Datasheet LM2592HV3.3MDC, LM2592HVSX-3.3, LM2592HVS-ADJ, LM2592HVS-3.3, LM2592HVADJMWC Datasheet (NSC)
...Page 1
LM2592HV
SIMPLE SWITCHER
®
Power Converter 150 kHz 2A
Step-Down Voltage Regulator
General Description
The LM2592HV series of regulators are monolithic integrated circuits that provide all the active functions for a
step-down (buck) switching regulator, capable of driving a
2A 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
LM2593HV, but without some of the supervisory and performance features of the latter.
Requiring a minimum numberofexternalcomponents, these
regulators are simple to use and include internal frequency
compensation
†
, improved line and load specifications and a
fixed-frequency oscillator.
The LM2592HV 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
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 2A 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, I
Q
typically 90 µA
n High Efficiency
n Thermal shutdown and current limit protection
Applications
n Simple high-efficiency step-down (buck) regulator
n Efficient pre-regulator for linear regulators
n On-card switching regulators
n Positive to Negative converter
Note:†Patent Number 5,382,918.
Typical Application (Fixed Output Voltage Versions)
10129401
SIMPLE SWITCHER®and
Switchers Made Simple
®
are registered trademarks of National Semiconductor Corporation.
August 2001
LM2592HV SIMPLE SWITCHER Power Converter 150 kHz 2A Step-Down Voltage Regulator
© 2001 National Semiconductor Corporation DS101294 www.national.com
Page 2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Supply Voltage (V
IN
) 63V
ON/OFF Pin Voltage
−0.3 ≤ V ≤ +25V
Feedback Pin Voltage −0.3 ≤ V ≤ +25V
Output Voltage to Ground
(Steady State) −1V
Power Dissipation Internally limited
Storage Temperature Range −65˚C to +150˚C
ESD Susceptibility
Human Body Model (Note 2) 2 kV
Lead Temperature
S Package
Vapor Phase (60 sec.) +215˚C
Infrared (10 sec.) +245˚C
T Package (Soldering, 10 sec.) +260˚C
Maximum Junction Temperature +150˚C
Operating Conditions
Temperature Range −40˚C ≤ TJ≤ +125˚C
Supply Voltage 4.5V to 60V
LM2592HV-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 LM2592HV-3.3 Units
(Limits)
Typ Limit
(Note 3) (Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit
Figure 1
V
OUT
Output Voltage 4.75V ≤ VIN≤ 60V, 0.2A ≤ I
LOAD
≤ 2A 3.3 V
3.168/3.135 V(min)
3.432/3.465 V(max)
η Efficiency V
IN
= 12V, I
LOAD
=2A 76
LM2592HV-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 LM2592HV-5.0 Units
(Limits)
Typ Limit
(Note 3) (Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit
Figure 1
V
OUT
Output Voltage 7V ≤ VIN≤ 60V, 0.2A ≤ I
LOAD
≤ 2A 5 V
4.800/4.750 V(min)
5.200/5.250 V(max)
η Efficiency V
IN
= 12V, I
LOAD
=2A 81 %
LM2592HV-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 LM2592HV-ADJ Units
(Limits)
Typ Limit
(Note 3) (Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit
Figure 1
V
FB
Feedback Voltage 4.5V ≤ VIN≤ 60V, 0.2A ≤ I
LOAD
≤ 2A 1.230 V
V
OUT
programmed for 3V. Circuit of
Figure 1
. 1.193/1.180 V(min)
1.267/1.280 V(max)
η Efficiency V
IN
= 12V, V
OUT
= 3V, I
LOAD
=2A 75 %
LM2592HV
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All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for TJ= 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, V
IN
= 12V for the 3.3V, 5V, and Adjustable version. I
LOAD
= 500 mA
Symbol Parameter Conditions LM2592HV-XX Units
(Limits)
Typ Limit
(Note 3) (Note 4)
DEVICE PARAMETERS
I
b
Feedback Bias Current Adjustable Version Only, VFB= 1.3V 10 nA
50/100 nA (max)
f
O
Oscillator Frequency (Note 6) 150 kHz
127/110 kHz(min)
173/173 kHz(max)
V
SAT
Saturation Voltage I
OUT
= 2A (Note 7) (Note 8) 1.10 V
1.3/1.4 V(max)
DC Max Duty Cycle (ON) (Note 8) 100 %
Min Duty Cycle (OFF) (Note 9) 0
I
CLIM
Switch current Limit Peak Current, (Note 7) (Note 8) 3.0 A
2.4/2.3 A(min)
3.7/4.0 A(max)
I
L
Output Leakage Current (Note 7) (Note 9) (Note 10) Output = 0V 50 µA(max)
Output = −1V 5 mA
30 mA(max)
I
Q
Operating Quiescent SD /SS Pin Open (Note 9) 5mA
Current 10 mA(max)
I
STBY
Standby Quiescent SD /SS pin = 0V (Note 10) 90 µA
Current 200/250 µA(max)
θ
JC
Thermal Resistance TO220 or TO263 Package, Junction to Case 2 ˚C/W
θ
JA
TO220 Package, Juncton to Ambient (Note 11) 50 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 12) 50 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 13) 30 ˚C/W
θ
JA
TO263 Package, Juncton to Ambient (Note 14) 20 ˚C/W
ON/OFF CONTROL Test Circuit
Figure 1
ON /OFF Pin Logic Input 1.3 V
V
IH
Threshold Voltage Low (Regulator ON) 0.6 V(max)
V
IL
High (Regulator OFF) 2.0 V(min)
I
H
ON /OFF Pin Input Current V
LOGIC
= 2.5V (Regulator OFF) 5 µA
15 µA(max)
I
L
V
LOGIC
= 0.5V (Regulator ON) 0.02 µA
5 µA(max)
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
LM2592HV 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: V
IN
= 60V.
