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LM5008
SNVS280I – APRIL 2004 – REVISED OCTOBER 2018
LM5008 95-V, 350-mA, Constant On-Time DC/DC Buck Switching Regulator
1 Features
3 Description
•
•
•
•
•
•
•
The LM5008 350-mA step-down switching converter
features all of the functions needed to implement a
low-cost and efficient buck regulator. This highvoltage converter has an integrated 100-V N-channel
buck switch and operates over an input voltage range
of 9 V to 95 V. The device is easy to implement and
is provided in the 8-pin VSSOP and the thermally
enhanced 8-pin WSON packages. The converter
uses a hysteretic control scheme with a PWM on-time
inversely proportional to VIN. This feature allows the
operating frequency to remain relatively constant. The
hysteretic control requires no loop compensation. An
intelligent current limit is implemented with forced offtime, which is inversely proportional to VOUT. This
scheme ensures short-circuit protection while
providing minimum foldback. Other protection
features include: thermal shutdown, VCC undervoltage
lockout, gate drive undervoltage lockout, and
maximum duty cycle limiter.
1
•
•
•
•
•
•
•
•
Operating Input Voltage Range: 6 V to 95 V
Integrated 100-V, N-Channel Buck Switch
Internal VCC Regulator
No Loop Compensation Required
Ultra-Fast Transient Response
On-Time Varies Inversely With Line Voltage
Operating Frequency Remains Constant With
Varying Line Voltage and Load Current
Adjustable Output Voltage
Highly Efficient Operation
Precision Internal Reference
Low Bias Current
Intelligent Current Limit Protection
Thermal Shutdown
8-Pin VSSOP and WSON Packages
Create a Custom Design Using the LM5008 With
the WEBENCH® Power Designer
Device Information(1)
PART NUMBER
2 Applications
•
•
•
PACKAGE
LM5008
Non-Isolated Telecommunication Buck Regulators
Secondary High-Voltage Post Regulators
48-V Automotive Systems
BODY SIZE (NOM)
VSSOP (8)
3.00 mm × 3.00 mm
WSON (8)
4.00 mm × 4.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application Circuit and Block Diagram
7V SERIES
REGULATOR
9.5 -95V
Input
LM5008
VCC 7
8
VIN
C1
SD
C5
C3
THERMAL
SHUTDOWN
UVLO
ON TIMER
START
RON
COMPLETE
6
SD /
RON
SHUTDOWN
BST
Ron
OVER-VOLTAGE
COMPARATOR
+
-
2.875V
START
UVLO
300 ns MIN OFF
TIMER
LEVEL
SHIFT
3
RCL
S
REGULATION
COMPARATOR
FB
4
R
SET
L1
SW 1
VOUT1
Q
Q
CLR
R1
COMPLETE
RCL
START
CURRENT LIMIT
OFF TIMER
RCL
C4
COMPLETE
+
FB
VIN
SD
DRIVER
2.5V
5
2
+
0.50A
BUCK
SWITCH
CURRENT
SENSE
R3
VOUT2
D1
RTN
R2
C2
Copyright © 2016, Texas Instruments Incorporated
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.
LM5008
SNVS280I – APRIL 2004 – REVISED OCTOBER 2018
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
6.7
4
4
4
4
5
5
6
Absolute Maximum Ratings ......................................
ESD Ratings ............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Switching Characteristics ..........................................
Typical Characteristics ..............................................
Detailed Description .............................................. 7
7.1 Overview ................................................................... 7
7.2 Functional Block Diagram ......................................... 7
7.3 Feature Description................................................... 8
7.4 Device Functional Modes ....................................... 11
8
Application and Implementation ........................ 12
8.1 Application Information............................................ 12
8.2 Typical Application .................................................. 12
9 Power Supply Recommendations...................... 16
10 Layout................................................................... 17
10.1 Layout Guidelines ................................................. 17
10.2 Layout Examples................................................... 17
11 Device and Documentation Support ................. 18
11.1
11.2
11.3
11.4
11.5
11.6
11.7
Device Support ....................................................
Documentation Support ........................................
Receiving Notification of Documentation Updates
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
18
18
19
19
19
19
19
12 Mechanical, Packaging, and Orderable
Information ........................................................... 19
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision H (December 2016) to Revision I
Page
•
Added links for WEBENCH ................................................................................................................................................... 1
•
Changed VSSOP-8 body size to 3 mm × 3 mm in Device Information.................................................................................. 1
•
Changed Layout Guidelines ................................................................................................................................................ 17
Changes from Revision G (March 2013) to Revision H
Page
•
Added 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
•
Deleted Lead temperature, soldering (260°C maximum) ....................................................................................................... 4
•
Changed RθJA values From: 200°C/W To: 139.7°C/W (VSSOP) and From: 40°C/W To: 42°C/W (WSON) .......................... 4
Changes from Revision F (March 2013) to Revision G
•
2
Page
Changed layout of National Semiconductor Data Sheet to TI format .................................................................................... 1
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SNVS280I – APRIL 2004 – REVISED OCTOBER 2018
5 Pin Configuration and Functions
DGK Package
8-Pin VSSOP
Top View
NGU Package
8-Pin WSON
Top View
SW
1
8
VIN
BST
2
7
VCC
RCL
3
6
RT/SD
RTN
4
5
FB
SW
1
BST
2
8
VIN
7
VCC
EP
RCL
3
6
RT/SD
RTN
4
5
FB
Not to scale
Not to scale
Pin Functions
PIN
NO.
