NSC LM2591HVT-5.0 Simple switcher power converter 150 khz 1a step-down voltage regulator Datasheet

LM2591HV
SIMPLE SWITCHER ® Power Converter 150 kHz 1A
Step-Down Voltage Regulator
General Description
The LM2591HV series of regulators are monolithic integrated circuits that provide all the active functions for a
step-down (buck) switching regulator, capable of driving a
1A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3V, 5V, and
an adjustable output version.
This series of switching regulators is similar to the
LM2590HV, but without some of the supervisory and performance features of the latter.
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation†, improved line and load specifications and a
fixed-frequency oscillator.
The LM2591HV 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 5-lead TO-220 package with several
different lead bend options, and a 5-lead TO-263 Surface
mount package.
Other features include a guaranteed ± 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 90 µA
standby current. Self protection features include a two stage
Typical Application
current limit for the output switch and an over temperature
shutdown for complete protection under fault conditions.
Features
n 3.3V, 5V, and adjustable output versions
n Adjustable version output voltage range, 1.2V to 57V
± 4% max over line and load conditions
n Guaranteed 1A output load current
n Available in 5-pin TO-220 and TO-263 (surface mount)
Package
n Input voltage range up to 60V
n 150 kHz fixed frequency internal oscillator
n On/Off control
n Low power standby mode, IQ typically 90 µA
n High Efficiency
n Thermal shutdown and current limit protection
Applications
n
n
n
n
Simple high-efficiency step-down (buck) regulator
Efficient pre-regulator for linear regulators
On-card switching regulators
Positive to Negative converter
Note: † Patent Number 5,382,918.
(Fixed Output Voltage Versions)
10129301
SIMPLE SWITCHER ® and Switchers Made Simple ® are registered trademarks of National Semiconductor Corporation.
© 2003 National Semiconductor Corporation
DS101293
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LM2591HV SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator
July 2003
LM2591HV
Absolute Maximum Ratings
Human Body Model (Note 2)
(Note 1)
Lead Temperature
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Supply Voltage (VIN)
S Package
63V
ON/OFF Pin Voltage
−0.3 ≤ V ≤ +25V
Feedback Pin Voltage
−0.3 ≤ V ≤ +25V
2 kV
Vapor Phase (60 sec.)
+215˚C
Infrared (10 sec.)
+245˚C
T Package (Soldering, 10 sec.)
+260˚C
Maximum Junction Temperature
+150˚C
Output Voltage to Ground
(Steady State)
−1V
Power Dissipation
Internally limited
Storage Temperature Range
−65˚C to +150˚C
Operating Conditions
−40˚C ≤ TJ ≤ +125˚C
Temperature Range
Supply Voltage
ESD Susceptibility
4.5V to 60V
LM2591HV-3.3
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range.
Symbol
Parameter
Conditions
LM2591HV-3.3
Typ
Limit
(Note 3)
(Note 4)
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT
η
Output Voltage
Efficiency
4.75V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 1A
VIN = 12V, ILOAD = 1A
3.3
V
3.168/3.135
V(min)
3.432/3.465
V(max)
77
LM2591HV-5.0
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range.
Symbol
Parameter
Conditions
LM2591HV-5.0
Typ
Limit
(Note 3)
(Note 4)
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT
η
Output Voltage
Efficiency
7V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 1A
VIN = 12V, ILOAD = 1A
5
V
4.800/4.750
V(min)
5.200/5.250
V(max)
82
%
LM2591HV-ADJ
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range.
Symbol
Parameter
Conditions
LM2591HV-ADJ
Typ
Limit
(Note 3)
(Note 4)
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VFB
Feedback Voltage
4.5V ≤ VIN ≤ 60V, 0.2A ≤ ILOAD ≤ 1A
1.230
VOUT programmed for 3V. Circuit of Figure 1.