LM2592HV
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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 in
2
.
Note 12: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in
2
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 in
2
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 in
2
of (1 oz.) copper area on
the LM2592HVS side of the board, and approximately 16 in
2
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.
LM2592HV
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Typical Performance Characteristics (Circuit of
Figure 1
)
Normalized
Output Voltage Line Regulation Efficiency
10129402
10129403 10129404
Switch Saturation
Voltage Switch Current Limit Dropout Voltage
10129405
10129406
10129407
Operating
Quiescent Current
Shutdown
Quiescent Current
Minimum Operating
Supply Voltage
10129408 10129409
10129410
LM2592HV
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Typical Performance Characteristics (Circuit of
Figure 1
) (Continued)
Feedback Pin
Bias Current Switching Frequency ON/OFF Threshold Voltage
10129411 10129413
10129479
ON/OFF Pin Current (Sinking) Internal Gain-Phase Characteristics
10129480
10129478
LM2592HV
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Typical Performance Characteristics (Circuit of
Figure 1
) (Continued)
Continuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
=2A
L = 32 µH, C
OUT
= 220 µF, C
OUT
ESR=50mΩ
Discontinuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA
L = 10 µH, C
OUT
= 330 µF, C
OUT
ESR=45mΩ
10129420
Horizontal Time Base: 2 µs/div.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 1A/div.
C: Output Ripple Voltage, 50 mV/div.
10129419
Horizontal Time Base: 2 µs/div.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 100 mV/div.
Load Transient Response for Continuous Mode
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA to 2A
L = 32 µH, C
OUT
= 220 µF, C
OUT
ESR=50mΩ
Load Transient Response for Discontinuous Mode
V
IN
= 20V, V
OUT
= 5V, I
LOAD
= 500 mA to 2A
L = 10 µH, C
OUT
= 330 µF, C
OUT
ESR=45mΩ
10129421
Horizontal Time Base: 50 µs/div.
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
10129422
Horizontal Time Base: 200 µs/div.
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
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)
10129481
Order Number LM2592HVT-3.3, LM2592HVT-5.0,
or LM2592HVT-ADJ
See NS Package Number T05D
10129482
Order Number LM2592HVS-3.3, LM2592HVS-5.0,
or LM2592HVS-ADJ
See NS Package Number TS5B
LM2592HV
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Test Circuit and Layout Guidelines
Fixed Output Voltage Versions
10129424
Component Values shown are for VIN= 15V,
V
OUT
= 5V, I
LOAD
= 2A.
C
IN
— 470 µF, 50V, Aluminum Electrolytic Nichicon “PM Series”
C
OUT
— 220 µF, 25V Aluminum Electrolytic, Nichicon “PM Series”
D1 — 3.3A, 60V Schottky Rectifier, 31DQ06 (International Rectifier)
L1 — 33 µH, See Inductor Selection Procedure
Adjustable Output Voltage Versions
10129425
Select R1to be approximately 1 kΩ, use a 1% resistor for best stability.
Component Values shown are for V
IN
= 20V,
V
OUT
= 10V, I
LOAD
= 2A.
