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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.
LM2599
SNVS123D APRIL 1998REVISED MAY 2016
LM2599 SIMPLE SWITCHER® Power Converter 150-kHz 3-A Step-Down Voltage Regulator,
With Features
1
1 Features
1 3.3-V, 5-V, 12-V, and Adjustable Output Versions
Adjustable Version Output Voltage Range: 1.2 V
to 37 V ±4% Maximum Over Line and Load
Conditions
3-A Output Current
Available in 7-Pin TO-220 and TO-263 (Surface-
Mount) Package
Input Voltage Range Up to 40 V
150-kHz Fixed-Frequency Internal Oscillator
Shutdown and Soft-Start
Out-of-Regulation Error Flag
Error Output Delay
Low Power Standby Mode, IQ, Typically 80 μA
High Efficiency
Uses Readily Available Standard Inductors
Thermal Shutdown and Current-Limit Protection
2 Applications
Simple High-Efficiency Step-Down (Buck)
Regulator
Efficient Preregulator for Linear Regulators
On-Card Switching Regulators
Positive to Negative Converter
3 Description
The LM2599 series of regulators are monolithic
integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of
driving a 3-A load with excellent line and load
regulation. These devices are available in fixed output
voltages of 3.3 V, 5 V, 12 V, and an adjustable output
version.
The LM2598 is a member of the LM259x family, with
additional supervisory and performance features
added.
Requiring a minimum number of external
components, these regulators are simple to use and
include internal frequency compensation, improved
line and load specifications, fixed-frequency oscillator,
Shutdown and Soft-start, error flag delay, and error
flag output.
Device Information(1)
PART NUMBER PACKAGE BODY SIZE (NOM)
LM2599 TO-220 (7) 14.986 mm × 10.16 mm
TO-263 (7) 10.10 mm × 8.89 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application
(fixed output voltage versions)
2
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Table of Contents
1 Features.................................................................. 1
2 Applications ........................................................... 1
3 Description............................................................. 1
4 Revision History..................................................... 2
5 Description (continued)......................................... 3
6 Pin Configuration and Functions......................... 3
7 Specifications......................................................... 4
7.1 Absolute Maximum Ratings ..................................... 4
7.2 ESD Ratings.............................................................. 4
7.3 Recommended Operating Conditions....................... 4
7.4 Thermal Information.................................................. 4
7.5 Electrical Characteristics 3.3-V Version................. 5
7.6 Electrical Characteristics 5-V Version.................... 5
7.7 Electrical Characteristics 12-V Version.................. 5
7.8 Electrical Characteristics Adjustable Voltage
Version....................................................................... 5
7.9 Electrical Characteristics All Output Voltage
Versions..................................................................... 6
7.10 Typical Characteristics............................................ 7
8 Detailed Description............................................ 10
8.1 Overview................................................................. 10
8.2 Functional Block Diagram....................................... 10
8.3 Feature Description................................................. 10
8.4 Device Functional Modes........................................ 15
9 Application and Implementation ........................ 16
9.1 Application Information............................................ 16
9.2 Typical Applications ................................................ 25
10 Power Supply Recommendations ..................... 34
11 Layout................................................................... 34
11.1 Layout Guidelines ................................................. 34
11.2 Layout Examples................................................... 34
11.3 Thermal Considerations........................................ 35
12 Device and Documentation Support................. 38
12.1 Community Resources.......................................... 38
12.2 Trademarks........................................................... 38
12.3 Electrostatic Discharge Caution............................ 38
12.4 Glossary................................................................ 38
13 Mechanical, Packaging, and Orderable
Information........................................................... 38
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision C (April 2013) to Revision D 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
Removed all references to design software Switchers Made Simple ................................................................................... 1
Changes from Revision B (April 2013) to Revision C Page
Changed layout of National Semiconductor Data Sheet to TI format .................................................................................. 35
3
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5 Description (continued)
The LM2599 series operates at a switching frequency of 150 kHz, thus allowing smaller sized filter components
than what would be needed with lower frequency switching regulators. Available in a standard 7-pin TO-220
package with several different lead bend options, and a 7-pin TO-263 surface-mount package.
A standard series of inductors (both through-hole and surface-mount types) are available from several different
manufacturers optimized for use with the LM2599 series. This feature greatly simplifies the design of switch-
mode power supplies.
Other features include a ±4% tolerance on output voltage under all conditions of input voltage and output load
conditions, and ±15% on the oscillator frequency. External shutdown is included, featuring typically 80-μA
standby current. Self-protection features include a two stage current limit for the output switch and an
overtemperature shutdown for complete protection under fault conditions.