η
Efficiency
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VIN = 12V, VOUT = 3V, ILOAD = 1A
2
76
V
1.193/1.180
V(min)
1.267/1.280
V(max)
%
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version. ILOAD = 500 mA
Symbol
Parameter
Conditions
LM2591HV-XX
Typ
Limit
(Note 3)
(Note 4)
Units
(Limits)
DEVICE PARAMETERS
Feedback Bias Current
Ib
fO
VSAT
DC
ICLIM
IL
Oscillator Frequency
Saturation Voltage
Adjustable Version Only, VFB = 1.3V
(Note 6)
IOUT = 1A (Note 7) (Note 8)
(Note 8)
100
(Note 9)
0
Switch current Limit
Peak Current, (Note 7) (Note 8)
(Note 7) (Note 9) (Note 10)
Output = 0V
SD /SS Pin Open (Note 9)
5
SD /SS pin = 0V
90
(Note 10)
Current
θJC
Thermal Resistance
kHz(min)
173/173
kHz(max)
1.2/1.3
V(max)
kHz
V
%
A
1.3/1.2
A(min)
2.8/3.0
A(max)
50
µA(max)
30
mA(max)
5
Current
Standby Quiescent
127/110
1.9
Output = −1V
ISTBY
nA (max)
0.95
Max Duty Cycle (ON)
Operating Quiescent
nA
50/100
150
Min Duty Cycle (OFF)
Output Leakage Current
IQ
10
mA
mA
10
mA(max)
200/250
µA(max)
µA
TO220 or TO263 Package, Junction to Case
2
˚C/W
θJA
TO220 Package, Juncton to Ambient (Note 11)
50
˚C/W
θJA
TO263 Package, Juncton to Ambient (Note 12)
50
˚C/W
θJA
TO263 Package, Juncton to Ambient (Note 13)
30
˚C/W
θJA
TO263 Package, Juncton to Ambient (Note 14)
20
˚C/W
1.3
V
ON/OFF CONTROL Test Circuit Figure 1
ON /OFF Pin Logic Input
VIH
Threshold Voltage
VIL
IH
ON /OFF Pin Input Current
IL
Low (Regulator ON)
0.6
V(max)
High (Regulator OFF)
2.0
V(min)
VLOGIC = 2.5V (Regulator OFF)
5
VLOGIC = 0.5V (Regulator ON)
0.02
µA
15
µA(max)
5
µA(max)
µA
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 3: Typical numbers are at 25˚C and represent the most likely norm.
Note 4: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used
to calculate Average Outgoing Quality Level (AOQL).
Note 5: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the
LM2591HV is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
Note 6: 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.
Note 7: No diode, inductor or capacitor connected to output pin.
Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
Note 9: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version to force the output transistor switch OFF.
Note 10: VIN = 60V.
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LM2591HV
All Output Voltage Versions
Electrical Characteristics
LM2591HV
All Output Voltage Versions
Electrical Characteristics (Continued)
Note 11: Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the leads soldered to
a printed circuit board with (1 oz.) copper area of approximately 1 in2.
Note 12: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in2 of (1 oz.) copper area.
Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in2 of (1 oz.) copper area.
Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in2 of (1 oz.) copper area on
the LM2591HVS side of the board, and approximately 16 in2 of copper on the other side of the p-c board. See application hints in this data sheet and the thermal
model in Switchers Made Simple available at http://power.national.com.
Typical Performance Characteristics
(Circuit of Figure 1)
Normalized
Output Voltage
Line Regulation
10129302
10129303
Switch Saturation
Voltage
Efficiency
10129305
10129304
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Switch Current Limit
LM2591HV
Typical Performance Characteristics (Circuit of Figure 1)
(Continued)
Dropout Voltage
10129306
10129307
Operating
Quiescent Current
Shutdown
Quiescent Current
10129308
10129309
Minimum Operating
Supply Voltage
Feedback Pin
Bias Current
10129311
10129310
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LM2591HV
Typical Performance Characteristics (Circuit of Figure 1)
Switching Frequency
(Continued)
ON/OFF Threshold Voltage
10129313
10129379
ON/OFF Pin Current (Sinking)
Internal Gain-Phase Characteristics
10129380
10129378
Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 250 mA
L = 15 µH, COUT = 150 µF, COUT ESR = 90 mΩ
Continuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 1A
L = 52 µH, COUT = 100 µF, COUT ESR = 100 mΩ
10129320
10129319
A: Output Pin Voltage, 10V/div.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
B: Inductor Current 0.25A/div.