C
IN
: — 470 µF, 35V, Aluminum Electrolytic Nichicon “PM Series”
C
OUT
: — 220 µF, 35V Aluminum Electrolytic, Nichicon “PM Series”
D1 — 3.3A, 60V Schottky Rectifier, 31DQ06 (International Rectifier)
L1 — 47 µH, See Inductor Selection Procedure
R
1
—1kΩ,1%
R
2
— 7.15k, 1%
C
FF
— 3.3 nF
Typical Values
CSS—0.1 µF
C
DELAY
—0.1 µF
R
PULL UP
— 4.7k (use 22k if V
OUT
is ≥ 45V)
†
Small signal Schottky diode to prevent damage to feedback pin by negative spike when output is shorted (CFFnot being able to discharge immediately will
drag feedback pin below ground). Required if V
IN
>
40V
FIGURE 1. Standard Test Circuits and Layout Guides
LM2592HV
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Page 9
Block Diagram
10129483
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 (+V
IN−VSAT
) and approxi-
mately −0.5V, with a duty cycle of V
OUT/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
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.
LM2592HV
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INDUCTOR VALUE SELECTION GUIDES
(For Continuous Mode Operation)
10129465
FIGURE 2. LM2592HV-3.3
10129466
FIGURE 3. LM2592HV-5.0
LM2592HV
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INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation) (Continued)
10129467
FIGURE 4. LM2592HV-ADJ
10129468
FIGURE 5. Current Ripple Ratio
LM2592HV
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Page 12
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation) (Continued)
Coilcraft Inc. Phone (USA): 1-800-322-2645
Web Address http://www.coilcraft.com
Coilcraft Inc., Europe Phone (UK): 1-236-730595
Web Address http://www.coilcraft-europe.com
Pulse Engineering Inc. 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
Schott Corp. 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
FIGURE 6. Contact Information for Suggested Inductor Manufacturers
LM2592HV
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Page 13
Application Information
INDUCTOR SELECTION PROCEDURE
Application NoteAN-1197titled ’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 2toFigure 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 I
CLIM
of the device, and this current
could be much higher than the rated load, I
LOAD
. 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 I
CLIM
. 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 I
CLIM
without saturating,
irrespective of the type of core structure/material
.
2. The Energy under steady operation is
where L is in µH and I
PEAK
is the peak of the inductor current
waveform with the regulator delivering I
LOAD
. These are the
energy values shown in the nomographs. See
Example 1
below.
3. The Energy under overload is
If V
IN
>
40V, the inductor should be sized to handle e
CLIM
instead of the steady energy values. The worst case I
CLIM
for
the LM2592HV is 4A. The Energy rating depends on the
Inductance. See
Example 2
below.
4. 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
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 (V
IN-VSAT-VOUT
) multiplied by the time for
which the switch is on in µsecs. See Example 3 below.
Example 1: (V
IN
≤ 40V) LM2592HV-5.0, VIN= 24V, Output
5V
@
1A
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 68µH/1A 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: (V
IN
>
40V) LM2592HV-5.0, VIN= 48V, Output
5V
@
1.5A
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 I
CLIM
. We therefore quick-select a
68µH/4A inductor (designed for 150 kHz operation) for this
application.
2. We should confirm that it is rated to handle e
CLIM
by the
procedure shown in AN-1197and that the losses are acceptable. Here e
CLIM
is:
Example 3: (VIN≤ 40V) LM2592HV-ADJ, VIN= 20V, Output
10V
@
2A
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 VDis the drop across the Catch Diode () 0.5V for a
Schottky) and V
SAT
the drop across the switch ()1.5V). So
And the switch ON time is
where f is the switching frequency in Hz. So
LM2592HV
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Page 14
Application Information (Continued)
Therefore, looking at
Figure 2
we quick-select a 47µH/2A
inductor (designed for 150 kHz operation) for this application.
2. We should confirm that it is rated to handle 200 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 e
CLIM
as in
Example 2 above).
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.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
C
FF
- A Feedforward Capacitor CFF, shown across R2 in
Figure 1
is used when the output voltage is greater than 10V
or when C
OUT
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, R
FF
, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the ripple voltage at
the feedback pin.
INPUT CAPACITOR
C
IN
—Alow 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.
OUTPUT CAPACITOR
C
OUT
—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.
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 LM2592HV
using short leads and short printed circuit traces.
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).
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 R
2
pulls the ON /OFF pin low, thus allowing the
circuit to start switching. Resistor R1is 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.
LM2592HV
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Page 15
Application Information (Continued)
UNDERVOLTAGE LOCKOUT
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
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.
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 LM2592HV-5.0 to generate a −5V
output, but other output voltages are possible by selecting
other output voltage versions, including the adjustable ver-
10129436
FIGURE 7. Delayed Startup
10129437
FIGURE 8. Undervoltage Lockout for Buck Regulator
10129484
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 9. Undervoltage Lockout for Inverting Regulator
LM2592HV
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Page 16
Application Information (Continued)
sion. 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:
where L is in µH and f is in Hz. The maximum possible load
current I
LOAD
is limited by the requirement that I
PEAK
≤ I
CLIM
.