6 Pin Configuration and Functions
NDZ Package
7-Pin TO-220
Top View KTW Package
7-Pin TO-263
Top View
(1) If any of the above three features (Shutdown/Soft-start, Error Flag, or Delay) are not used, the respective pins must be left open.
Pin Functions(1)
PIN I/O DESCRIPTION
NO. NAME
1 +VIN IThis is the positive input supply for the IC switching regulator. A suitable input bypass
capacitor must be present at this pin to minimize voltage transients and to supply the
switching currents needed by the regulator.
2 Output O Internal switch. The voltage at this pin switches between approximately (+VIN VSAT) and
approximately 0.5 V, with a duty cycle of VOUT/VIN. To minimize coupling to sensitive
circuitry, the PCB copper area connected to this pin must be kept to a minimum.
3 Error Flag O
Open-collector output that provides a low signal (flag transistor ON) when the regulated
output voltage drops more than 5% from the nominal output voltage. On start up, Error Flag
is low until VOUT reaches 95% of the nominal output voltage and a delay time determined by
the Delay pin capacitor. This signal can be used as a reset to a microprocessor on power-
up.
4 Ground Circuit ground.
5 Delay O At power-up, this pin can be used to provide a time delay between the time the regulated
output voltage reaches 95% of the nominal output voltage, and the time the error flag output
goes high.
6 Feedback I Senses the regulated output voltage to complete the feedback loop.
7 Shutdown/Soft-start I This dual function pin provides the following features: (a) Allows the switching regulator
circuit to be shut down using logic level signals thus dropping the total input supply current
to approximately 80 μA. (b) Adding a capacitor to this pin provides a soft-start feature which
minimizes start-up current and provides a controlled ramp up of the output voltage.
4
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(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.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
(3) Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.
7 Specifications
7.1 Absolute Maximum Ratings (1)(2)
MIN MAX UNIT
Maximum supply voltage, VIN 45 V
SD/SS pin input voltage(3) 6 V
Delay pin voltage(3) 1.5 V
Flag pin voltage –0.3 45 V
Feedback pin voltage –0.3 25 V
Output voltage to ground (steady-state) –1 V
Power dissipation Internally limited
Lead temperature KTW package Vapor phase (60 s) 215 °CInfrared (10 s) 245
NDZ package (soldering, 10 s) 260
Maximum junction temperature 150 °C
Storage temperature, Tstg 65 150 °C
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
7.2 ESD Ratings VALUE UNIT
V(ESD) Electrostatic
discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1) ±2000 V
7.3 Recommended Operating Conditions MIN MAX UNIT
Supply voltage 4.5 40 V
Temperature –40 125 °C
(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
(2) The package thermal impedance is calculated in accordance to JESD 51-7.
(3) Thermal resistances were simulated on a 4-layer, JEDEC board.
(4) Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the
leads soldered to a PCB with 1-oz copper area of approximately 1 in2.
(5) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single-sided PCB with 0.5 in2of 1-oz copper area.
(6) Junction to ambient thermal resistance with the TO-263 package tab soldered to a single-sided PCB with 2.5 in2of 1-oz copper area.
(7) Junction to ambient thermal resistance with the TO-263 package tab soldered to a double-sided PCB with 3 in2of 1-oz copper area on
the LM2599S side of the board, and approximately 16 in2of copper on the other side of the PCB.
7.4 Thermal Information
THERMAL METRIC(1)
LM2599
UNITKTW (TO-263) NDZ (TO-220)
7 PINS 7 PINS
RθJA Junction-to-ambient thermal resistance(2)(3)
See(4) 50
°C/W
See(5) 50
See(6) 30
See(7) 20
RθJC(top) Junction-to-case (top) thermal resistance 2 2 °C/W
5
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(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.
7.5 Electrical Characteristics 3.3-V Version
Specifications are for TJ= 25°C (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX (1) UNIT
SYSTEM PARAMETERS(3) (see Figure 43 for test circuit)
VOUT Output voltage 4.75 V VIN 40 V,
0.2 A ILOAD 3 A TJ= 25°C 3.168 3.3 3.432 V
–40°C TJ125°C 3.135 3.465
ηEfficiency VIN = 12 V, ILOAD = 3 A 73%
(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.
