramp for PWM and is usually less susceptible. Voltage-mode control with input line feedforward also has excellent line transient response. When configuring for voltage mode control, a dc blocking
capacitor can be placed in series with the primary winding of the power transformer to avoid any flux imbalance
that may cause transformer core saturation.
7.4.2 Voltage Mode Control Using the LM5045
To configure the LM5045 for voltage mode control, an external resistor (RFF) and capacitor (CFF) connected to
VIN, AGND, and the RAMP pins is required to create a saw-tooth modulation ramp signal shown in Figure 17.
The slope of the signal at RAMP will vary in proportion to the input line voltage. The varying slope provides line
feed-forward information necessary to improve line transient response with voltage mode control. With a constant
error signal, the on-time (TON) varies inversely with the input voltage (VIN) to stabilize the Volt- Second product
of the transformer primary. Using a line feed-forward ramp for PWM control requires very little change in the
voltage regulation loop to compensate for changes in input voltage, as compared to a fixed slope oscillator ramp.
Furthermore, voltage mode control is less susceptible to noise and does not require leading edge filtering.
Therefore, it is a good choice for wide input range power converters. Voltage mode control requires a Type-III
compensation network, due to the complex-conjugate poles of the L-C output filter.
22
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Device Functional Modes (continued)
SLOPE
PROPORTIONAL
TO VIN
VIN
5V
COMP
RFF
VIN
5k
R
R
1V
Gate Drive
1:1
RAMP
CLK
CFF
LM5045
Figure 17. Feed-Forward Voltage Mode Configuration
The recommended capacitor value range for CFF is from 100 pF to 1800 pF. Referring to Figure 17, it can be
seen that CFF value must be small enough to be discharged with in the clock pulse-width which is typically within
50ns. The RDS(ON) of the internal discharge FET is 5.5 Ω.
The value of RFF required can be calculated from
RFF =
-1
FOSC x CFF x In (1-
VRAMP
)
VINMIN
(4)
For example, assuming a VRAMP of 1.5 V (a good compromise of signal range and noise immunity), at VINMIN of
36 V (oscillator frequency of 400 kHz and CFF = 470 pF results in a value for RFF of 125 kΩ.
7.4.3 Current Mode Control Using the LM5045
The LM5045 can be configured for current mode control by applying a signal proportional to the primary current
to the RAMP pin. One way to achieve this is shown in Figure 18. The primary current can be sensed using a
current transformer or sense resistor, the resulting signal is filtered and applied to the RAMP pin through a
resistor used for slope compensation. It can be seen that the signal applied to the RAMP pin consists of the
primary current information from the CS pin plus an additional ramp for slope compensation, added by the
resistor RSLOPE.
The current sense resistor is selected such that during over current condition, the voltage across the current
sense resistor is above the minimum CS threshold of 728 mV.
In general, the amount of slope compensation required to avoid sub-harmonic oscillation is equal to at least onehalf the down-slope of the output inductor current, transformed to the primary. To mitigate sub-harmonic
oscillation after one switching period, the slope compensation must be equal to one times the down slope of the
filter inductor current transposed to primary. This is known as deadbeat control. The slope compensation resistor
required to implement dead-beat control can be calculated as follows:
where
•
NTR is the turns-ratio with respect to the secondary
(5)
For example, for a 3.3 V output converter with a turns-ratio between primary and secondary of 9:1, an output
filter inductance (LFILTER) of 800 nH and a current sense resistor (RSENSE) of 150 mΩ, RSLOPE of 1.67 kΩ will
suffice.
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Device Functional Modes (continued)
LM5045
100 PA
SLOPE
0
RAMP
CLK
Current
Sense
RSLOPE
RFILTER
CS
CLK + LEB
RCS
CFILTER
Figure 18. Current Mode Configuration
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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 LM5045 is a highly integrated PWM controller that contains all of the features necessary for implementing
full-bridge topology power converters using either current mode or voltage mode control. The device targets DCDC converter applications with input voltages of up to 100 Vdc and output power in the range 100 W to 1 kW.
