Product
Folder
Order
Now
Support &
Community
Tools &
Software
Technical
Documents
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1 42-V 2-A Constant On-Time Switching
Regulator With Adjustable Current Limit
1 Features
3 Description
•
The LM25011 constant on-time step-down switching
regulator features all the functions needed to
implement a low-cost, efficient, buck bias regulator
capable of supplying up to 2 A of load current. This
high-voltage regulator contains an N-Channel Buck
switch, a startup regulator, current limit detection, and
internal ripple control. The constant on-time
regulation principle requires no loop compensation,
results in fast load transient response, and simplifies
circuit implementation. The operating frequency
remains constant with line and load. The adjustable
valley current limit detection results in a smooth
transition from constant voltage to constant current
mode when current limit is reached, without the use
of current limit foldback. The PGD output indicates
the output voltage has increased to within 5% of the
expected regulation value. Additional features
include: Low output ripple, VIN under-voltage lockout, adjustable soft-start timing, thermal shutdown,
gate drive pre-charge, gate drive under-voltage lockout, and maximum duty cycle limit.
1
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
LM25011-Q1 is an Automotive Grade Product that
is AEC-Q100 Grade 1 Qualified (–40°C to +125°C
Operating Junction Temperature)
LM25011A Allows Low-Dropout Operation at High
Switching Frequency
Input Operating Voltage Range: 6 V to 42 V
Absolute Maximum Input Rating: 45 V
Integrated 2-A N-Channel Buck Switch
Adjustable Current Limit Allows for Smaller
Inductor
Adjustable Output Voltage from 2.51 V
Minimum Ripple Voltage at VOUT
Power Good Output
Switching Frequency Adjustable to 2 MHz
COT Topology Features:
– Switching Frequency Remains Nearly
Constant with Load Current and Input Voltage
Variations
– Ultra-Fast Transient Response
– No Loop Compensation Required
– Stable Operation with Ceramic Output
Capacitors
– Allows for Smaller Output Capacitor and
Current Sense Resistor
Adjustable Soft-Start Timing
Thermal Shutdown
Precision 2% Feedback Reference
Package: 10-Pin, HVSSOP
Create a Custom Design Using the LM25011
Family with the WEBENCH Power Designer
2 Applications
•
•
•
•
Automotive Safety
Infotainment
Telecommunication
Front Camera
The LM25011A has a shorter minimum off-time than
the LM25011, which allows for higher frequency
operation at low input voltages.
Device Information(1)
PART NUMBER
LM25011 / -Q1
LM25011A / -Q1
PACKAGE
BODY SIZE (NOM)
HVSSOP (10)
3.00 mm × 3.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application
6V to 42V
Input
VIN
BST
CBST
LM25011
CIN
RT
D1
RT
VOUT
CS
RPGD
VPGD
Power
Good
L1
SW
RS
PGD
CSG
COUT
RFB2
SS
CSS
SGND
FB
RFB1
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.
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
6.7
4
4
4
4
5
6
7
Absolute Maximum Ratings .....................................
Handling Ratings: LM25011......................................
Handling Ratings: LM25011-Q1................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Detailed Description ............................................ 10
7.1 Overview ................................................................. 10
7.2 Functional Block Diagram ....................................... 10
7.3 Feature Description................................................. 10
7.4 Device Functional Modes........................................ 15
8
Application and Implementation ........................ 16
8.1 Application Information............................................ 16
8.2 Typical Application .................................................. 16
9 Power Supply Recommendations...................... 23
10 Layout................................................................... 23
10.1 Layout Guidelines ................................................. 23
10.2 Layout Example .................................................... 23
10.3 Power Dissipation ................................................. 23
11 Device and Documentation Support ................. 24
11.1
11.2
11.3
11.4
11.5
11.6
Custom Design with WEBENCH Tools.................
