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LM2590HV
SIMPLE SWITCHER
®
Power Converter 150 kHz 1A
Step-Down Voltage Regulator, with Features
December 2001
LM2590HV SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator, with
Features
General Description
The LM2590HV 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
LM2591HV withadditionalsupervisory and performance features.
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation
fixed-frequency oscillator, Shutdown/Soft-start, output error
flag and flag delay.
The LM2590HV 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-lead TO-220 package with several
different lead bend options, and a 7-lead TO-263 Surface
mount package.
Other features include a guaranteed
put voltage under all conditions of input voltage and output
load conditions, and
ternal 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.
†
, improved line and load specifications,
±
4% tolerance on out-
±
15% on the oscillator frequency. Ex-
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 7-pin TO-220 and TO-263 (surface mount)
Package
n Input voltage range up to 60V
n 150 kHz fixed frequency internal oscillator
n Shutdown/Soft-start
n Out of regulation error flag
n Error flag delay
n Low power standby mode, I
n High Efficiency
n Thermal shutdown and current limit protection
typically 90 µA
Q
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)
10134701
SIMPLE SWITCHER®and
© 2001 National Semiconductor Corporation DS101347 www.national.com
Switchers Made Simple
®
are registered trademarks of National Semiconductor Corporation.
Page 2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
LM2590HV
Distributors for availability and specifications.
Maximum Supply Voltage (V
SD /SS Pin Input Voltage (Note 2)
Delay Pin Voltage (Note 2) 1.5V
Flag Pin Voltage −0.3 ≤ V ≤45V
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
) 63V
IN
6V
ESD Susceptibility
Human Body Model (Note 3) 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
LM2590HV-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 LM2590HV-3.3 Units
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
V
OUT
η Efficiency V
Output Voltage 4.75V ≤ VIN≤ 60V, 0.2A ≤ I
= 12V, I
IN
Figure 1
≤ 1A 3.3 V
LOAD
=1A 77
LOAD
3.168/3.135 V(min)
3.432/3.465 V(max)
(Limits)
LM2590HV-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 LM2590HV-5.0 Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit
V
OUT
η Efficiency V
Output Voltage 7V ≤ VIN≤ 60V, 0.2A ≤ I
= 12V, I
IN
Typ Limit
(Note 4) (Note 5)
Figure 1
≤ 1A 5 V
LOAD
4.800/4.750 V(min)
5.200/5.250 V(max)
=1A 82 %
LOAD
LM2590HV-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 LM2590HV-ADJ Units
(Limits)
SYSTEM PARAMETERS (Note 6) Test Circuit
V
FB
Feedback Voltage 4.5V ≤ VIN≤ 60V, 0.2A ≤ I
V
programmed for 3V. Circuit of
OUT
Figure 1
Typ Limit
(Note 4) (Note 5)
≤ 1A 1.230 V
LOAD
Figure 1
. 1.193/1.180 V(min)
1.267/1.280 V(max)
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LM2590HV-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 LM2590HV-ADJ Units
η Efficiency V
= 12V, V
IN
(Continued)
= 3V, I
OUT
Typ Limit
(Note 4) (Note 5)
=1A 76 %
LOAD
(Limits)
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
Symbol Parameter Conditions LM2590HV-XX Units
DEVICE PARAMETERS
I
b
f
O
V
SAT
Feedback Bias Current Adjustable Version Only, VFB= 1.3V 10 nA
Oscillator Frequency (Note 7) 150 kHz
Saturation Voltage I
DC Max Duty Cycle (ON) (Note 9) 100 %
Min Duty Cycle (OFF) (Note 10) 0
I
I
I
CLIM
L
Q
Switch current Limit Peak Current, (Note 8) (Note 9) 1.9 A
Output Leakage Current (Note 8) (Note 10) (Note 11) Output = 0V 50 µA(max)
Operating Quiescent SD /SS Pin Open (Note 10) 5mA
Current 10 mA(max)
I
STBY
Standby Quiescent SD /SS pin = 0V (Note 11) 90 µA
Current 200/250 µA(max)
θ
JC
θ
JA
θ
JA
θ
JA
θ
JA
Thermal Resistance TO220 or TO263 Package, Junction to Case 2 ˚C/W
SHUTDOWN/SOFT-START CONTROL Test Circuit of
V
SD
Shutdown Threshold 1.3 V
Voltage Low, (Shutdown Mode) 0.6 V(max)
V
SS
I
SD
I
SS
Soft-start Voltage V