NAME
TYPE
DESCRIPTION
1
SW
P
Switching node: power switching node. Connect to the output inductor, re-circulating diode, and bootstrap
capacitor.
2
BST
I
Boost pin (bootstrap capacitor input): an external capacitor is required between the BST and the SW pins. A
0.01-µF ceramic capacitor is recommended. An internal diode charges the capacitor from VCC.
3
RCL
I
Current limit OFF time set pin: a resistor between this pin and RTN sets the off-time when current limit is
detected. The off-time is preset to 35 µs if FB = 0 V.
Toff = 10–5 / (0.285 + (FB / 6.35 × 10−6 × RCL))
4
RTN
G
Ground pin: ground for the entire circuit.
5
FB
I
Feedback input from regulated output: this pin is connected to the inverting input of the internal regulation
comparator. The regulation threshold is 2.5 V.
I
On-time set pin: a resistor between this pin and VIN sets the switch on-time as a function of VIN. The
minimum recommended on-time is 400 ns at the maximum input voltage. This pin can be used for remote
shutdown.
Ton = 1.25 × 10–10 RON / VIN
6
RON/SD
7
VCC
P
Output from the internal high voltage series pass regulator. Regulated at 7 V. If an auxiliary voltage is
available to raise the voltage on this pin, above the regulation set point (7 V), the internal series pass
regulator will shutdown, reducing the IC power dissipation. Do not exceed 14 V. This voltage provides gate
drive power for the internal buck switch. An internal diode is provided between this pin and the BST pin. A
local 0.1-µF decoupling capacitor is recommended. Series pass regulator is current limited to 10 mA.
8
VIN
P
Input voltage: recommended operating range is 9.5 V to 95 V.
—
EP
G
Exposed pad: the exposed pad has no electrical contact. Connect to system ground plane for reduced
thermal resistance (WSON package only).
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SNVS280I – APRIL 2004 – REVISED OCTOBER 2018
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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 to GND
–0.3
100
V
BST to GND
–0.3
114
V
SW to GND (steady-state)
–1
V
BST to VCC
100
V
BST to SW
14
V
VCC to GND
14
V
All other inputs to GND
–0.3
7
V
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)
(3)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001
(1) (2)
UNIT
±2000
Charged-device model (CDM), per JEDEC specification JESD22-C101 (3)
V
±750
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
The human body model is a 100-pF capacitor discharged through a 1.5-kΩ resistor into each pin.
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
MAX
VIN
9.5
95
UNIT
V
Operating junction temperature, TJ
–40
125
°C
6.4 Thermal Information
LM5008
THERMAL METRIC (1)
DGK (VSSOP)
NGU (WSON)
8 PINS
8 PINS
UNIT
RθJA
Junction-to-ambient thermal resistance
139.7
42
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
51.2
27.6
°C/W
RθJB
Junction-to-board thermal resistance
70.5
18.5
°C/W
ψJT
Junction-to-top characterization parameter
3.4
0.3
°C/W
ψJB
Junction-to-board characterization parameter
69.5
18.5
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
—
4.3
°C/W
(1)
4
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
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6.5 Electrical Characteristics
Specifications are for TJ = 25°C and VIN = 48 V (unless otherwise stated) (1).
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
VCC SUPPLY
VCC Reg
TJ = 25°C
VCC regulator output
VCC current limit
7
TJ = –40°C to 125°C
6.6
(2)
7.4
9.5
VCC undervoltage lockout voltage
VCC increasing
VCC undervoltage hysteresis
VCC UVLO delay (filter)
100-mV overdrive
IIN operating current
Non-switching, FB = 3 V
IIN shutdown current
RON/SD = 0 V
TJ = 25°C
mA
6.3
V
200
mV
10
µs
485
TJ = –40°C to 125°C
675
TJ = 25°C
V
76
TJ = –40°C to 125°C
150
µA
µA
CURRENT LIMIT
TJ = 25°C
Current limit threshold
0.51
TJ = –40°C to 125°C
0.41
Current limit response time
Iswitch overdrive = 0.1 A, time to switch off
OFF time generator (test 1)
FB = 0 V, RCL = 100 K
OFF time generator (test 2)
FB = 2.3 V, RCL = 100 K
0.61
A
400
ns
35
µs
2.56
µs
ON-TIME GENERATOR
TON – 1
VIN = 10 V, RON = 200 K
TON – 2
VIN = 95 V, RON = 200 K
Remote shutdown threshold
Rising
TJ = 25°C
TJ = –40°C to 125°C
2.77
2.15
TJ = 25°C
TJ = –40°C to 125°C
300
200
TJ = 25°C
TJ = –40°C to 125°C
3.5
420
0.7
0.4
Remote shutdown hysteresis
1.05
µs
ns
V
35
mV
300
ns
MINIMUM OFF-TIME
Minimum off-timer
FB = 0 V
REGULATION AND OV COMPARATORS
FB reference threshold
Internal reference, trip point for
switch ON
FB overvoltage threshold
Trip point for switch OFF
TJ = 25°C
TJ = –40°C to 125°C
2.5
2.445
FB bias current
2.55
V
2.875
V
100
nA
165
°C
25
°C
THERMAL SHUTDOWN
Tsd
Thermal shutdown temperature
Thermal shutdown hysteresis
(1)
(2)
All electrical characteristics having room temperature limits are tested during production with TA = TJ = 25°C. All hot and cold limits are
specified by correlating the electrical characteristics to process and temperature variations and applying statistical process control.