Receiving Notification of Documentation Updates
Related Links ........................................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
24
24
24
24
24
24
12 Mechanical, Packaging, and Orderable
Information ........................................................... 24
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision G (February 2013) to Revision H
•
Page
Added Pin Configuration and Functions section, Handling Rating table, Feature Description section, Device
Functional Modes, Application and Implementation section, Power Supply Recommendations section, Layout
section, Device and Documentation Support section, and Mechanical, Packaging, and Orderable Information
section ................................................................................................................................................................................... 1
Changes from Revision F (February 2013) to Revision G
•
2
Page
Changed layout of National Semiconductor Data Sheet to TI format .................................................................................. 23
Submit Documentation Feedback
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
5 Pin Configuration and Functions
10-Pin
HVSSOP Package
Top View
Exposed Pad on Bottom
Connect to Ground
VIN
1
10
BST
RT
2
9
SW
PGD
3
8
CS
SS
4
7
CSG
SGND
5
6
FB
Pin Functions
PIN
I/O
DESCRIPTION
APPLICATION INFORMATION
NUMBER
NAME
1
VIN
I
Input supply voltage
Operating input range is 6 V to 42 V. Transient capability is 45 V.
A low ESR capacitor must be placed as close as possible to the
VIN and SGND pins.
2
RT
I
On-time Control
An external resistor from VIN to this pin sets the buck switch ontime and the switching frequency.
3
PGD
–
Power Good
Logic output indicates when the voltage at the FB pin has
increased to above 95% of the internal reference voltage.
Hysteresis is provided. An external pull-up resistor to a voltage
less than 7 V is required.
4
SS
I
Soft-Start
An internal current source charges an external capacitor to
provide the soft-start function.
5
SGND
Signal Ground
Ground for all internal circuitry other than the current limit sense
circuit.
6
FB
I
Feedback
Internally connected to the regulation comparator. The regulation
level is 2.51 V.
7
CSG
–
Current Sense Ground
Ground connection for the current limit sensing circuit. Connect to
ground and to the current sense resistor.
8
CS
I
Current sense
Connect to the current sense resistor and the anode of the freewheeling diode.
9
SW
O
Switching Node
Internally connected to the buck switch source. Connect to the
external inductor, cathode of the free-wheeling diode, and
bootstrap capacitor.
10
BST
I
Bootstrap capacitor connection of Connect a 0.1-µF capacitor from SW to this pin. The capacitor is
the buck switch gate driver.
charged during the buck switch off-time via an internal diode.
-
EP
–
Exposed Pad
Copyright © 2009–2014, Texas Instruments Incorporated
Exposed pad on the underside of the package. This pad should
be soldered to the PC board ground plane to aid in heat
dissipation.
Submit Documentation Feedback
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
3
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
6 Specifications
6.1 Absolute Maximum Ratings (1)
MAX
UNIT
VIN to SGND (TJ = 25°C)
MIN
45
V
BST to SGND
52
V
45
V
SW to SGND (Steady State)
–1.5
BST to SW
–0.3
7
V
CS to CSG
–0.3
0.3
V
CSG to SGND
–0.3
0.3
V
PGD to SGND
–0.3
7
V
SS to SGND
–0.3
3
V
RT to SGND
–0.3
1
V
FB to SGND
–0.3
7
V
150
°C
For soldering specs, see www.ti.com/packaging.
Junction Temperature
(1)
(1)
Absolute Maximum Ratings are limits beyond which damage to the device may occur. Recommended Operating Conditions are
conditions under which operation of the device is intended to be functional. For specifications and test conditions, see the Electrical
Characteristics .
6.2 Handling Ratings: LM25011
Tstg
Storage temperature range
V(ESD)
(1)
(2)
Electrostatic discharge
MIN
MAX
UNIT
–65
150
°C
Human body model (HBM), per ANSI/ESDA/JEDEC JS-001,
all pins (1)
2000
Charged device model (CDM), per JEDEC specification
JESD22-C101, all pins (2)
750
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 Handling Ratings: LM25011-Q1
Tstg
Storage temperature range
MIN
MAX
UNIT
–65
150
°C
Human body model (HBM), per AEC Q100-002 (1)
V(ESD)
(1)
Electrostatic discharge
Charged device model (CDM), per
AEC Q100-011
2000
Corner pins (1, 5, 6,
and 10)
750
Other pins
750
V
AEC Q100-002 indicates HBM stressing is done in accordance with the ANSI/ESDA/JEDEC JS-001 specification.
6.4 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted) (1)
VIN Voltage
Junction Temperature
(1)
4
MIN
MAX
6.0
42
UNIT
V
–40
125
°C
(1)
Absolute Maximum Ratings are limits beyond which damage to the device may occur. Recommended Operating Conditions are
conditions under which operation of the device is intended to be functional. For specifications and test conditions, see the Electrical
Characteristics .