8.2 Typical Application
The following schematic shows an example of a 100W full-bridge converter controlled by LM5045. The operating
input voltage range is 36 V to 75 V, and the output voltage is 3.3 V. The output current capability is 30 A. The
converter is configured for current mode control with external slope compensation. An auxiliary winding is used to
raise the VCC voltage to reduce the controller power dissipation.
Figure 19. Evaluation Board Schematic
8.2.1 Design Requirements
PARAMETERS
VALUE
Input operating range
36 V to 75 V
Output voltage
3.3 V
Measured efficiency at 48 V
92% at 30A
Frequency of operation
420 kHz
Board size
2.28 x 1.45 x 0.5 inches
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Typical Application (continued)
PARAMETERS
VALUE
Load Regulation
0.2%
Line Regulation
0.1%
Line UVLO
34V/32V on/off
Hiccup Mode
Current Limit
8.2.2 Detailed Design Procedure
8.2.2.1 VIN and VCC
The voltage applied to the VIN pin, which may be the same as the system voltage applied to the power
transformer’s primary (VPWR), can vary in the range of the 14 to 100 V. It is recommended that the filter shown in
Figure 20 be used to suppress the transients that may occur at the input supply. This is particularly important
when VIN is operated close to the maximum operating rating of the LM5045. The current into VIN depends
primarily on the operating current of the LM5045, the switching frequency, and any external loads on the VCC
pin, that typically include the gate capacitances of the power MOSFETs. In typical applications, an auxiliary
transformer winding is connected through a diode to the VCC pin. This pin must raise VCC voltage above 8 V to
shut off the internal start-up regulator.
After the outputs are enabled and the external VCC supply voltage has begun supplying power to the IC, the
current into the VIN pin drops below 1mA. VIN should remain at a voltage equal to or above the VCC voltage to
avoid reverse current through the internal body diode of the internal VCC regulator.
8.2.2.2 For Applications With > 100 VIN
For applications where the system input voltage exceeds 100 V, VIN can be powered from an external start-up
regulator as shown in Figure 21. In this configuration, the VIN and VCC pins should be connected together. The
voltage at the VCC and VIN pins must be greater than 10 V (> Max VCC reference voltage) yet not exceed 16 V.
To enable operation the VCC voltage must be raised above 10 V. The voltage at the VCC pin must not exceed
16 V. The voltage source at the right side of Figure 21 is typically derived from the power stage, and becomes
active once the LM5045’s outputs are active.
VPWR
50
VIN
LM5045
0.1 PF
Figure 20. Input Transient Protection
10V - 16V
(from aux winding)
VPWR
VIN
11V
VCC
LM5045
Figure 21. Start-Up Regulator For VPWR > 100 V
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8.2.2.3 UVLO and OVP Voltage Divider Selection
Two dedicated comparators connected to the UVLO and OVP pins are used to detect undervoltage and
overvoltage conditions. The threshold values of both these comparators are set at 1.25 V. The two functions can
be programmed independently with two separate voltage dividers from VIN to AGND as shown in Figure 22 and
Figure 23, or with a three-resistor divider as shown in Figure 24. Independent UVLO and OVP pins provide
greater flexibility for the user to select the operational voltage range of the system. When the UVLO pin voltage is
below 0.4 V, the controller is in a low current shutdown mode. For a UVLO pin voltage greater than 0.4 V but
less than 1.25 V the controller is in standby mode. Once the UVLO pin voltage is greater than 1.25 V, the
controller is fully enabled. Two external resistors can be used to program the minimum operational voltage for the
power converter as shown in Figure 22. When the UVLO pin voltage falls below the 1.25 V threshold, an internal
20 µA current sink is enabled to lower the voltage at the UVLO pin, thus providing threshold hysteresis.
Resistance values for R1 and R2 can be determined from the following equations:
R1 =
R2 =
VHYS
20 PA
1.25V x R1
VPWR-OFF -1.25V - (20 PA x R1)
where
•
•
VPWR is the desired turnon voltage
VHYS is the desired UVLO hysteresis at VPWR
(6)
For example, if the LM5045 is to be enabled when VPWR reaches 33 V, and disabled when VPWR is decreased to
31 V, R1 should be 100 kΩ, and R2 should be 4.2 kΩ. The voltage at the UVLO pin should not exceed 7 V at any
time.