C: Output Ripple Voltage, 50 mV/div.
C: Output Ripple Voltage, 100 mV/div.
Horizontal Time Base: 2 µs/div.
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Horizontal Time Base: 2 µs/div.
6
(Continued)
Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 250 mA to 1A
L = 15 µH, COUT = 150 µF, COUT ESR = 90 mΩ
Load Transient Response for Continuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 250 mA to 1A
L = 52 µH, COUT = 100 µF, COUT ESR = 100 mΩ
10129322
A: Output Voltage, 100 mV/div. (AC)
10129321
B: 250 mA to 1A Load Pulse
A: Output Voltage, 100 mV/div. (AC)
Horizontal Time Base: 200 µs/div.
B: 250 mA to 1A Load Pulse
Horizontal Time Base: 50 µs/div.
Connection Diagrams and Order Information
Bent and Staggered Leads, Through Hole Package
5-Lead TO-220 (T)
Surface Mount Package
5-Lead TO-263 (S)
10129382
10129381
Order Number LM2591HVS-3.3, LM2591HVS-5.0,
or LM2591HVS-ADJ
See NS Package Number TS5B
Order Number LM2591HVT-3.3, LM2591HVT-5.0,
or LM2591HVT-ADJ
See NS Package Number T05D
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LM2591HV
Typical Performance Characteristics (Circuit of Figure 1)
LM2591HV
Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
10129324
Component Values shown are for VIN = 15V,
VOUT = 5V, ILOAD = 1A.
— 470 µF, 50V, Aluminum Electrolytic Nichicon “PM Series”
CIN
—
COUT
220 µF, 25V Aluminum Electrolytic, Nichicon “PM Series”
D1
—
2A, 60V Schottky Rectifier, 21DQ06 (International Rectifier)
L1
—
68 µH, See Inductor Selection Procedure
Adjustable Output Voltage Versions
10129325
Select R1 to be approximately 1 kΩ, use a 1% resistor for best stability.
Component Values shown are for VIN = 20V,
VOUT = 10V, ILOAD = 1A.
CIN: — 470 µF, 35V, Aluminum Electrolytic Nichicon “PM Series”
COUT:
— 220 µF, 35V Aluminum Electrolytic, Nichicon “PM Series”
D1 — 2A, 60V Schottky Rectifier, 21DQ06 (International Rectifier)
L1 — 100 µH, See Inductor Selection Procedure
R1 — 1 kΩ, 1%
R2 — 7.15k, 1%
CFF — 3.3 nF
Typical Values
CSS — 0.1 µF
CDELAY — 0.1 µF
RPULL UP — 4.7k (use 22k if VOUT is ≥ 45V)
† Small signal Schottky diode to prevent damage to feedback pin by negative spike when output is shorted (CFF not being able to discharge immediately will
drag feedback pin below ground). Required if VIN > 40V
FIGURE 1. Standard Test Circuits and Layout Guides
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LM2591HV
Block Diagram
10129383
on this pin whenever the output is shorted. This happens
because the feedforward capacitor cannot discharge fast
enough, and since one end of it is dragged to Ground, the
other end goes momentarily negative. To prevent the energy
rating of this pin from being exceeded, a small-signal Schottky diode to Ground is recommended for DC input voltages
above 40V whenever a feedforward capacitor is present
(See Figure 1). Feedforward capacitor values larger than 0.1
µF are not recommended for the same reason, whatever be
the DC input voltage.
ON /OFF (Pin 5) — The regulator is in shutdown mode,
drawing about 90 µA, when this pin is driven to a high level
(≥ 2.0V), and is in normal operation when this Pin is left
floating or driven to a low level (≤ 0.6V). The typical value of
the threshold is 1.3V and the voltage on this pin must not
exceed 25V.
Pin Functions
+VIN (Pin 1) — This 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.