7.6 Electrical Characteristics 5-V Version
Specifications are for TJ= 25°C (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
SYSTEM PARAMETERS(3) (see Figure 43 for test circuit)
VOUT Output voltage 7 V VIN 40 V,
0.2 A ILOAD 3 A TJ= 25°C 4.8 5 5.2 V
–40°C TJ125°C 4.75 5.25
ηEfficiency VIN = 12 V, ILOAD = 3 A 80%
(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.
7.7 Electrical Characteristics 12-V Version
Specifications are for TJ= 25°C (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
SYSTEM PARAMETERS(3) (see Figure 43 for test circuit)
VOUT Output voltage 15 V VIN 40 V,
0.2 A ILOAD 3 A TJ= 25°C 11.52 12 12.48 V
–40°C TJ125°C 11.4 12.6
ηEfficiency VIN = 25 V, ILOAD = 3 A 90%
(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.
7.8 Electrical Characteristics Adjustable Voltage Version
Specifications are for TJ= 25°C (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
SYSTEM PARAMETERS(3) (see Figure 43 for test circuit)
VFB Feedback voltage 4.5 V VIN 40 V, 0.2 A ILOAD 3 A 1.23 V
VOUT programmed for 3 V,
circuit of Figure 43 TJ= 25°C 1.193 1.267
–40°C TJ125°C 1.18 1.28
ηEfficiency VIN = 12 V, VOUT = 3 V, ILOAD = 3 A 73%
6
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(1) All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
(2) Typical numbers are at 25°C and represent the most likely norm.
(3) The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the
severity of current overload.
(4) No diode, inductor or capacitor connected to output pin.
(5) Feedback pin removed from output and connected to 0 V to force the output transistor switch ON.
(6) Feedback pin removed from output and connected to 12 V for the 3.3-V, 5-V, and the adjustable versions, and 15 V for the 12-V
version, to force the output transistor switch OFF.
7.9 Electrical Characteristics All Output Voltage Versions
Specifications are for TJ= 25°C, ILOAD = 500 mA, VIN = 12 V for the 3.3-V, 5-V, and Adjustable version, and VIN = 24 V for the
12-V version (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN(1) TYP(2) MAX(1) UNIT
DEVICE PARAMETERS
IbFeedback bias current Adjustable voltage
version only, VFB = 1.3
V
TJ= 25°C 10 50 nA
–40°C TJ125°C 100
fOOscillator frequency(3) TJ= 25°C 127 150 173 kHz
–40°C TJ125°C 110 173
VSAT Saturation voltage IOUT = 3 A(4)(5) TJ= 25°C 1.16 1.4 V
–40°C TJ125°C 1.5
DC Max duty cycle (ON)(5) 100%
Min duty cycle (OFF)(6) 0%
ICL Current limit Peak current(4) (5) TJ= 25°C 3.6 4.5 6.9 A
–40°C TJ125°C 3.4 7.5
ILOutput leakage current Output = 0 V, VIN = 40 V(4) (6) 50 μA
Output = 1 V 2 30 mA
IQOperating quiescent current SD/SS pin open(6) 5 10 mA
ISTBY Current standby quiescent SD/SS pin = 0 V, VIN =
40 V TJ= 25°C 80 200 μA
–40°C TJ125°C 250 μA
SHUTDOWN/SOFT-START CONTROL See Figure 43
VSD Shutdown threshold voltage 1.3 VLow (Shutdown mode), –40°C TJ125°C 0.6
High (Soft-start mode), 40°C TJ125°C 2
VSS Soft-start voltage VOUT = 20% of nominal output voltage 2 V
VOUT = 100% of nominal output voltage 3
ISD Shutdown current VSHUTDOWN = 0.5 V 5 10 μA
ISS Soft-start current VSoft-start = 2.5 V 1.6 5 μA
FLAG/DELAY CONTROL See Figure 43
Regulator dropout detector
threshold voltage Low (flag ON) 92% 96% 98%
VFSAT Voltage flag output saturation ISINK = 3 mA 0.3 V
VDELAY = 0.5 V TJ= 25°C 0.7 V
–40°C TJ125°C 1
IFLFlag output leakage current VFLAG = 40 V 0.3 μA
Voltage delay pin threshold 1.25 V
Low (flag ON) 1.21 V
High (flag OFF) and VOUT regulated 1.29
Delay pin source current VDELAY = 0.5 V 3 6 μA
Delay pin saturation Low (flag ON) TJ= 25°C 55 350 mV
–40°C TJ125°C 400
7
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7.10 Typical Characteristics
See Figure 43 for test circuit
Figure 1. Normalized Output Voltage Figure 2. Line Regulation
Figure 3. Efficiency Figure 4. Switch Saturation Voltage
Figure 5. Switch Current Limit Figure 6. Dropout Voltage
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Typical Characteristics (continued)
See Figure 43 for test circuit
Figure 7. Operating Quiescent Current Figure 8. Shutdown Quiescent Current
Figure 9. Minimum Operating Supply Voltage Figure 10. Feedback Pin Bias Current
Figure 11. Flag Saturation Voltage Figure 12. Switching Frequency
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Typical Characteristics (continued)
See Figure 43 for test circuit
Figure 13. Soft-Start Figure 14. Shutdown/Soft-Start Current
Figure 15. Daisy Pin Current Figure 16. Soft-Start Response
Figure 17. Shutdown/Soft-Start Threshold Voltage
10
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8 Detailed Description
8.1 Overview
The LM2599 SIMPLE SWITCHER®regulator is an easy-to-use, nonsynchronous, step-down DC-DC converter
with a wide input voltage range up to 40 V. The regulator is capable of delivering up to 3-A DC load current with
excellent line and load regulation. These devices are available in fixed output voltages of 3.3 V, 5 V, 12 V, and
an adjustable output version. The family requires few external components, and the pin arrangement was
designed for simple, optimum PCB layout.