The VCC output is intended as a self bias for the internal gate drive power and control circuits. Device thermal limitations limit external
loading.
6.6 Switching Characteristics
over operating free-air temperature range (unless otherwise noted)
PARAMETER
TEST CONDITIONS
Buck switch RDS(on)
ITEST = 200 mA (1)
Gate drive UVLO
VBST – VSW rising
TYP
2.47
TJ = 25°C
TJ = –40°C to 125°C
MAX
1.15
TJ = –40°C to 125°C
Gate drive UVLO hysteresis
(1)
MIN
TJ = 25°C
4.5
3.4
5.5
430
UNIT
Ω
V
mV
For devices procured in the 8-pin WSON package the RDS(on) limits are specified by design characterization data only.
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6.7 Typical Characteristics
2.0
8
RON = 500k
6
5
1.6
TON (Ps)
ICC INPUT CURRENT (mA)
7
1.8
1.4
300k
4
3
2
100k
1.2
1
0
1.0
9
8
10
11
12
13
0
14
20
40
EXTERNALLY APPLIED VCC (V)
80
100
VIN (V)
Figure 1. ICC Current vs Applied VCC Voltage
Figure 2. On-Time vs Input Voltage and RON
700
35
48V
60V
80V
600
Max VIN
= 30V
500
95V
400
300
200
100
CURRENT LIMIT OFF TIME (Ps)
MAXIMUNM FREQUENCY (kHz)
60
30
25
20
15
RCL = 500k
300k
10
100k
5
50k
0
0
0
2.5 5.0
10
15
20
0
0.5
1.0
Figure 3. Maximum Frequency vs VOUT and VIN
2.5
Figure 4. Current Limit Off-Time vs VFB and RCL
10.2
100
VIN = 15V
VOUT1
10.0
VIN = 48V
80
9.8
VOUT (V)
EFFICIENCY (%)
2.0
VFB (V)
VOUT (V)
90
1.5
VIN = 95V
70
VOUT2
9.6
60
9.4
50
9.2
40
100
9.0
VIN = 48V
200
300
LOAD CURRENT (mA)
100
200
300
LOAD CURRENT (mA)
Figure 5. Efficiency vs Load Current vs VIN
(Circuit of Figure 10)
6
0
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Figure 6. Output Voltage vs Load Current
(Circuit of Figure 10)
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7 Detailed Description
7.1 Overview
The LM5008 regulator is an easy-to-use buck DC-DC converter that operates from 9.5-V to 95-V supply voltage.
The device is intended for step-down conversions from 12-V, 24-V, and 48-V unregulated, semi-regulated and
fully-regulated supply rails. With integrated buck power MOSFET, the LM5008 delivers up to 350-mA DC load
current with exceptional efficiency and low input quiescent current in a very small solution size.
Designed for simple implementation, a nearly fixed-frequency, constant on-time (COT) operation with
discontinuous conduction mode (DCM) at light loads is ideal for low-noise, high current, fast transient load
requirements. Control loop compensation is not required reducing design time and external component count.
The LM5008 incorporates other features for comprehensive system requirements, including VCC undervoltage
lockout (UVLO), gate drive undervoltage lockout, maximum duty cycle limiter, intelligent current limit off timer, a
precharge switch, and thermal shutdown with automatic recovery. These features enable a flexible and easy-touse platform for a wide range of applications. The pin arrangement is designed for simple and optimized PCB
layout, requiring only a few external components.
7.2 Functional Block Diagram
7V SERIES
REGULATOR
9.5 -95V
Input
LM5008
VCC 7
8
VIN
C1
SD
C5
C3
THERMAL
SHUTDOWN
UVLO
ON TIMER
START
RON
COMPLETE
6
SD /
RON
SHUTDOWN
BST
Ron
OVER-VOLTAGE
COMPARATOR
+
-
2.875V
START
UVLO
300 ns MIN OFF
TIMER
LEVEL
SHIFT
RCL
S
REGULATION
COMPARATOR
FB
3
4
R
SET
L1
SW 1
VOUT1
Q
Q
CLR
R1
COMPLETE
RCL
START
CURRENT LIMIT
OFF TIMER
RCL
C4
DRIVER
+
FB
VIN
SD
COMPLETE
2.5V
5
2
+
0.50A
BUCK
SWITCH
CURRENT
SENSE
R3
VOUT2
D1
RTN
R2
C2
Copyright © 2016, Texas Instruments Incorporated
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7.3 Feature Description
7.3.1 Hysteretic Control Circuit Overview
The LM5008 is a buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with
line voltage (VIN). Control is based on a comparator and the on-time one-shot, with the output voltage feedback
(FB) compared to an internal reference (2.5 V). If the FB level is below the reference the buck switch is turned on
for a fixed time determined by the line voltage and a programming resistor (RON). Following the ON period, the
switch remains off for at least the minimum off-timer period of 300 ns. If FB is still below the reference at that
time, the switch turns on again for another on-time period. This will continue until regulation is achieved.