While checking for this, take I
CLIM
to be the lowest possible
current limit value (min across tolerance and temperature is
2.3A for the LM2592HV). Also to account for inductor tolerances, 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 as-
suming 100% efficiency, which is never so. Therefore expect
I
PEAK
to be an additional 10-20% higher than calculated from
the above equation.
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 LM2592HV 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 C
IN
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 C
IN
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.
10129439
This circuit has hysteresis
Regulator starts switching at V
IN
= 13V
Regulator stops switching at V
IN
=8V
FIGURE 10. Undervoltage Lockout with Hysteresis for Inverting Regulator
10129440
CIN—68 µF/25V Tant. Sprague 595D
470 µF/50V Elec. Panasonic HFQ
C
OUT
—47 µF/20V Tant. Sprague 595D
220 µF/25V Elec. Panasonic HFQ
FIGURE 11. Inverting −5V Regulator with Delayed Startup
LM2592HV
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Page 17
Application Information (Continued)
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,
4A 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 LM2592HV 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, R
1
and R2) shown in
Figure
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 (C
IN
). For severe start up conditions, the input
capacitor can be made much larger than normal.
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
THERMAL CONSIDERATIONS
The LM2592HV is available in two packages, a 5-pin TO-220
(T) and a 5-pin surface mount TO-263 (S).
The TO-220 package needs a heat sink under most conditions. The size of the heatsink depends on the input voltage,
the output voltage, the load current and the ambient temperature. Higher ambient temperatures require more heat
sinking.
The TO-263 surface mount package tab is designed to be
soldered to the copper on a printed circuit board. The copper
and the board are the heat sink for this package and the
other heat producing components, such as the catch diode
and inductor. The PC board copper area that the package is
soldered to should be at least 0.4 in
2
, and ideally should
have 2 or more square inches of 2 oz. (0.0028) in) copper.
Additional copper area improves the thermal characteristics,
but with copper areas greater than approximately 6 in
2
, only
small improvements in heat dissipation are realized. If further thermal improvements are needed, double sided, multilayer PC board with large copper areas and/or airflow are
recommended.
The curves shown in
Figure 14
show the LM2592HVS
(TO-263 package) junction temperature rise above ambient
temperature with a 2A load for various input and output
voltages. This data was taken with the circuit operating as a
buck switching regulator with all components mounted on a
10129442
FIGURE 12. Inverting Regulator Ground Referenced Shutdown
10129486
FIGURE 13. Inverting Regulator Ground Referenced Shutdown using Opto Device
LM2592HV
www.national.com17
Page 18
Application Information (Continued)
PC board to simulate the junction temperature under actual
operating conditions. This curve can be used for a quick
check for the approximate junction temperature for various
conditions, but be aware that there are many factors that can
affect the junction temperature. When load currents higher
than 2A are used, double sided or multilayer PC boards with
large copper areas and/or airflow might be needed, especially for high ambient temperatures and high output voltages.
For the best thermal performance, wide copper traces and
generous amounts of printed circuit board copper should be
used in the board layout. (One exception to this is the output
(switch) pin, which should not have large areas of copper.)
Large areas of copper provide the best transfer of heat
(lower thermal resistance) to the surrounding air, and moving
air lowers the thermal resistance even further.
Package thermal resistance and junction temperature rise
numbers are all approximate, and there are many factors
that will affect these numbers. Some of these factors include
board size, shape, thickness, position, location, and even
board temperature. Other factors are, trace width, total
printed circuit copper area, copper thickness, single- or
double-sided, multilayer board and the amount of solder on
the board. The effectiveness of the PC board to dissipate
heat also depends on the size, quantity and spacing of other
components on the board, as well as whether the surrounding air is still or moving. Furthermore, some of these components such as the catch diode will add heat to the PC
board and the heat can vary as the input voltage changes.
For the inductor, depending on the physical size, type of core
material and the DC resistance, it could either act as a heat
sink taking heat away from the board, or it could add heat to
the board.
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.
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 C
OUT
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.
10129438
FIGURE 14. Junction Temperature Rise, TO-263
LM2592HV
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Page 19
Physical Dimensions inches (millimeters)
unless otherwise noted
5-Lead TO-220 Bent and Staggered Package
Order Number LM2592HVT-3.3, LM2592HVT-5.0
or LM2592HVT-ADJ
NS Package Number T05D
LM2592HV
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Page 20
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
5-Lead TO-263 Bent and Formed Package
Order Number LM2592HVS-3.3, LM2592HVS-5.0 or LM2592HVS-ADJ
NS Package Number TS5B
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LM2592HV SIMPLE SWITCHER Power Converter 150 kHz 2A Step-Down Voltage Regulator
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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