Submit Documentation Feedback
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
6.5 Thermal Information
THERMAL METRIC (1)
HVSSOP (DGQ)
10 PINS
RθJA
Junction-to-ambient thermal resistance
RθJC(top)
Junction-to-case (top) thermal resistance
54.3
RθJB
Junction-to-board thermal resistance
34.2
ψJT
Junction-to-top characterization parameter
4.0
ψJB
Junction-to-board characterization parameter
33.9
RθJC(bot)
Junction-to-case (bottom) thermal resistance
10
(1)
UNIT
48
°C/W
For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report, SPRA953.
Copyright © 2009–2014, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
5
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
6.6 Electrical Characteristics
Typical values correspond to TJ = 25°C. Minimum and maximum limits apply over –40°C to 125°C junction temperature range
unless otherwise stated. Unless otherwise stated, the following conditions apply: VIN = 12 V, RT = 50 kΩ. (1) (2) (3)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
1200
1600
µA
5.3
5.9
INPUT (VIN PIN)
IIN
UVLOVIN
Input operating current
Non-switching, FB = 3 V
VIN undervoltage lock-out threshold
VIN increasing
4.6
VIN undervoltage lock-out threshold hysteresis
200
V
mV
SWITCH CHARACTERISTICS
RDS(ON)
Buck Switch RDS(ON)
ITEST = 200 mA
UVLOGD
Gate Drive UVLO
BST-SW
2.4
UVLOGD Hysteresis
0.3
0.6
3.4
4.4
350
Pre-charge switch voltage
ITEST = 10 mA into SW pin
Pre-charge switch on-time
Ω
V
mV
1.4
V
120
ns
SOFT-START PIN
VSS
Pullup voltage
ISS
Internal current source
VSS-SH
Shutdown threshold
2.51
V
10
µA
70
140
mV
–146
–130
CURRENT LIMIT
VILIM
Threshold voltage at CS
–115
mV
CS bias current
FB = 3 V
–120
µA
CSG bias current
FB = 3 V
–35
µA
ON TIMER, RT PIN
tON - 1
On-time
VIN = 12 V, RT = 50 kΩ
tON - 2
On-time
VIN = 32 V, RT = 50 kΩ
150
200
75
250
ns
ns
tON - 3
On-time (current limit) LM25011
VIN = 12 V, RT = 50 kΩ
100
ns
tON - 3
On-time (current limit) LM25011A
VIN = 12 V, RT = 50 kΩ
200
ns
tON - 4
On-time
VIN = 12 V, RT = 301 kΩ
1020
tON - 5
On-time
VIN = 9 V, RT = 30.9 kΩ
130
171
215
ns
tON - 6
On-time
VIN = 12 V, RT = 30.9 kΩ
105
137
170
ns
tON - 7
On-time
VIN = 16 V, RT = 30.9 kΩ
79
109
142
ns
Minimum off-time (LM25011)
90
150
208
ns
Minimum off-time (LM25011A)
52
75
93
2.46
2.51
2.56
ns
OFF TIMER
tOFF
REGULATION COMPARATOR (FB PIN)
VREF
FB regulation threshold
SS pin = steady state
FB bias current
FB = 3 V
100
V
nA
POWER GOOD (PGD PIN)
Threshold at FB, with respect to VREF
FB increasing
91%
Threshold hysteresis
95%
3.3%
PGDVOL
Low state voltage
IPGD = 1 mA, FB = 0 V
125
PGDLKG
Off state leakage
VPGD = 7 V, FB = 3 V
0.1
180
mV
µA
Junction temperature increasing
155
°C
20
°C
THERMAL SHUTDOWN
TSD
Thermal shutdown
Thermal shutdown hysteresis
(1)
(2)
(3)
6
Current flow out of a pin is indicated as a negative number.
All hot and cold limits are specified by correlating the electrical characteristics to process and temperature variations and applying
statistical process control.
The junction temperature (TJ in °C) is calculated from the ambient temperature (TA in °C) and power dissipation (PD in watts) as follows:
TJ = TA + (PD × RθJA ) where RθJA (in °C/W) is the package thermal impedance provided in the Thermal Information section.