The LM5008 operates in discontinuous conduction mode at light load currents, and continuous conduction mode
at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero and
ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time
period starts when the voltage at FB falls below the internal reference; until then, the inductor current remains
zero. In this mode the operating frequency is lower than in continuous conduction mode, and varies with load
current. Therefore at light loads the conversion efficiency is maintained, because the switching losses reduce
with the reduction in load and frequency. The discontinuous operating frequency can be calculated with
Equation 1.
VOUT2 x L x 1.28 x 1020
F=
RL x (RON)2
where
•
RL = the load resistance
(1)
In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero.
In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively
constant with load and line variations. The approximate continuous mode operating frequency can be calculated
with Equation 2.
VOUT
F=
1.25 x 10-10 x RON
(2)
The output voltage (VOUT) can be programmed by two external resistors as shown in Functional Block Diagram.
The regulation point can be calculated with Equation 3.
VOUT = 2.5 × (R1 + R2) / R2
(3)
All hysteretic regulators regulate the output voltage based on ripple voltage at the feedback input, requiring a
minimum amount of ESR for the output capacitor C2. A minimum of 25 mV to 50 mV of ripple voltage at the
feedback pin (FB) is required for the LM5008. In cases where the capacitor ESR is too small, additional series
resistance may be required (R3 in Functional Block Diagram).
For applications where lower output voltage ripple is required the output can be taken directly from a low-ESR
output capacitor, as shown in Figure 7. However, R3 slightly degrades the load regulation.
L1
SW
LM5008
R1
R3
FB
VOUT2
R2
C2
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Figure 7. Low-Ripple Output Configuration
8
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Feature Description (continued)
7.3.2 High Voltage Start-Up Regulator
The LM5008 contains an internal high voltage start-up regulator. The input pin (VIN) can be connected directly to
the line voltages up to 95 Volts, with transient capability to 100 V. The regulator is internally current limited to 9.5
mA at VCC. Upon power up, the regulator sources current into the external capacitor at VCC (C3). When the
voltage on the VCC pin reaches the undervoltage lockout threshold of 6.3 V, the buck switch is enabled.
In applications involving a high value for VIN, where power dissipation in the VCC regulator is a concern, an
auxiliary voltage can be diode connected to the VCC pin. Setting the auxiliary voltage to 8 V to 14 V shuts off the
internal regulator, reducing internal power dissipation. See Figure 8. The current required into the VCC pin is
shown in Figure 1.
VCC
C3
BST
C4
LM5008
L1
D2
SW
D1
R1
R3
FB
VOUT2
R2
C2
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Figure 8. Self-Biased Configuration
7.3.3 Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5-V reference. In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at FB falls below 2.5 V. The buck switch stays on for
the on-time, causing the FB voltage to rise above 2.5 V. After the on-time period, the buck switch stays off until
the FB voltage again falls below 2.5 V. During start-up, the FB voltage is below 2.5 V at the end of each on-time,
resulting in the minimum off-time of 300 ns. Bias current at the FB pin is nominally 100 nA.
7.3.4 Overvoltage Comparator
The feedback voltage at FB is compared to an internal 2.875-V reference. If the voltage at FB rises above 2.875
V, the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load,
change suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5 V.
7.3.5 On-Time Generator and Shutdown
The on-time for the LM5008 is determined by the RON resistor, and is inversely proportional to the input voltage
(VIN), resulting in a nearly constant frequency as VIN is varied over its range. Equation 4 shows the on-time
equation for the LM5008.
TON = 1.25 × 10–10 × RON / VIN
(4)
See Figure 2. RON should be selected for a minimum on-time (at maximum VIN) greater than 400 ns for proper
current limit operation. This requirement limits the maximum frequency for each application, depending on VIN
and VOUT. See Figure 3.
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Feature Description (continued)
The LM5008 can be remotely disabled by taking the RON/SD pin to ground. See Figure 9. The voltage at the
RON/SD pin is between 1.5 and 3 volts, depending on VIN and the value of the RON resistor.
Input
Voltage
VIN
RON
LM5008
RON/SD
STOP
RUN
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Figure 9. Shutdown Implementation
7.3.6 Current Limit
The LM5008 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 0.5 A the
present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is
controlled by an external resistor (RCL) and the FB voltage (see Figure 4). When FB = 0 V, a maximum off-time is
required, and the time is preset to 35 µs. This condition occurs when the output is shorted, and during the initial
part of start-up. This amount of time ensures safe short-circuit operation up to the maximum input voltage of 95
V. In cases of overload where the FB voltage is above zero volts (not a short circuit), the current limit off-time will
be less than 35 µs. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery
time, and the start-up time. The off-time is calculated from Equation 5.
10
TOFF =
-5
VFB
0.285 +
-6
(6.35 x 10 x RCL)
(5)
The current limit sensing circuit is blanked for the first 50-70 ns of each on-time so it is not falsely tripped by the
current surge which occurs at turnon. The current surge is required by the re-circulating diode (D1) for its turnoff
recovery.
7.3.7 N-Channel Buck Switch and Driver
The LM5008 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.01µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver during
the on-time.
During each off-time, the SW pin is at approximately 0 V, and the bootstrap capacitor charges from VCC through
the internal diode. The minimum off-timer, set to 300 ns, ensures a minimum time each cycle to recharge the
bootstrap capacitor.