8.2 Functional Block Diagram
8.3 Feature Description
8.3.1 Shutdown/Soft-Start
The circuit shown in Figure 20 is a standard buck regulator with 20-VIN, 12-VOUT, 1-A load using a 0.068-μF soft-
start capacitor. Figure 18 and Figure 19 show the effects of soft-start on the output voltage, the input current,
with, and without a soft-start capacitor. The reduced input current required at startup is very evident when
comparing the two photos. The soft-start feature reduces the start-up current from 2.6 A down to 650 mA, and
delays and slows down the output voltage rise time.
11
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Feature Description (continued)
Figure 18. Output Voltage and Input Current
at Start-Up With Soft-Start Figure 19. Output Voltage and Input Current
at Start-Up Without Soft-Start
This reduction in start-up current is useful in situations where the input power source is limited in the amount of
current it can deliver. In some applications soft-start can be used to replace undervoltage lockout or delayed
start-up functions.
If a very slow output voltage ramp is desired, the soft-start capacitor can be made much larger. Many seconds or
even minutes are possible.
If only the shutdown feature is needed, the soft-start capacitor can be eliminated.
Figure 20. Typical Circuit Using Shutdown/Soft-Start and Error Flag Features
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Feature Description (continued)
Figure 21. Inverting 5-V Regulator With Shutdown and Soft-Start
8.3.2 Inverting Regulator
The circuit in Figure 21 converts a positive input voltage to a negative output voltage with a common ground. The
circuit operates by bootstrapping the ground pin of the regulator to the negative output voltage, then grounding
the feedback pin, the regulator senses the inverted output voltage and regulates it.
This example uses the LM2599-5 to generate a 5-V output, but other output voltages are possible by selecting
other output voltage versions, including the adjustable version. Because this regulator topology can produce an
output voltage that is either greater than or less than the input voltage, the maximum output current greatly
depends on both the input and output voltage. The curve shown in Figure 22 provides a guide as to the amount
of output load current possible for the different input and output voltage conditions.
The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and
this must be limited to a maximum of 40 V. In this example, when converting 20 V to 5 V, the regulator would
see 25 V between the input pin and ground pin. The LM2599 has a maximum input voltage rating of 40 V.
Figure 22. Maximum Load Current for Inverting Regulator Circuit
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Feature Description (continued)
An additional diode is required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or
noise from coupling through the CIN capacitor to the output, under light or no load conditions. Also, this diode
isolation changes the topology to closely resemble a buck configuration, thus providing good closed-loop stability.
TI recommends a Schottky diode for low input voltages, (because of its lower voltage drop) but for higher input
voltages, a IN5400 diode could be used.
Because of differences in the operation of the inverting regulator, the standard design procedure is not used to
select the inductor value. In the majority of designs, a 33-μH, 3.5-A inductor is the best choice. Capacitor
selection can also be narrowed down to just a few values. Using the values shown in Figure 21 provides good
results in the majority of inverting designs.