Submit Documentation Feedback
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
6.7 Typical Characteristics
Figure 1. Efficiency (Circuit of Figure 19)
Figure 2. Efficiency at 2 MHz
Figure 3. On-Time vs VIN and RT
Figure 4. Voltage at the RT Pin
Figure 5. Shutdown Current into VIN
Figure 6. Operating Current into VIN
Copyright © 2009–2014, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
7
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
Typical Characteristics (continued)
8
Figure 7. PGD Low Voltage vs Sink Current
Figure 8. Reference Voltage vs Temperature
Figure 9. Current Limit Threshold vs Temperature
Figure 10. Operating Current vs Temperature
Figure 11. VIN UVLO vs Temperature
Figure 12. SS Pin Shutdown Threshold vs Temperature
Submit Documentation Feedback
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
Typical Characteristics (continued)
190
MINIMUM OFF-TIME (ns)
170
LM25011
150
130
110
90
LM25011A
70
50
-40 -20 0 20 40 60 80 100 120
JUNCTION TEMPERATURE (°C)
Figure 13. On-Time vs Temperature
Copyright © 2009–2014, Texas Instruments Incorporated
Figure 14. Minimum Off-Time vs Temperature
Submit Documentation Feedback
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
9
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
7 Detailed Description
7.1 Overview
The LM25011 constant on-time step-down switching regulator features all the functions needed to implement a
low-cost, efficient buck bias power converter capable of supplying up to 2.0 A to the load. This high-voltage
regulator contains an N-Channel buck switch, is easy to implement, and is available in a 10-pin VSSOP,
PowerPAD power enhanced package. The operation of the regulator is based on a constant on-time control
principle with the on-time inversely proportional to the input voltage. This feature results in the operating
frequency remaining relatively constant with load and input voltage variations. The constant on-time feedback
control principle requires no loop compensation resulting in very fast load transient response. The adjustable
valley current limit detection results in a smooth transition from constant voltage to constant current when current
limit is reached. To aid in controlling excessive switch current due to a possible saturating inductor, the on-time is
reduced by approximately 40% when the current limit is detected. The Power Good output (PGD pin) indicates
when the output voltage is within 5% of the expected regulation voltage.
The LM25011 can be implemented to efficiently step-down higher voltages in non-isolated applications.
Additional features include: low output ripple, VIN under-voltage lock-out, adjustable soft-start timing, thermal
shutdown, gate drive pre-charge, gate drive under-voltage lock-out, and maximum duty-cycle limit.
7.2 Functional Block Diagram
6V to 42V
LM25011(A)
VIN
5V REGULATOR
Input
CIN
CBYP
UVLO
CL
RT
THERMAL
SHUTDOWN
OFF TIMER
ON TIMER
RT
FINISH
START
START
FINISH
BST
2.5V
Gate Drive
10
PA
SD
UVLO
VIN
CBST
SS
LOGIC
CSS
LEVEL
SHIFT
L1
+
FB
CL
REGULATION
COMPARATOR
VOUT
SW
FCIC
CONTROL
CURRENT
LIMIT COMPARATOR
D1
+
COUT
Pre - Chg
-
RFB2
CS
RPGD
Power
Good
0.8V
PGD
+
SGND
CURRENT LIMIT
THRESHOLD
+
125 mV
RS
CSG
2.375V
RFB2
7.3 Feature Description
7.3.1 Control Circuit Overview
The LM25011 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with
the output voltage feedback (FB) compared to an internal reference (2.51 V). If the FB voltage is below the
reference, the internal buck switch is switched on for the one-shot timer period which is a function of the input
voltage and the programming resistor (RT). Following the on-time, the switch remains off until the FB voltage falls
below the reference, but never less than the minimum off-time forced by the off-time one-shot timer. When the
FB pin voltage falls below the reference and the off-time one-shot period expires, the buck switch is then turned
on for another on-time one-shot period.
10
Submit Documentation Feedback
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
Feature Description (continued)
When in regulation, the LM25011 operates in continuous conduction mode at heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode, the inductor current is
always greater than zero and the operating frequency remains relatively constant with load and line variations.
The minimum load current for continuous conduction mode is one-half of the ripple current amplitude of the
inductor. The approximate operating frequency is calculated as follows:
VOUT
FS =
-11
(4.1 x 10 x (RT + 0.5k)) + (VIN x 15 ns)
(1)
The buck switch duty cycle is approximately equal to:
DC =
tON
VOUT
= tON x FS =
tON + tOFF
VIN
(2)
When the load current is less than one-half of the ripple current amplitude of the inductor, the circuit operates in
discontinuous conduction mode. The off-time is longer than in continuous conduction mode while the inductor
current is zero, causing the switching frequency to reduce as the load current is reduced. Conversion efficiency is
maintained at light loads because the switching losses are reduced with the reduction in load and frequency. The
approximate discontinuous operating frequency can be calculated as follows:
FS =
VOUT2 x L1 x 1.19 x 1021
2
RL x R T
(3)
where RL = the load resistance, and L1 is the inductor in the circuit.