An external re-circulating diode (D1) carries the inductor current after the internal Buck switch turns off. This
diode must be of the ultra-fast or Schottky type to minimize turnon losses and current overshoot.
7.3.8 Thermal Protection
The LM5008 must be operated so the junction temperature does not exceed 125°C during normal operation. An
internal thermal shutdown circuit is provided to protect the LM5008 in the event of a higher than normal junction
temperature. When activated, typically at 165°C, the controller is forced into a low power reset state, disabling
the buck switch and the VCC regulator. This feature prevents catastrophic failures from accidental device
overheating. When the junction temperature reduces below 140°C (typical hysteresis = 25°C), the VCC regulator
is enabled, and normal operation is resumed.
10
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7.4 Device Functional Modes
7.4.1 Shutdown Mode
The RON/SD pin provides ON and OFF control for the LM5008. When VSD is below approximately 0.7 V, the
device is in shutdown mode. Both the internal LDO and the switching regulator are off. The quiescent current in
shutdown mode drops to 76 µA (typical) at VIN = 48 V. The LM5008 also employs VCC bias rail undervoltage
protection. If the VCC bias supply voltage is below its UV threshold, the regulator remains off.
7.4.2 Active Mode
LM5008 is in active mode when the internal bias rail, VCC, is above its UV threshold. Depending on the load
current, the device operates in either DCM or CCM mode.
Whenever the load current is reduced to a level less than half the peak-to-peak inductor ripple current, the device
enters discontinuous conduction mode (DCM). Calculate the critical conduction boundary using Equation 6.
IBOUNDARY
'IL
2
VOUT ˜ 1 D
2 ˜ LF ˜ FSW
(6)
When the inductor current reaches zero, the SW node becomes high impedance. Resonant ringing occurs at SW
as a result of the LC tank circuit formed by the buck inductor and the parasitic capacitance at the SW node. At
light loads, several pulses may be skipped in between switching cycles, effectively reducing the switching
frequency and further improving light-load efficiency.
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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 final circuit is shown in Figure 10. The circuit was tested, and the resulting performance is shown in
Figure 12 through Figure 6.
8.1.1 Minimum Load Current
A minimum load current of 1 mA is required to maintain proper operation. If the load current falls below that level,
the bootstrap capacitor may discharge during the long off-time, and the circuit will either shutdown or cycle on
and off at a low frequency. If the load current is expected to drop below 1 mA in the application, the feedback
resistors should be chosen low enough in value so they provide the minimum required current at nominal VOUT.
8.2 Typical Application
12 - 95V
Input
VCC
VIN
7
8
C1
1.0 µF
C3
0.1 µF
C5
0.1 µF
BST
RON
357k
2
RON / SD
6
LM5008
C4
0.01 µF
L1
220 µH
10.0V
SW
VOUT1
1
SHUTDOWN
D1
RCL
R1
R3
3.01k
2.0
3
RCL
267k
RTN
VOUT2
FB
R2
5
1.0k
4
C2
15 µF
GND
Copyright © 2016, Texas Instruments Incorporated
Figure 10. LM5008 Example Circuit
8.2.1 Design Requirements
A guide for determining the component values will be illustrated with a design example. Table 1 lists the bill of
materials for this application. The following steps will configure the LM5008 for:
• Input voltage range (VIN): 12 V to 95 V
• Output voltage (VOUT1): 10 V
• Load current (for continuous conduction mode): 100 mA to 300 mA
• Maximum ripple at VOUT2: 100 mVp-p at maximum input voltage
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Typical Application (continued)
Table 1. Bill of Materials (Circuit of Figure 10)
ITEM
DESCRIPTION
PART NUMBER
VALUE
C1
Ceramic Capacitor
TDK C4532X7R2A105M
1 µF, 100 V
C2
Ceramic Capacitor
TDK C4532X7R1E156M
15 µF, 25 V
C3
Ceramic Capacitor
Kemet C1206C104K5RAC
0.1 µF, 50 V
C4
Ceramic Capacitor
Kemet C1206C103K5RAC
0.01 µF, 50 V
C5
Ceramic Capacitor
TDK C3216X7R2A104M
0.1 µF, 100 V
D1
Ultra-Fast Power Diode
ON Semi MURA110T3
100 V, 1 A
L1
Power Inductor
Coilcraft DO3316-224 or
220 µH
TDK SLF10145T-221MR65
R1
Resistor
Vishay CRCW12063011F
3.01 kΩ
R2
Resistor
Vishay CRCW12061001F
1 kΩ
R3
Resistor
Vishay CRCW12062R00F
2Ω
RON
Resistor
Vishay CRCW12063573F
357 kΩ
RCL
Resistor
Vishay CRCW12062673F
267 kΩ
U1
Switching Regulator
Texas Instruments LM5008
8.2.2 Detailed Design Procedure
8.2.2.1 Custom Design With WEBENCH® Tools
Click here to create a custom design using the LM5008 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.
R1 and R2: From Functional Block Diagram, VOUT1 = VFB × (R1 + R2) / R2, and because VFB = 2.5 V, the ratio of
R1 to R2 calculates as 3:1. Standard values of 3.01 kΩ (R1) and 1.00 kΩ (R2) are chosen. Other values could
be used as long as the 3:1 ratio is maintained. The selected values, however, provide a small amount of output
loading (2.5 mA) in the event the main load is disconnected. This allows the circuit to maintain regulation until the
main load is reconnected.