This type of inverting regulator can require relatively large amounts of input current when starting up, even with
light loads. Input currents as high as the LM2599 current limit (approximately 4.5 A) are needed for 2 ms or
more, until the output reaches its nominal output voltage. The actual time depends on the output voltage and the
size of the output capacitor. Input power sources that are current limited or sources that can not deliver these
currents without getting loaded down, may not work correctly. Because of the relatively high startup currents
required by the inverting topology, the soft-start feature shown in Figure 21 is recommended.
Also shown in Figure 21 are several shutdown methods for the inverting configuration. With the inverting
configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but
is now at the negative output voltage. The shutdown methods shown accept ground referenced shutdown
signals.
8.3.3 Undervoltage Lockout
Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage.
Figure 23 contains a undervoltage lockout circuit for a buck configuration, while Figure 24 and Figure 25 are for
the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 23 uses a Zener
diode to establish the threshold voltage when the switcher begins operating. When the input voltage is less than
the Zener voltage, resistors R1 and R2 hold the shutdown/soft-start pin low, keeping the regulator in the
shutdown mode. As the input voltage exceeds the Zener voltage, the Zener conducts, pulling the shutdown/soft-
start pin high, allowing the regulator to begin switching. The threshold voltage for the undervoltage lockout
feature is approximately 1.5 V greater than the Zener voltage.
Figure 23. Undervoltage Lockout for a Buck Regulator
Figure 24 and Figure 25 apply the same feature to an inverting circuit. Figure 24 features a constant threshold
voltage for turn on and turn off (Zener voltage plus approximately one volt). If hysteresis is needed, the circuit in
Figure 25 has a turn ON voltage which is different than the turn OFF voltage. The amount of hysteresis is
approximately equal to the value of the output voltage. Because the SD/SS pin has an internal 7-V Zener clamp,
R2 is needed to limit the current into this pin to approximately 1 mA when Q1 is on.
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Feature Description (continued)
Figure 24. Undervoltage Lockout Without Hysteresis for an Inverting Regulator
Figure 25. Undervoltage Lockout With Hysteresis for an Inverting Regulator
8.3.4 Negative Voltage Charge Pump
Occasionally a low-current negative voltage is needed for biasing parts of a circuit. A simple method of
generating a negative voltage using a charge pump technique and the switching waveform present at the OUT
pin, is shown in Figure 26. This unregulated negative voltage is approximately equal to the positive input voltage
(minus a few volts), and can supply up to a 600 mA of output current. There is a requirement however, that there
be a minimum load of 1.2 A on the regulated positive output for the charge pump to work correctly. Also, resistor
R1 is required to limit the charging current of C1 to some value less than the LM2599 current limit (typically
4.5 A).
This method of generating a negative output voltage without an additional inductor can be used with other
members of the SIMPLE SWITCHER Family, using either the buck or boost topology.
Figure 26. Charge Pump for Generating a Low-Current, Negative-Output Voltage
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8.4 Device Functional Modes
8.4.1 Discontinuous Mode Operation
The selection guide chooses inductor values suitable for continuous mode operation, but for low current
applications or high input voltages, a discontinuous mode design may be a better choice. The design would use
an inductor that would be physically smaller, and would need only one half to one third the inductance value
needed for a continuous mode design. The peak switch and inductor currents is higher in a discontinuous design,
but at these low load currents (1 A and below), the maximum switch current is still less than the switch current
limit.
Discontinuous operation can have voltage waveforms that are considerably different than a continuous design.
The output pin (switch) waveform can have some damped sinusoidal ringing present (see Figure 44). This
ringing is normal for discontinuous operation, and is not caused by feedback loop instabilities. In discontinuous
operation, there is a period of time where neither the switch nor the diode are conducting, and the inductor
current has dropped to zero. During this time, a small amount of energy can circulate between the inductor and
the switch and diode parasitic capacitance causing this characteristic ringing. Normally this ringing is not a
problem, unless the amplitude becomes great enough to exceed the input voltage, and even then, there is very
little energy present to cause damage.
Different inductor types or core materials produce different amounts of this characteristic ringing. Ferrite core
inductors have very little core loss and therefore produce the most ringing. The higher core loss of powdered iron
inductors produce less ringing. If desired, a series RC could be placed in parallel with the inductor to dampen the
ringing.
Figure 27. Post Ripple Filter Waveform
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9 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.
9.1 Application Information
9.1.1 Soft-Start Capacitor (CSS)
The capacitor on this pin provides the regulator with a soft-start feature (slow start-up). When the DC input
voltage is first applied to the regulator, or when the Shutdown/Soft-start pin is allowed to go high, a constant
current (approximately 5 μA begins charging this capacitor). As the capacitor voltage rises, the regulator goes
through four operating regions (see the bottom curve in Figure 28):
1. Regulator in Shutdown: When the SD/SS pin voltage is between 0 V and 1.3 V, the regulator is in shutdown,
the output voltage is zero, and the IC quiescent current is approximately 85 μA.