The output voltage is set by the two feedback resistors (RFB1, RFB2 in the Functional Block Diagram ). The
regulated output voltage is calculated as follows:
VOUT = 2.51 V × (RFB1 + RFB2) / RFB1
(4)
Ripple voltage, which is required at the input of the regulation comparator for proper output regulation, is
generated internally in the LM25011, and externally when the LM25011A is used. In the LM25011 the ERM
(emulated ripple mode) control circuit generates the required internal ripple voltage from the ripple waveform at
the CS pin. The LM25011A, which is designed for higher frequency operation, requires additional ripple voltage
which must be generated externally and provided to the FB pin. This is described in the Application and
Implementation section.
7.3.2 On-Time Timer
The on-time for the LM25011/LM25011A is determined by the RT resistor and the input voltage (VIN), calculated
from:
tON =
4.1 x 10
-11
x (RT + 500:)
(VIN)
+ 15 ns
(5)
The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a specific
continuous conduction mode switching frequency (FS), the RT resistor is determined from the following:
VOUT - (VIN x FS x 15 ns)
- 500:
RT =
-11
FS x 4.1 x 10
(6)
The on-time must be chosen greater than 90 ns for proper operation. Equation 1, Equation 5, and Equation 6 are
valid only during normal operation; that is, the circuit is not in current limit. When the LM25011 operates in
current limit, the on-time is reduced by approximately 40% (this feature is not present in LM25011A). This feature
reduces the peak inductor current which may be excessively high if the load current and the input voltage are
simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the
Copyright © 2009–2014, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
11
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
Feature Description (continued)
output voltage resumes its normal regulated value. The maximum continuous current into the RT pin must be
less than 2 mA. For high-frequency applications, the maximum switching frequency is limited at the maximum
input voltage by the minimum on-time one-shot period (90 ns). At minimum input voltage the maximum switching
frequency is limited by the minimum off-time one-shot period which, if reached, prevents achievement of the
proper duty cycle.
7.3.3 Current Limit
Current limit detection occurs during the off-time by monitoring the voltage across the external current sense
resistor RS. Referring to the Functional Block Diagram , during the off-time the recirculating current flows through
the inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across
the sense resistor exceeds the threshold (VILIM), the current limit comparator output switches to delay the start of
the next on-time period. The next on-time starts when the recirculating current decreases such that the voltage
across RS reduces to the threshold and the voltage at FB is below 2.51 V. The operating frequency is typically
lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by
approximately 40% (only in LM25011) if the voltage at the FB pin is below its threshold when the voltage across
RS reduces to its threshold (VOUT is low due to current limiting).
Figure 15 illustrates the inductor current waveform during normal operation and in current limit. During the first
normal operation, the load current is I01, the average of the inductor current waveform. As the load resistance is
reduced, the inductor current increases until the lower peak of the inductor ripple current exceeds the threshold.
During the current limited portion of Figure 15, each on-time is reduced by approximately 40%, resulting in lower
ripple amplitude for the inductor current. During this time the LM25011 is in a constant-current mode with an
average load current equal to the current limit threshold plus half the ripple amplitude (IOCL), and the output
voltage is below the normal regulated value. Normal operation resumes when the load current is reduced (to IO2),
allowing VOUT and the on-time to return to their normal values. Note that in the second period of normal
operation, even though the peak current of the inductor exceeds the current limit threshold during part of each
cycle, the circuit is not in current limit because the inductor current falls below the current limit threshold during
each off-time. The peak current allowed through the buck switch is 3.5 A and the maximum allowed average
current is 2.0 A.
IPK
IOCL
Current
Limit Threshold
IO2
'I
Inductor
Current
IO1
0V
Voltage at the CS Pin
Voltage at the FB Pin
2.51V
Load
Current
Increases
Normal
Operation
Current
Limited
Load Current
Decreases
Normal
Operation
Figure 15. Normal and Current Limit Operation
12
Submit Documentation Feedback
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
Feature Description (continued)
7.3.4 Ripple Requirements
The LM25011 requires about 25 mVP-P of ripple voltage at the CS pin. Higher switching frequencies may require
more ripple. That ripple voltage is generated by the decreasing recirculating current (the inductor ripple current)
through RS during the off-time. See Figure 16.