Fs and RON: The recommended operating frequency range for the LM5008 is 50 kHz to 600 kHz. Unless the
application requires a specific frequency, the choice of frequency is generally a compromise because it affects
the size of L1 and C2, and the switching losses. The maximum allowed frequency, based on a minimum on-time
of 400 ns, is calculated from Equation 7:
FMAX = VOUT / (VINMAX × 400 ns)
(7)
For this exercise, FMAX = 263 kHz. From Equation 2, RON calculates to 304 kΩ. A standard value 357-kΩ resistor
is used to allow for tolerances in Equation 2, resulting in a frequency of 224 kHz.
L1: The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor
value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum
ripple current occurs at maximum VIN.
a. Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude
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(IOR) must be less than 200 mAp-p so the lower peak of the waveform does not reach zero. L1 is calculated
using Equation 8.
VOUT1 x (VIN - VOUT1)
L1 =
IOR x Fs x VIN
(8)
At VIN = 95 V, L1 (minimum) calculates to 200 µH. The next larger standard value (220 µH) is chosen and
with this value IOR calculates to 181 mAp-p at VIN = 95 V, and 34 mAp-p at VIN = 12 V.
b. Maximum load current: At a load current of 300 mA, the peak of the ripple waveform must not reach the
minimum value of the LM5008’s current limit threshold (410 mA). Therefore the ripple amplitude must be less
than 220 mAp-p, which is already satisfied in Equation 8. With L1 = 220 µH, at maximum VIN and IO, the
peak of the ripple will be 391 mA. While L1 must carry this peak current without saturating or exceeding its
temperature rating, it also must be capable of carrying the maximum value of the LM5008’s current limit
threshold (610 mA) without saturating, because the current limit is reached during start-up.
The DC resistance of the inductor should be as low as possible. For example, if the inductor’s DCR is 1 Ω,
the power dissipated at maximum load current is 0.09 W. While small, it is not insignificant compared to the
load power of 3 W.
C3: The capacitor on the VCC output provides not only noise filtering and stability, but its primary purpose is to
prevent false triggering of the VCC UVLO at the buck switch ON/OFF transitions. For this reason, C3 should be
no smaller than 0.1 µF.
C2, and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its
ESR, ripple voltage due to its capacitance, and the nature of the load.
a. ESR and R3: A low ESR for C2 is generally desirable so as to minimize power losses and heating within the
capacitor. However, a hysteretic regulator requires a minimum amount of ripple voltage at the feedback input
for proper loop operation. For the LM5008 the minimum ripple required at pin 5 is 25 mVp-p, requiring a
minimum ripple at VOUT1 of 100 mV. Because the minimum ripple current (at minimum VIN) is 34 mAp-p, the
minimum ESR required at VOUT1 is 100 mV / 34 mA = 2.94 Ω. Because quality capacitors for SMPS
applications have an ESR considerably less than this, R3 is inserted as shown in Functional Block Diagram.
R3’s value, along with C2’s ESR, must result in at least 25 mVp-p ripple at pin 5. Generally, R3 will be 0.5 to
3 Ω.
b. Nature of the Load: The load can be connected to VOUT1 or VOUT2. VOUT1 provides good regulation, but with
a ripple voltage which ranges from 100 mV (at VIN = 12 V) to 500 mV (at VIN = 95 V). Alternatively, VOUT2
provides low ripple, but lower regulation due to R3.
For a maximum allowed ripple voltage of 100 mVp-p at VOUT2 (at VIN = 95 V), assume an ESR of 0.4 Ω for
C2. At maximum VIN, the ripple current is 181 mAp-p, creating a ripple voltage of 72 mVp-p. This leaves 28
mVp-p of ripple due to the capacitance. The average current into C2 due to the ripple current is calculated
using the waveform in Figure 11.
L1 Current
391 mA
300 mA
209 mA
0 mA
1/Freq.
= Ts
Ts/2
Figure 11. Inductor Current Waveform
Starting when the current reaches Io (300 mA in Figure 11) half way through the on-time, the current
continues to increase to the peak (391 mA), and then decreases to 300 mA half way through the off-time.
The average value of this portion of the waveform is 45.5 mA, and will cause half of the voltage ripple, or 14
mV. The interval is one half of the frequency cycle time, or 2.23 µs. Using the capacitor’s basic equation (see
Equation 9), the minimum value for C2 is 7.2 µF.
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The ripple due to C2’s capacitance is 90° out of phase from the ESR ripple, and the two numbers do not add
directly. However, this calculation provides a practical minimum value for C2 based on its ESR and the target
spec. To allow for the capacitor’s tolerance, temperature effects, and voltage effects, a 15-µF, X7R capacitor
is used.
c. In summary: The above calculations provide a minimum value for C2 and a calculation for R3. The ESR is
just as important as the capacitance. The calculated values are guidelines, and should be treated as starting
points. For each application, experimentation is needed to determine the optimum values for R3 and C2.
C = I × Δt / ΔV
(9)
RCL: When a current limit condition is detected, the minimum off-time set by this resistor must be greater than the
maximum normal off-time which occurs at maximum VIN. Using Equation 4, the minimum on-time is 0.47 µs,
yielding a maximum off-time of 3.99 µs. This is increased by 117 ns (to 4.11 µs) due to a ±25% tolerance of the
on-time. This value is then increased to allow for:
The response time of the current limit detection loop (400 ns).