Two external resistors can be used to program the maximum operational voltage for the power converter as
shown in Figure 23. When the OVP pin voltage rises above the 1.25 V threshold, an internal 20-µA current
source is enabled to raise the voltage at the OVP pin, thus providing threshold hysteresis. Resistance values for
R1 and R2 can be determined from the following equations:
R1 =
R2 =
VHYS
20 PA
1.25V x R1
VPWR -1.25V + (20 PA x R1)
(7)
If the LM5045 is to be disabled when VPWR-OFF reaches 80 V and enabled when it is decreased to 78 V. R1
should be 100 kΩ, and R2 should be 1.5 kΩ. The voltage at the OVP pin should not exceed 7 V at any time.
VPWR
LM5045
R1
1.25V
UVLO
STANDBY
20 PA
R2
0.4V
SHUTDOWN
Figure 22. Basic UVLO Configuration
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LM5045
5V
VPWR
20 PA
R1
OVP
STANDBY
1.25V
R2
Figure 23. Basic OVP Configuration
VPWR
R1
UVLO
1.25V
STANDBY
LM5045
20 PA
0.4V
R2
SHUTDOWN
5V
20 PA
OVP
1.25V
STANDBY
R3
Figure 24. UVLO/OVP Divider
The UVLO and OVP can also be set together using a 3 resistor divider ladder as shown in Figure 24. R1 is
calculated as explained in the basic UVLO divider selection. Using the same values, as in the above two
examples, for the UVLO and OVP set points, R1 and R3 remain the same at 100 kΩ and 1.5 kΩ. The R2 is 2.7
kΩ obtained by subtracting R3 from 4.2 kΩ.
Remote configuration of the controller’s operational modes can be accomplished with open drain device(s)
connected to the UVLO pin as shown in Figure 25.
Figure 26 shows an application of the OVP comparator for Remote Thermal Protection using a thermistor (or
multiple thermistors) which may be located near the main heat sources of the power converter. The negative
temperature coefficient (NTC) thermistor is nearly logarithmic, and in this example a 100 kΩ thermistor with the β
material constant of 4500 Kelvin changes to approximately 2 kΩ at 130ºC. Setting R1 to one-third of this
resistance (665 Ω) establishes 130ºC as the desired trip point (for VREF = 5 V). In a temperature band from
20ºC below to 20ºC above the OVP threshold, the voltage divider is nearly linear with 25mV per ºC sensitivity.
R2 provides temperature hysteresis by raising the OVP comparator input by R2 x 20µA. For example, if a 22kΩ
resistor is selected for R2, then the OVP pin voltage will increase by 22 kΩ x 20 µA = 506 mV. The NTC
temperature must therefore fall by 506 mV / 25 mV per ºC = 20ºC before the LM5045 switches from standby
mode to the normal mode.
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VPWR
LM5045
R1
1.25V
STANDBY
UVLO
STANDBY
R2
SHUTDOWN
20 PA
0.4V
SHUTDOWN
Figure 25. Remote Standby and Disable Control
NTC
THERMISTOR
LM5045
5V
VPWR
20 PA
T
R1
OVP
STANDBY
1.25V
R2
Figure 26. Remote Thermal Protection
8.2.2.4 Current Sense
The CS pin receives an input signal representative of its transformer’s primary current, either from a current
sense transformer or from a resistor located at the junction of source pin of the primary switches, as shown in
Figure 27 and Figure 28, respectively. In both the cases, the filter components RF and CF should be located as
close to the IC as possible, and the ground connection from the current sense transformer, or RSENSE should be
a dedicated trace to the appropriate GND pin. Please refer to the Layout section for more layout tips.
The current sense components must provide a signal > 710 mV at the CS pin during an over-load event. Once
the voltage on the CS pin crosses the current limit threshold, the current sense comparator terminates the PWM
pulse and starts to charge the RES pin. Depending on the configuration of the RES pin, the LM5045 will
eventually initiate a hiccup mode restart or be in continuous current limit.