Inductor
Current
'I
0V
Voltage
at CS
VRIPPLE
tOFF
tON
Figure 16. CS Pin Waveform
The ripple voltage is equal to:
VRIPPLE = ΔI × RS
(7)
where ΔI is the inductor current ripple amplitude, and RS is the current-sense resistor at the CS pin.
More ripple can be achieved by decreasing the inductor value.
The LM25011A, with its shorter minimum off-time, typically will require more ripple than the LM25011. An
external circuit to increase the effective ripple voltage may be needed. Different methods of generating this ripple
are explained in the External Components section.
7.3.5 N-Channel Buck Switch and Driver
The LM25011 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 (CBST) and an internal high-voltage diode.
A 0.1-µF capacitor connected between BST and SW provides the supply voltage for the driver during the ontime. During each off-time, the SW pin is at approximately –1 V, and CBST is recharged from the internal 5-V
regulator for the next on-time. The minimum off-time ensures a sufficient time each cycle to recharge the
bootstrap capacitor.
In applications with relatively high output voltage and low minimum load current, the internal pre-charge device of
the LM25011 may not pull the SW pin sufficiently low during the off-time to maintain the voltage on the bootstrap
capacitor. If the bootstrap capacitor (CBST) discharges during the long off-times, and the regulator will cycle on
and off at a low frequency. Decreasing the values of the feedback resistors RFB1 and RFB2 to provide a minimum
load of typically 1mA at nominal VOUT will increase the minimum switching frequency and maintain sufficient
bootstrap capacitor voltage.
7.3.6 Soft-Start
The soft-start feature allows the converter to gradually reach a steady-state operating point, thereby reducing
startup stresses and current surges. Upon turn-on, when VIN reaches its undervoltage lock-out threshold an
internal 10-µA current source charges the external capacitor at the SS pin to 2.51 V (t1 in Figure 17). The
ramping voltage at SS ramps the non-inverting input of the regulation comparator and the output voltage, in a
controlled manner. For proper operation, the soft-start capacitor should be no smaller than 1000 pF.
Copyright © 2009–2014, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
13
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
Feature Description (continued)
The LM25011 can be employed as a tracking regulator by applying the controlling voltage to the SS pin. The
output voltage of the regulator tracks the applied voltage, gained up by the ratio of the feedback resistors. The
applied voltage at the SS pin must be within the range of 0.5 V to 2.6 V. The absolute maximum rating for the SS
pin is 3.0 V. If the tracking function causes the voltage at the FB pin to go below the thresholds for the PGD pin,
the PGD pin will switch low (see the Power Good Output (PGD) section). An internal switch grounds the SS pin if
the input voltage at VIN is below its undervoltage lock-out threshold or if the thermal shutdown activates. If the
tracking function (described above) is used, the tracking voltage applied to the SS pin must be current limited to
a maximum of 1 mA.
UVLO
VIN
SW Pin
Inductor
Current
SS Pin
VOUT
PGD
t1
Figure 17. Startup Sequence
7.3.7 Power Good Output (PGD)
The Power Good output (PGD) indicates when the voltage at the FB pin is close to the internal 2.51-V reference
voltage. The rising threshold at the FB pin for the PGD output to switch high is 95% of the internal reference. The
falling threshold for the PGD output to switch low is approximately 3.3% below the rising threshold.
The PGD pin is internally connected to the drain of an N-channel MOSFET switch. An external pull-up resistor
(RPGD), connected to an appropriate voltage not exceeding 7 V, is required at PGD to indicate the LM25011
status to other circuitry. When PGD is low, the pin voltage is determined by the current into the pin. See Figure 7,
PGD Low Voltage vs Sink Current.
14
Submit Documentation Feedback
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
Feature Description (continued)
Upon powering up the LM25011, the PGD pin is high until the voltage at VIN reaches 2 V, at which time PGD
switches low. As VIN is increased, PGD stays low until the output voltage takes the voltage at the FB pin above
95% of the internal reference voltage, at which time PGD switches high. As VIN is decreased (during shutdown),
PGD remains high until either the voltage at the FB pin falls below approximately 92% of the internal reference or
when VIN falls below its lower UVLO threshold, whichever occurs first. PGD then switches low, and remains low
until VIN falls below 2 V, at which time PGD switches high. If the LM25011 is used as a tracking regulator (see
the Soft-Start section), the PGD output is high as long as the voltage at the FB pin is above the thresholds
mentioned above.