The off-time determined by Equation 5 has a ±25% tolerance.
tOFFCL(MIN) = (4.11 µs + 0.40 µs) × 1.25 = 5.64 µs
(10)
Using Equation 5, RCL calculates to 264 kΩ (at VFB = 2.5 V). The closest standard value is 267 kΩ.
D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time
determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage
drop is significant in the event the output is short-circuited as it is only this diode’s voltage which forces the
inductor current to reduce during the forced off-time. For this reason, a higher voltage is better, although that
affects efficiency. A good choice is an ultra-fast power diode, such as the MURA110T3 from ON Semiconductor.
Its reverse recovery time is 30 ns, and its forward voltage drop is approximately 0.72 V at 300 mA at 25°C. Other
types of diodes may have a lower forward voltage drop, but may have longer recovery times, or greater reverse
leakage. D1’s reverse voltage rating must be at least as great as the maximum VIN, and its current rating be
greater than the maximum current limit threshold (610 mA).
C1: This capacitor’s purpose is to supply most of the switch current during the on-time, and limit the voltage
ripple at VIN, on the assumption that the voltage source feeding VIN has an output impedance greater than zero.
At maximum load current when the buck switch turns on, the current into pin 8 will suddenly increase to the lower
peak of the output current waveform, ramp up to the peak value, then drop to zero at turnoff. The average input
current during this on-time is the load current (300 mA). For a worst case calculation, C1 must supply this
average load current during the maximum on-time. To keep the input voltage ripple to less than 2 V (for this
exercise), C1 is calculated with Equation 11.
C1 =
I x tON
'V
=
0.3A x 3.72 Ps
= 0.56 PF
2.0V
(11)
Quality ceramic capacitors in this value have a low ESR which adds only a few millivolts to the ripple. It is the
capacitance which is dominant in this case. To allow for the capacitor’s tolerance, temperature effects, and
voltage effects, a 1.0-µF, 100-V, X7R capacitor will be used.
C4: The recommended value is 0.01 µF for C4, as this is appropriate in the majority of applications. A highquality ceramic capacitor, with low ESR is recommended as C4 supplies the surge current to charge the buck
switch gate at turnon. A low ESR also ensures a quick recharge during each off-time. At minimum VIN, when the
on-time is at maximum, it is possible during start-up that C4 will not fully recharge during each 300-ns off-time.
The circuit will not be able to complete the start-up, and achieve output regulation. This can occur when the
frequency is intended to be low (for example, RON = 500 K). In this case C4 should be increased so it can
maintain sufficient voltage across the buck switch driver during each on-time.
C5: This capacitor helps avoid supply voltage transients and ringing due to long lead inductance at VIN. A lowESR, 0.1-µF ceramic chip capacitor is recommended, placed close to the LM5008.
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8.2.3 Application Curves
100
100
90
90
80
80
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 15V
70
60
VIN = 95V
70
60
IOUT = 300 mA
50
50
40
100
40
0
20
40
VIN = 48V
60
80
100
VIN (V)
200
300
LOAD CURRENT (mA)
Figure 12. Efficiency vs VIN
Figure 13. Efficiency vs Load Current vs VIN
9 Power Supply Recommendations
The LM5008 converter is designed to operate from a wide input voltage range from 9.5 V to 95 V. The
characteristics of the input supply must be compatible with the Absolute Maximum Ratings and Recommended
Operating Conditions. In addition, the input supply must be capable of delivering the required input current to the
fully-loaded regulator. Estimate the average input current with Equation 12.
VOUT ˜ IOUT
VIN ˜ K
IIN
where
•
η is the efficiency
(12)
If the converter is connected to an input supply through long wires or PCB traces with large impedance,
sachieving stable performance requires special care. The parasitic inductance and resistance of the input cables
may have an adverse affect on converter operation. The parasitic inductance in combination with the low-ESR
ceramic input capacitors form an underdamped resonant circuit. This circuit can cause overvoltage transients at
VIN each time the input supply is cycled ON and OFF. The parasitic resistance causes the input voltage to dip
during a load transient. If the regulator is operating close to the minimum input voltage, this dip can cause false
UVLO fault triggering and a system reset. The best way to solve such issues is to reduce the distance from the
input supply to the regulator and use an aluminum or tantalum input capacitor in parallel with the ceramics. The
moderate ESR of the electrolytic capacitors helps to damp the input resonant circuit and reduce any voltage
overshoots. A capacitance in the range of 10 µF to 47 µF is usually sufficient to provide input damping and helps
to hold the input voltage steady during large load transients.
An EMI input filter is often used in front of the regulator that, unless carefully designed, can lead to instability as
well as some of the effects mentioned above. The user's guide Simple Success with Conducted EMI for DC-DC
Converters (SNVA489) provides helpful suggestions when designing an input filter for any switching regulator.
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10 Layout
10.1 Layout Guidelines
The LM5008 regulation and overvoltage comparators are very fast, and as such responds to short-duration noise
pulses. Layout considerations are therefore critical for optimum performance:
1. Minimize the area of the high di/dt switching current loop consisting of the VIN pin, input ceramic capacitor,
SW node and freewheeling power diode. Keep the input capacitor as close as possible to the VIN pin and
route a short, direct connection to the RTN pin using polygon copper pours.