Output (Pin 2) — Internal switch. The voltage at this pin
switches between approximately (+VIN − VSAT) and approximately −0.5V, with a duty cycle of VOUT/VIN.
Ground (Pin 3) — Circuit ground.
Feedback (Pin 4) — Senses the regulated output voltage to
complete the feedback loop. This pin is directly connected to
the Output for the fixed voltage versions, but is set to 1.23V
by means of a resistive divider from the output for the
Adjustable version. If a feedforward capacitor is used (Adjustable version), then a negative voltage spike is generated
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LM2591HV
Inductor Value Selection Guides
(For Continuous Mode Operation)
10129365
FIGURE 2. LM2591HV-3.3
10129366
FIGURE 3. LM2591HV-5.0
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LM2591HV
Inductor Value Selection Guides (For Continuous Mode Operation)
(Continued)
10129367
FIGURE 4. LM2591HV-ADJ
10129368
FIGURE 5. Current Ripple Ratio
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LM2591HV
Inductor Value Selection Guides (For Continuous Mode Operation)
Coilcraft Inc.
(Continued)
Phone
(USA): 1-800-322-2645
Web Address
http://www.coilcraft.com
Phone
(UK): 1-236-730595
Web Address
http://www.coilcraft-europe.com
Phone
(USA): 1-858-674-8100
Web Address
http://www.pulseeng.com
Pulse Engineering Inc.,
Phone
(UK): 1-483-401700
Europe
Web Address
http://www.pulseeng.com
Renco Electronics Inc.
Phone
(USA): 1-321-637-1000
Web Address
http://www.rencousa.com
Phone
(USA): 1-952-475-1173
Web Address
http://www.shottcorp.com
Cooper Electronic Tech.
(Coiltronics)
Phone
(USA): 1-888-414-2645
Web Address
http://www.cooperet.com
TDK
Phone
(USA): 1-847-803-6100
Web Address
http://www.componet.tdk.com
Coilcraft Inc., Europe
Pulse Engineering Inc.
Schott Corp.
FIGURE 6. Contact Information for Suggested Inductor Manufacturers
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INDUCTOR SELECTION PROCEDURE
Application Note AN-1197 titled "Selecting Inductors for Buck
Converters" provides detailed information on this topic. For a
quick-start the designer may refer to the nomographs provided in Figure 2 to Figure 4. To widen the choice of the
Designer to a more general selection of available inductors,
the nomographs provide the required inductance and also
the energy in the core expressed in microjoules (µJ), as an
alternative to just prescribing custom parts. The following
points need to be highlighted:
1. The Energy values shown on the nomographs apply to
steady operation at the corresponding x-coordinate
(rated maximum load current). However under start-up,
without soft-start, or a short-circuit on the output, the
current in the inductor will momentarily/repetitively hit
the current limit ICLIM of the device, and this current
could be much higher than the rated load, ILOAD. This
represents an overload situation, and can cause the
Inductor to saturate (if it has been designed only to
handle the energy of steady operation). However most
types of core structures used for such applications have
a large inherent air gap (for example powdered iron
types or ferrite rod inductors), and so the inductance
does not fall off too sharply under an overload. The
device is usually able to protect itself by not allowing the
current to ever exceed ICLIM. But if the DC input voltage
to the regulator is over 40V, the current can slew up so
fast under core saturation, that the device may not be
able to act fast enough to restrict the current. The current can then rise without limit till destruction of the
device takes place. Therefore to ensure reliability, it is
recommended, that if the DC Input Voltage exceeds
40V, the inductor must ALWAYS be sized to handle an
instantaneous current equal to ICLIM without saturating,
irrespective of the type of core structure/material.
2.
consider the rather wide tolerance on the nominal inductance of commercial inductors.
5. Figure 4 shows the inductor selection curves for the
Adjustable version. The y-axis is ’Et’, in Vµsecs. It is the
applied volts across the inductor during the ON time of
the switch (VIN-VSAT-VOUT) multiplied by the time for
which the switch is on in µsecs. See Example 3 below.