7.3.8 Thermal Shutdown
The LM25011 should be operated so the junction temperature does not exceed 125°C. If the junction
temperature increases above that, an internal thermal shutdown circuit activates (typically) at 155°C, taking the
controller to a low-power reset state by disabling the buck switch and taking the SS pin to ground. This feature
helps prevent catastrophic failures from accidental device overheating. When the junction temperature decreases
below 135°C (typical hysteresis = 20°C), normal operation resumes.
7.4 Device Functional Modes
7.4.1 Shutdown Function
The SS pin can be used to shutdown the LM25011 by grounding the SS pin as shown in Figure 18. Releasing
the pin allows normal operation to resume.
SS
STOP
RUN
LM25011
CSS
Figure 18. Shutdown Implementation
Copyright © 2009–2014, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
15
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
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 LM25011/LM25011-Q1 is a non-synchronous buck regulator designed to operate over a wide input voltage
range and output current. Spreadsheet-based quick-start calculation tools and the on-line WEBENCH® software
can be used to create a buck design with the bill of materials, estimated efficiency, and the complete solution
cost.
8.2 Typical Application
8.2.1 LM25011 Example Circuit
The final circuit is shown in Figure 19, and its performance is shown in Figure 20 and Figure 21. The current limit
measures approximately 1.62 A at VIN = 8 V, and 1.69 A at VIN = 36 V.
8V to 36V
Input
RT
118 k:
CIN
4.7 PF
BST
VIN
CBYP
0.1 PF
CBST 0.1 PF
L1 10 PH
LM25011
SW
RT
VOUT
D1
5V
VPGD
CS
RPGD
10 k:
Power
Good
COUT
10 PF
RS
80 m:
PGD
RFB2
4.99 k:
CSG
SS
CSS
0.022 PF
SGND
FB
RFB1
4.99 k:
Figure 19. Example Circuit
8.2.1.1 Design Requirements
Table 1 shows the design parameters.
Table 1. Design Parameters
DESIGN PARAMETER
Input voltage range
Output voltage
5V
Maximum load current (IOUT(max))
1.5 A
Minimum load current (IOUT(min))
300 mA
Switching frequency (FSW)
1 MHz
Soft-start time
16
VALUE
8 V to 36 V
Submit Documentation Feedback
5 ms
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 Custom Design with WEBENCH Tools
Click here to create a custom design using the LM25011 device with the WEBENCH® Power Designer.
1. Start by entering your VIN, VOUT and IOUT requirements.
2. Optimize your design for key parameters like efficiency, footprint and cost using the optimizer dial and
compare this design with other possible solutions from Texas Instruments.
3. WEBENCH Power Designer provides you with a customized schematic along with a list of materials with real
time pricing and component availability.
4. In most cases, you will also be able to:
– Run electrical simulations to see important waveforms and circuit performance,
– Run thermal simulations to understand the thermal performance of your board,
– Export your customized schematic and layout into popular CAD formats,
– Print PDF reports for the design, and share your design with colleagues.
5. Get more information about WEBENCH tools at www.ti.com/webench.
8.2.1.2.2 External Components
The procedure for calculating the external components is illustrated with a design example using the LM25011.
Referring to the Functional Block Diagram , the circuit is to be configured for the following specifications:
• VOUT = 5 V
• VIN = 8 V to 36 V
• Minimum load current for continuous conduction mode IOUT(min) = 300 mA
• Maximum load current IOUT(max) = 1.5 A
• Switching frequency (FSW) = 1.0 MHz
• Soft-start time = 5 ms
RFB2 and RFB1: These resistors set the output voltage, and their ratio is calculated from:
RFB2/RFB1 = (VOUT / 2.51 V) – 1
(8)
For this example, RFB2/RFB1 = 0.992. RFB1 and RFB2 should be chosen from standard value resistors in the range
of 1.0 kΩ to 10 kΩ which satisfy the above ratio. For this example, 4.99 kΩ is chosen for both resistors, providing
a 5.02-V output.
RT: This resistor sets the on-time and (by default) the switching frequency. First check that the desired frequency
does not require an on-time or off-time shorter than the minimum allowed values (90 ns and 150, respectively).