2. Regulator ON, but the output voltage is zero: With the SD/SS pin voltage between approximately 1.3 V and
1.8 V, the internal regulator circuitry is operating, the quiescent current rises to approximately 5 mA, but the
output voltage is still zero. Also, as the 1.3-V threshold is exceeded, the soft-start capacitor charging current
decreases from 5 μA down to approximately 1.6 μA. This decreases the slope of capacitor voltage ramp.
3. Soft-start Region: When the SD/SS pin voltage is between 1.8 V and 2.8 V (at 25°C), the regulator is in a
soft-start condition. The switch (Pin 2) duty cycle initially starts out very low, with narrow pulses and gradually
get wider as the capacitor SD/SS pin ramps up towards 2.8 V. As the duty cycle increases, the output
voltage also increases at a controlled ramp up (see the center curve in Figure 28) The input supply current
requirement also starts out at a low level for the narrow pulses and ramp up in a controlled manner. This is a
very useful feature in some switcher topologies that require large start-up currents (such as the inverting
configuration) which can load down the input power supply.
NOTE
The lower curve shown in Figure 28 shows the soft-start region from 0% to 100%. This is
not the duty cycle percentage, but the output voltage percentage. Also, the soft-start
voltage range has a negative temperature coefficient associated with it. See the soft-start
section in Electrical Characteristics All Output Voltage Versions.
4. Normal operation: Above 2.8 V, the circuit operates as a standard Pulse Width Modulated switching
regulator. The capacitor continues to charge up until it reaches the internal clamp voltage of approximately 7
V. If this pin is driven from a voltage source, the current must be limited to about 1 mA.
If the part is operated with an input voltage at or below the internal soft-start clamp voltage of approximately 7 V,
the voltage on the SD/SS pin tracks the input voltage and can be disturbed by a step in the voltage. To maintain
proper function under these conditions, TI strongly recommends clamping the SD/SS pin externally between the
3-V maximum soft-start threshold and the 4.5-V minimum input voltage. Figure 30 is an example of an external
3.7-V (approximately) clamp that prevents a line-step related glitch but does not interfere with the soft-start
behavior of the device.
17
LM2599
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SNVS123D APRIL 1998REVISED MAY 2016
Product Folder Links: LM2599
Submit Documentation FeedbackCopyright © 1998–2016, Texas Instruments Incorporated
Application Information (continued)
Figure 28. Soft-Start, Delay, Error, Output
Figure 29. Timing Diagram for 5-V Output
Z1
3V
CSS
SD/SS
VIN
LM2599
5Q1
18
LM2599
SNVS123D APRIL 1998REVISED MAY 2016
www.ti.com
Product Folder Links: LM2599
Submit Documentation Feedback Copyright © 1998–2016, Texas Instruments Incorporated
Application Information (continued)
Figure 30. External 3.7-V Soft-Start Clamp
9.1.2 Delay Capacitor (CDELAY)
This capacitor provides delay for the error flag output (see Figure 28 and Figure 29). A capacitor on this pin
provides a time delay between the time the regulated output voltage (when it is increasing in value) reaches 95%
of the nominal output voltage, and the time the error flag output goes high. A 3-μA constant current from the
delay pin charges the delay capacitor resulting in a voltage ramp. When this voltage reaches a threshold of
approximately 1.3 V, the open-collector error flag output (or power OK) goes high. This signal can be used to
indicate that the regulated output has reached the correct voltage and has stabilized.
If, for any reason, the regulated output voltage drops by 5% or more, the error output flag (Pin 3) immediately
goes low (internal transistor turns on). The delay capacitor provides very little delay if the regulated output is
dropping out of regulation. The delay time for an output that is decreasing is approximately a 1000 times less
than the delay for the rising output. For a 0.1-μF delay capacitor, the delay time would be approximately 50 ms
when the output is rising and passes through the 95% threshold, but the delay for the output dropping would only
be approximately 50 μs.
9.1.2.1 RPULLUP
The error flag output (or power OK) is the collector of a NPN transistor, with the emitter internally grounded. To
use the error flag, a pullup resistor to a positive voltage is needed. The error flag transistor is rated up to a
maximum of 45 V and can sink approximately 3 mA. If the error flag is not used, it can be le