Example 1: (VIN ≤ 40V) LM2591HV-5.0, VIN = 24V, Output
5V @ 0.8A
1. A first pass inductor selection is based upon Inductance
and rated max load current. We choose an inductor with the
Inductance value indicated by the nomograph (Figure 3) and
a current rating equal to the maximum load current. We
therefore quick-select a 100µH/0.8A inductor (designed for
150 kHz operation) for this application.
2. We should confirm that it is rated to handle 50 µJ (see
Figure 3) by either estimating the peak current or by a
detailed calculation as shown in AN-1197, and also that the
losses are acceptable.
Example 2: (VIN > 40V) LM2591HV-5.0, VIN = 48V, Output
5V @ 1A
1. A first pass inductor selection is based upon Inductance
and the switch currrent limit. We choose an inductor with the
Inductance value indicated by the nomograph (Figure 3) and
a current rating equal to ICLIM. We therefore quick-select a
100µH/3A inductor (designed for 150 kHz operation) for this
application.
2. We should confirm that it is rated to handle eCLIM by the
procedure shown in AN-1197 and that the losses are acceptable. Here eCLIM is:
The Energy under steady operation is
Example 3: (VIN ≤ 40V) LM2591HV-ADJ, VIN = 20V, Output
10V @ 1A
1. Since input voltage is less than 40V, a first pass inductor
selection is based upon Inductance and rated max load
current. We choose an inductor with the Inductance value
indicated by the nomograph Figure 4 and a current rating
equal to the maximum load. But we first need to calculate Et
for the given application. The Duty cycle is
where L is in µH and IPEAK is the peak of the inductor
current waveform with the regulator delivering ILOAD.
These are the energy values shown in the nomographs.
See Example 1 below.
3.
The Energy under overload is
If VIN > 40V, the inductor should be sized to handle
eCLIM instead of the steady energy values. The worst
case ICLIM for the LM2591HV is 3A. The Energy rating
depends on the Inductance. See Example 2 below.
4.
where VD is the drop across the Catch Diode () 0.5V for a
Schottky) and VSAT the drop across the switch ()1.5V). So
The nomographs were generated by allowing a greater
amount of percentage current ripple in the Inductor as
the maximum rated load decreases (see Figure 5). This
was done to permit the use of smaller inductors at light
loads. Figure 5 however shows only the ’median’ value
of the current ripple. In reality there may be a great
spread around this because the nomographs approximate the exact calculated inductance to standard available values. It is a good idea to refer to AN-1197 for
detailed calculations if a certain maximum inductor current ripple is required for various possible reasons. Also
And the switch ON time is
where f is the switching frequency in Hz. So
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LM2591HV
Application Information
LM2591HV
Application Information
OUTPUT CAPACITOR
COUT — An output capacitor is required to filter the output
and provide regulator loop stability. Low impedance or low
ESR Electrolytic or solid tantalum capacitors designed for
switching regulator applications must be used. When selecting an output capacitor, the important capacitor parameters
are; the 100 kHz Equivalent Series Resistance (ESR), the
RMS ripple current rating, voltage rating, and capacitance
value. For the output capacitor, the ESR value is the most
important parameter. The ESR should generally not be less
than 100 mΩ or there will be loop instability. If the ESR is too
large, efficiency and output voltage ripple are effected. So
ESR must be chosen carefully.
(Continued)
Therefore, looking at Figure 2 we quick-select a 100µH/1A
inductor (designed for 150 kHz operation) for this application.
2. We should confirm that it is rated to handle 100 µJ (see
Figure 4) by the procedure shown in AN-1197 and that the
losses are acceptable. (If the DC Input voltage had been
greater than 40V we would need to consider eCLIM as in
Example 2 above).
Note that we have taken VSAT as 1.5V which includes an
estimated resistive drop across the inductor.
CATCH DIODE
Buck regulators require a diode to provide a return path for
the inductor current when the switch turns off. This must be
a fast diode and must be located close to the LM2591HV
using short leads and short printed circuit traces.
This completes the simplified inductor selection procedure.
For more general applications and better optimization, the
designer should refer to AN-1197. Figure 6 provides helpful
contact information on suggested Inductor manufacturers
who may be able to recommend suitable parts, if the requirements are known.
Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance,
especially in low output voltage applications (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
good choice, but some types with an abrupt turnoff characteristic may cause instability or EMI problems. Ultra-fast
recovery diodes typically have reverse recovery times of 50
ns or less. The diode must be chosen for its average/RMS
current rating and maximum voltage rating. The voltage
rating of the diode must be greater than the DC input voltage
(not the output voltage).
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
CFF - A Feedforward Capacitor CFF, shown across R2 in
Figure 1 is used when the output voltage is greater than 10V
or when COUT has a very low ESR. This capacitor adds lead
compensation to the feedback loop and increases the phase
margin for better loop stability.
If the output voltage ripple is large ( > 5% of the nominal
output voltage), this ripple can be coupled to the feedback
pin through the feedforward capacitor and cause the error
comparator to trigger the error flag. In this situation, adding a
resistor, RFF, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the ripple voltage at
the feedback pin.
DELAYED STARTUP
The circuit in Figure 7 uses the the ON /OFF pin to provide
a time delay between the time the input voltage is applied
and the time the output voltage comes up (only the circuitry
pertaining to the delayed start up is shown). As the input
voltage rises, the charging of capacitor C1 pulls the ON /OFF
pin high, keeping the regulator off. Once the input voltage
reaches its final value and the capacitor stops charging, and
resistor R2 pulls the ON /OFF pin low, thus allowing the
circuit to start switching. Resistor R1 is included to limit the
maximum voltage applied to the ON /OFF pin (maximum of
25V), reduces power supply noise sensitivity, and also limits
the capacitor, C1, discharge current. When high input ripple
voltage exists, avoid long delay time, because this ripple can
be coupled into the ON /OFF pin and cause problems.
This delayed startup feature is useful in situations where the
input power source is limited in the amount of current it can
deliver. It allows the input voltage to rise to a higher voltage
before the regulator starts operating. Buck regulators require
less input current at higher input voltages.
INPUT CAPACITOR
CIN — A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground pin. It must be
located near the regulator using short leads. This capacitor
prevents large voltage transients from appearing at the input, and provides the instantaneous current needed each
time the switch turns on.
The important parameters for the Input capacitor are the
voltage rating and the RMS current rating. Because of the
relatively high RMS currents flowing in a buck regulator’s
input capacitor, this capacitor should be chosen for its RMS
current rating rather than its capacitance or voltage ratings,
although the capacitance value and voltage rating are directly related to the RMS current rating. The voltage rating of
the capacitor and its RMS ripple current capability must
never be exceeded.
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LM2591HV
Application Information
(Continued)
10129336
FIGURE 7. Delayed Startup
UNDERVOLTAGE LOCKOUT
needed, the circuit in Figure 10 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. If zener voltages greater than 25V are used, an additional 47 kΩ resistor is needed from the ON /OFF pin to the
ground pin to stay within the 25V maximum limit of the ON
/OFF pin.
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. An undervoltage lockout feature applied to a buck regulator is
shown in Figure 8, while Figure 9 and Figure 10 applies the
same feature to an inverting circuit. The circuit in Figure 9
features a constant threshold voltage for turn on and turn off
(zener voltage plus approximately one volt). If hysteresis is
10129337
FIGURE 8. Undervoltage Lockout for Buck Regulator
15
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LM2591HV
Application Information
(Continued)
10129384
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 9. Undervoltage Lockout for Inverting Regulator
ances, we should take the min value of Inductance for L in
the equation above (typically 20% less than the nominal
value). Further, the above equation disregards the drop
across the Switch and the diode. This is equivalent to assuming 100% efficiency, which is never so. Therefore expect
IPEAK to be an additional 10-20% higher than calculated from
the above equation.
lNVERTING REGULATOR
The circuit in Figure 11 converts a positive input voltage to a
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulator’s ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regulates it.
This example uses the LM2591HV-5.0 to generate a −5V
output, but other output voltages are possible by selecting
other output voltage versions, including the adjustable version. Since 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.