VPWR
Q1
Q3
VIN
NS1
RF
CS
NP
LM5045
CF
AGND
RCS
NS2
Q2
Q4
Figure 27. Transformer Current Sense
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Q3
Q1
NP
Q4
Q2
VIN
CS
RF
LM5045
CF
RCS
AGND
Figure 28. Resistor Current Sense
8.2.2.5 Hiccup Mode Current Limit Restart
The operation of the hiccup mode restart circuit is explained in the section. During a continuous current limit
condition, the RES pin is charged with 30 µA current source. The restart delay time required to reach the 1.0 V
threshold is given by:
TCS =
CRES x 1.0V
30 PA
(8)
This establishes the time allowed before the IC initiates a hiccup restart sequence. For example, if the CRES =
0.01 µF, the time TCS as noted in Figure 29 below is 334 µs. Once the RES pin reaches 1.0 V, the 30 µA
current source is turned-off and a 10 µA current source is turned-on during the ramp up to 4 V and a 5 µA is
turned on during the ramp down to 2 V. The hiccup mode off-time is given by:
THICCUP =
CRES x (2.0Vx8) CRES x ((2.0Vx8) + 1.0V)
+
10 µA
5 µA
(9)
With a CRES = 0.01 µF, the hiccup time is 49 ms. Once the hiccup time is finished, the RES pin is pulled low and
the SS pin is released allowing a soft-start sequence to commence. Once the SS pin reaches 1 V, the PWM
pulses will commence. The hiccup mode provides a cool-down period for the power converter in the event of a
sustained overload condition thereby lowering the average input current and temperature of the power
components during such an event.
4V
2V
1V
Count to Eight
Restart delay
Soft-Start
1V
Hiccup Mode off-time
Figure 29. Hiccup Mode Delay and Soft-Start Timing Diagram
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8.2.2.6 Augmenting the Gate Drive Strength
The LM5045 includes powerful 2-A integrated gate drivers. However, in certain high-power applications (> 500
W), it might be necessary to augment the strength of the internal gate driver to achieve higher efficiency and
better thermal performance. In high power applications, typically, the I2xR loss in the primary MOSFETs is
significantly higher than the switching loss. To minimize the I2xR loss, either the primary MOSFETs are paralleled
or MOSFETs with low RDS (on) are employed. Both these scenarios increase the total gate charge to be driven by
the controller IC. An increase in the gate charge increases the FET transition time and hence increases the
switching losses. Therefore, to keep the total losses within a manageable limit the transition time must be
reduced.
Generally, during the Miller capacitance charge/discharge the total available driver current is lower during the
turnoff process than during the turnon process and often it is enough to speed-up the turnoff time to achieve the
efficiency and thermal goals. This can be achieved simply by employing a PNP device, as shown in Figure 30,
from gate to source of the power FET. During the turnon process, when the LO1 goes high, the current is
sourced through the diode D1 and the BJT Q1 provides the path for the turnoff current. Q1 should be located as
close to the power FET as possible so that the turnoff current has the shortest possible path to the ground and
does not have to pass through the controller.
VIN
LM5045
BST1
D1
HO1
Q1
HS1
VCC
LO1
PGND
Figure 30. Circuit to Speed-Up the Turnoff Process
Depending on the gate charge characteristics of the primary FET, if it is required to speed up both the turnon and
the turnoff time, a bipolar totem pole structure as shown in Figure 31 can be used. When LO1 goes high, the
gate to source current is sourced through the NPN transistor Q1 and similar to the circuit shown in Figure 30
when LO1 goes low, the PNP transistor Q2 expedites the turnoff process.
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VIN
LM5045
BST1
Q1
HO1
Q2
HS1
VCC
LO1
PGND
Figure 31. Bipolar Totem Pole Arrangement
Alternatively, a low-side gate driver such as LM5112 can be used instead of the discrete totem pole. The
LM5112 comes in a small package with a 3A source and a 7A sink capability. While driving the high-side FET,
the HS1 acts as a local ground and the boot capacitor between the BST and HS pins acts as VCC.