The minimum on-time occurs at the maximum input voltage. For this example:
VOUT
tON(min) =
VIN(max) x FS
=
5V
= 139 ns
36V x 1 MHz
(9)
The minimum off-time occurs at the minimum input voltage. For this example:
tOFF(min) =
VIN(min) - VOUT
VIN(min) x FS
=
8V - 5V
= 375 ns
8V x 1 MHz
(10)
Both the on-time and off-time are acceptable because they are significantly greater than the minimum value for
each. The RT resistor is calculated from Equation 6 using the minimum input voltage:
5 - (8V x 1MHz x 15 ns)
- 500: = 118.5 k:
RT =
-11
1MHz x 4.1 x 10
(11)
A standard value 118-kΩ resistor is selected. The minimum on-time calculates to 152 ns at VIN = 36 V, and the
maximum on-time calculates to 672 ns at VIN = 8 V.
Copyright © 2009–2014, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1
17
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
www.ti.com
L1: The parameters controlled by the inductor are the inductor current ripple amplitude (IOR), and the ripple
voltage amplitude across the current sense resistor RS. The minimum load current is used to determine the
maximum allowable ripple to maintain continuous conduction mode (the lower peak does not reach 0 mA). This
is not a requirement of the LM25011, but serves as a guideline for selecting L1. For this example, the maximum
ripple current should be less than:
IOR(max) = 2 × IOUT(min) = 600 mAP-P
(12)
For applications where the minimum load current is zero, a good starting point for allowable ripple is 20% of the
maximum load current. In this case substitute 20% of IOUT(max) for IOUT(min) in Equation 12. The ripple amplitude
calculated in Equation 12 is then used in Equation 13:
L1(min) =
tON(min) x (VIN(max) - VOUT)
= 7.85 PH
IOR(max)
(13)
A standard value 10-µH inductor is chosen. Using this inductor value, the maximum ripple current amplitude,
which occurs at maximum VIN, calculates to 472 mAP-P, and the peak current is 1736 mA at maximum load
current. Ensure the selected inductor is rated for this peak current. The minimum ripple current, which occurs at
minimum VIN, calculates to 200 mAP-P.
RS: The minimum current limit threshold is calculated at maximum load current using the minimum ripple current
calculated above. The current limit threshold is the lower peak of the inductor current waveform when in current
limit (see Figure 15).
ILIM = 1.5 A – (0.2 A / 2) = 1.4 A
(14)
Current limit detection occurs when the voltage across the sense resistor (RS) reaches the current limit threshold.
To allow for tolerances, the sense resistor value is calculated using the minimum threshold specification:
RS = 115 mV / 1.4 A = 82 mΩ
(15)
The next smaller standard value, 80 mΩ, is selected. The next step is to ensure that sufficient ripple voltage
occurs across RS with this value sense resistor. As mentioned in the Ripple Requirements section, a minimum of
15-mVP-P voltage ripple is required across the RS sense resistor during the off-time to ensure the regulation
circuit operates properly. The ripple voltage is the product of the inductor ripple current amplitude and the sense
resistor value. In this case, the minimum ripple voltage calculates to:
VRIPPLE = ΔI × RS = 200 mA × 0.080 Ω = 16 mV
(16)
If the ripple voltage had calculated to less than 15 mVP-P, the inductor value would have to be reduced to
increase the ripple current amplitude. This would have required a recalculation of ILIM and RS in the above
equations. Because the minimum requirement is satisfied in this case, no change is necessary.
The nominal current limit threshold calculates to 1.63 A. The minimum and maximum thresholds calculate to 1.44
A and 1.83 A, respectively, using the minimum and maximum limits for the current limit threshold specification.
The load current is equal to the threshold current plus one-half of the ripple current. Under normal load
conditions, the maximum power dissipation in RS occurs at maximum load current, and at maximum input voltage
where the on-time duty cycle is minimum. In this design example, the minimum on-time duty cycle is:
Duty Cycle = D =
VOUT
5V
= 13.9%
=
36V
VIN
(17)
At maximum load current, the power dissipation in RS is equal to:
P(RS) = (1.5 A)2 × 0.080 Ω × (1 – 0.139) = 155 mW
(18)
When in current limit the maximum power dissipation in RS calculates to
P(RS) = (1.83 A + 0.472 A / 4)2 × 0.080 Ω = 304 mW
(19)
Duty cycle is not included in this power calculation because the on-time duty cycle is typically