To determine how much load current is possible before the
internal device current limit is reached (and power limiting
occurs), the system must be evaluated as a buck-boost
configuration rather than as a buck. The peak switch current
in Amperes, for such a configuration is given as:
The reader is also referred to Application Note AN-1157 for
examples based on positive to negative configuration.
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 60V. For example, when converting +20V to −12V, the regulator would see 32V between the
input pin and ground pin. The LM2591HV has a maximum
input voltage spec of 60V.
Additional diodes are 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 closley resemble a buck configuration thus providing good closed loop stability. A Schottky diode is recommended for low input voltages, (because of its lower voltage
drop) but for higher input voltages, a fast recovery diode
could be used.
Without diode D3, when the input voltage is first applied, the
charging current of CIN can pull the output positive by several volts for a short period of time. Adding D3 prevents the
output from going positive by more than a diode voltage.
where L is in µH and f is in Hz. The maximum possible load
current ILOAD is limited by the requirement that IPEAK ≤ ICLIM.
While checking for this, take ICLIM to be the lowest possible
current limit value (min across tolerance and temperature is
1.2A for the LM2591HV). Also to account for inductor toler-
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16
LM2591HV
Application Information
(Continued)
10129339
This circuit has hysteresis
Regulator starts switching at VIN = 13V
Regulator stops switching at VIN = 8V
FIGURE 10. Undervoltage Lockout with Hysteresis for Inverting Regulator
10129340
CIN
470
COUT
220
— 68 µF/25V Tant. Sprague 595D
µF/50V Elec. Panasonic HFQ
— 47 µF/20V Tant. Sprague 595D
µF/25V Elec. Panasonic HFQ
FIGURE 11. Inverting −5V Regulator with Delayed Startup
11 is recommended. By delaying the regulator startup, the
input capacitor is allowed to charge up to a higher voltage
before the switcher begins operating. A portion of the high
input current needed for startup is now supplied by the input
capacitor (CIN). For severe start up conditions, the input
capacitor can be made much larger than normal.
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,
3A inductor is the best choice. Capacitor selection can also
be narrowed down to just a few values.
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 LM2591HV current limit
(approx 4A) are needed for at least 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 delayed startup feature (C1, R1 and R2) shown in Figure
lNVERTING REGULATOR SHUTDOWN METHODS
To use the ON /OFF pin in a standard buck configuration is
simple, pull it below 1.3V (@25˚C, referenced to ground) to
turn regulator ON, pull it above 1.3V to shut the regulator
OFF. With the inverting configuration, some level shifting is
required, because the ground pin of the regulator is no
longer at ground, but is now setting at the negative output
voltage level. Two different shutdown methods for inverting
regulators are shown in Figure 12 and Figure 13
17
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LM2591HV
Application Information
(Continued)
10129342
FIGURE 12. Inverting Regulator Ground Referenced Shutdown
10129386
FIGURE 13. Inverting Regulator Ground Referenced Shutdown using Opto Device
LAYOUT SUGGESTIONS
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance can
generate voltage transients which can cause problems. For
minimal inductance and ground loops, with reference to
Figure 1, the wires indicated by heavy lines should be wide
printed circuit traces and should be kept as short as
possible. For best results, external components should be
located as close to the switcher lC as possible using ground
plane construction or single point grounding.
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If open core inductors are used, special care must be
taken as to the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback,
lC groundpath and COUT wiring can cause problems.
When using the adjustable version, special care must be
taken as to the location of the feedback resistors and the
associated wiring. Physically locate both resistors near the
IC, and route the wiring away from the inductor, especially an
open core type of inductor.
18
LM2591HV
Physical Dimensions
inches (millimeters)
unless otherwise noted
5-Lead TO-220 Bent and Staggered Package
Order Number LM2591HVT-3.3, LM2591HVT-5.0
or LM2591HVT-ADJ
NS Package Number T05D
19
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LM2591HV SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
5-Lead TO-263 Bent and Formed Package
Order Number LM2591HVS-3.3, LM2591HVS-5.0 or LM2591HVS-ADJ
NS Package Number TS5B
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