VIN
LM5045
BST1
HO1
LM5112
HS1
VCC
LO1
LM5112
PGND
Figure 32. Using a Low-Side Gate Driver to Augment Gate Drive Strength
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8.2.3 Application Curve
Figure 33. Application Board Efficiency
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9 Power Supply Recommendations
The LM5045 can be used to control power levels up to 1 kW. Therefore the current levels can be considerable.
Care should be taken that components with the correct current rating are chosen. This would include magnetic
components, power MOSFETS and diodes, connectors and wire sizes. Input and output capacitors should have
the correct ripple current rating. The use of a multilayer PCB is recommended with a copper area chosen to
ensure the LM5045 is operating below its maximum junction temperature.
Full power loading should never be attempted with providing with providing adequate cooling.
10 Layout
10.1 Layout Guidelines
The LM5045 current sense and PWM comparators are very fast and respond to short duration noise pulses. The
components at the CS, COMP, SLOPE, RAMP, SS, SSSR, RES, UVLO, OVP, RD1, RD2, and RT pins should
be physically close as possible to the IC, thereby minimizing noise pickup on the PC board trace inductance.
Eliminating or minimizing via’s in these critical connections are essential. Layout consideration is critical for the
current sense filter. If a current sense transformer is used, both leads of the transformer secondary should be
routed to the sense filter components and to the IC pins. The ground side of the transformer should be
connected via a dedicated PC board trace to the AGND pin, rather than through the ground plane. If the current
sense circuit employs a sense resistor in the drive transistor source, low inductance resistors should be used. In
this case, all the noise sensitive, low-current ground trace should be connected in common near the IC, and then
a single connection made to the power ground (sense resistor ground point).
The gate drive outputs of the LM5045 should have short, direct paths to the power MOSFETs to minimize
inductance in the PC board. The boot-strap capacitors required for the high side gate drivers should be located
very close to the IC and connected directly to the BST and HS pins. The VCC and REF capacitors should also
be placed close to their respective pins with short trace inductance. Low ESR and ESL ceramic capacitors are
recommended for the boot-strap, VCC and the REF capacitors. The two ground pins (AGND, PGND) must be
connected together directly underneath the IC with a short, direct connection, to avoid jitter due to relative ground
bounce.
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10.2 Layout Example
RAMP
To Current Sensing
Network
UVLO
VIN
OVP
HS1
RAMP
HO1
CS
R_FILTER
C_FILTER
To Voltage Feedback
Network
SLOPE
SR1
COMP
LO1
REF
RD1
RD2
RES
SS
PGND
RT
VCC
AGND
LO2
RD1
SR2
RD2
BST2
RES
HO2
SS
HS2
SSSR
RT
BST1
SS OFF
SSSR
Figure 34. Layout of Components Around RAMP, CS, SLOPE, COMP, RT, RD1, RD2, RES, SS, and SSR
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Layout Example (continued)
INPUT DC VOLTAGE
R1
R2
UVLO
VIN
OVP
HS1
RAMP
HO1
CS
R3
C2
BST1
SLOPE
SR1
COMP
LO1
REF
C4
PGND
RT
VCC
AGND
LO2
RD1
SR2
RD2
BST2
RES
HO2
SS
HS2
SSSR
C1
C3
C4
SS OFF
Figure 35. Layout of Components Around VIN, VCC, AGND, PGND UVLO, OVP, REF, BST1, BST2, HS1,
and HS2
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11 Device and Documentation Support
11.1 Trademarks
PowerPAD is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.2 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.
11.3 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)
LM5045MH/NOPB
ACTIVE
HTSSOP
PWP
28
48
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM5045
MH
LM5045MHX/NOPB
ACTIVE
HTSSOP
PWP
28
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
LM5045
MH
LM5045SQ/NOPB
ACTIVE
WQFN
RSG
28
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
L5045
LM5045SQX/NOPB
ACTIVE
WQFN
RSG
28
4500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
L5045
(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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