2. Minimize SW copper area to reduce radiated noise related to high dv/dt.
3. Locate all components as physically close as possible to their respective pins, thereby minimizing noise
pickup in the printed-circuit tracks.
4. Locate the FB trace away from noise sources and inductors. Place the resistor close to the FB pin to
minimize the length of the FB trace.
If the internal dissipation of the LM5008 converter produces excessive junction temperatures during normal
operation, optimal use of the PCB ground plane can help considerably to dissipate heat. The exposed pad on the
bottom of the WSON-8 package can be soldered to a ground plane on the PCB, and that plane should extend
out from beneath the IC to help dissipate the heat. Additionally, the use of wide PCB traces for power connection
can also help conduct heat away from the IC. Judicious positioning of the LM5008 converter within the end
product, along with use of any available air flow (forced or natural convection), can help reduce the operating
junction temperature.
10.2 Layout Examples
VOUT
CA
COUT
GND
L1
CIN
RA
D1
CBST
SW
VIN
SW
VCC
BST
VIN
CVCC
LM5008
RT/SD
RTN
FB
RT
RCL
RCL
RFB1
Via
to VIN
RFB2
CB
Via to Ground Plane
Figure 14. LM5008 Evaluation Board Top Layer
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11 Device and Documentation Support
11.1 Device Support
11.1.1 Third-Party Products Disclaimer
TI'S PUBLICATION OF INFORMATION REGARDING THIRD-PARTY PRODUCTS OR SERVICES DOES NOT
CONSTITUTE AN ENDORSEMENT REGARDING THE SUITABILITY OF SUCH PRODUCTS OR SERVICES
OR A WARRANTY, REPRESENTATION OR ENDORSEMENT OF SUCH PRODUCTS OR SERVICES, EITHER
ALONE OR IN COMBINATION WITH ANY TI PRODUCT OR SERVICE.
11.1.2 Custom Design With WEBENCH® Tools
Click here to create a custom design using the LM5008 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.
11.1.3 Development Support
For development support see the following:
• For TI's reference design library, visit TI Designs
• For TI's WEBENCH Design Environments, visit WEBENCH® Design Center
11.2 Documentation Support
11.2.1 Related Documentation
For related documentation see the following:
• LM5008 Quick-start Calculator
• AN-1330 LM5008 Evaluation Board (SNVA380)
• AN-1925 LM5008A Evaluation Board (SNVA380)
• Buck Regulator Topologies for Wide Input/Output Voltage Differentials (SNVA594)
11.2.1.1 PCB Layout Resources
• AN-1149 Layout Guidelines for Switching Power Supplies (SNVA021)
• AN-1229 Simple Switcher PCB Layout Guidelines (SNVA054)
• Constructing Your Power Supply – Layout Considerations (SLUP230)
• Low Radiated EMI Layout Made SIMPLE with LM4360x and LM4600x (SNVA721)
• AN-2162 Simple Success With Conducted EMI From DC-DC Converters (SNVA489)
• Reduce Buck-Converter EMI and Voltage Stress by Minimizing Inductive Parasitics (SLYT682)
• White Papers:
– Valuing Wide VIN, Low EMI Synchronous Buck Circuits for Cost-driven, Demanding Applications
– An Overview of Conducted EMI Specifications for Power Supplies
– An Overview of Radiated EMI Specifications for Power Supplies
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Documentation Support (continued)
11.2.1.2 Thermal Design Resources
• AN-2020 Thermal Design By Insight, Not Hindsight (SNVA419)
• AN-1520 A Guide to Board Layout for Best Thermal Resistance for Exposed Pad Packages (SNVA183)
• Semiconductor and IC Package Thermal Metrics (SPRA953)
• Thermal Design Made Simple with LM43603 and LM43602 (SNVA719)
• PowerPAD™Thermally Enhanced Package (SLMA002)
• PowerPAD Made Easy (SLMA004)
• Using New Thermal Metrics (SBVA025)
• Power House Blogs:
– High-Density PCB Layout of DC/DC Converters
11.3 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.
11.4 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.
11.5 Trademarks
PowerPAD, E2E are trademarks of Texas Instruments.
WEBENCH is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.6 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.
11.7 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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30-Sep-2021
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)
LM5008MM
NRND
VSSOP
DGK
8
1000
Non-RoHS
& Green
Call TI
Level-1-260C-UNLIM
-40 to 125
SAYB
LM5008MM/NOPB
ACTIVE
VSSOP
DGK
8
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
SAYB
LM5008MMX
NRND
VSSOP
DGK
8
3500
Non-RoHS
& Green
Call TI
Level-1-260C-UNLIM
-40 to 125
SAYB
LM5008MMX/NOPB
ACTIVE
VSSOP
DGK
8
3500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
SAYB
LM5008SD
NRND
WSON
NGU
8
1000
Non-RoHS
& Green
Call TI
Level-1-260C-UNLIM
LM5008SDC/NOPB
ACTIVE
WSON
NGU
8
1000
RoHS & Green
NIPDAU | SN
Level-1-260C-UNLIM
-40 to 125
L5008SD
LM5008SDCX/NOPB
ACTIVE
WSON
NGU
8
4500
RoHS & Green
NIPDAU | SN
Level-1-260C-UNLIM
-40 to 125
L5008SD
L00040B
(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.
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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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