Shutdown Current V
Soft-start Current V
= 12V for the 3.3V, 5V, and Adjustable version. I
IN
= 500 mA
LOAD
Typ Limit
(Limits)
(Note 4) (Note 5)
50/100 nA (max)
127/110 kHz(min)
173/173 kHz(max)
= 1A (Note 8) (Note 9) 0.95 V
OUT
1.2/1.3 V(max)
1.3/1.2 A(min)
2.8/3.0 A(max)
Output = −1V 5 mA
30 mA(max)
TO220 Package, Juncton to Ambient (Note 12) 50 ˚C/W
TO263 Package, Juncton to Ambient (Note 13) 50 ˚C/W
TO263 Package, Juncton to Ambient (Note 14) 30 ˚C/W
TO263 Package, Juncton to Ambient (Note 15) 20 ˚C/W
Figure 1
High, (Soft-start Mode) 2 V(min)
= 20% of Nominal Output Voltage 2 V
OUT
V
= 100% of Nominal Output Voltage 3
OUT
SHUTDOWN
= 0.5V 5µA
10 µA(max)
Soft-start
= 2.5V 1.5 µA
5 µA(max)
LM2590HV
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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 Tempera-
LM2590HV
ture Range. Unless otherwise specified, V
Symbol Parameter Conditions LM2590HV-XX Units
FLAG/DELAY CONTROL Test Circuit of
Regulator Dropout Detector Low (Flag ON) 96 %
Threshold Voltage 92 %(min)
VF
SAT
IF
L
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: Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 4: Typical numbers are at 25˚C and represent the most likely norm.
Note 5: 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 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the
LM2590HV is used as shown in the
Note 7: 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 8: No diode, inductor or capacitor connected to output pin.
Note 9: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
Note 10: 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 11: V
Note 12: 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
Note 13: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in
Note 14: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in
Note 15: Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in
the LM2590HVS side of the board, and approximately 16 in
model in Switchers Made Simple available at http://power.national.com.
Flag Output Saturation I
Voltage V
Flag Output Leakage Current V
Delay Pin Threshold 1.25 V
Voltage Low (Flag ON) 1.21 V(min)
Delay Pin Source Current V
Delay Pin Saturation Low (Flag ON) 70 mV
Figure 1
= 60V.
IN
(Continued)
= 12V for the 3.3V, 5V, and Adjustable version. I
IN
LOAD
= 500 mA
Typ Limit
(Note 4) (Note 5)
Figure 1
98 %(max)
= 3 mA 0.3 V
SINK
= 0.5V 0.7/1.0 V(max)
DELAY
= 60V 0.3 µA
FLAG
High (Flag OFF) and V
= 0.5V 3 µA
DELAY
Regulated 1.29 V(max)
OUT
6 µA(max)
350/400 mV(max)
test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
2
.
2
of copper on the other side of the p-c board. See application hints in this data sheet and the thermal
2
of (1 oz.) copper area.
2
of (1 oz.) copper area.
2
of (1 oz.) copper area on
(Limits)
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LM2590HV
Typical Performance Characteristics (Circuit of
Figure 1
)
NormalizedOutput Voltage Line Regulation Efficiency
10134702
10134703 10134704
Switch SaturationVoltage Switch Current Limit Dropout Voltage
10134705
Operating
Quiescent Current Shutdown Quiescent Current
10134708 10134709
10134706
10134707
Minimum Operating
Supply Voltage
10134710
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Typical Performance Characteristics (Circuit of
Figure 1
) (Continued)
LM2590HV
Feedback Pin Bias Current Flag Saturation Voltage Switching Frequency
Soft-start
10134711
10134712
Shutdown /Soft-start
Current Delay Pin Current
10134713
Soft-start Response
10134714
10134718
10134715
10134716
Shutdown/Soft-start
Threshold Voltage Internal Gain-Phase Characteristics
10134753
10134778
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Typical Performance Characteristics (Circuit of
Figure 1
) (Continued)
Continuous Mode Switching Waveforms
V
L = 52 µH, C
IN
= 20V, V
OUT
= 5V, I
OUT
= 100 µF, C
=1A
LOAD
ESR = 100 mΩ
OUT
Horizontal Time Base: 2 µs/div.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 50 mV/div.
Load Transient Response for Continuous Mode
V
= 20V, V
IN
L = 52 µH, C
OUT
= 100 µF, C
OUT
= 5V, I
= 250 mA to 1A
LOAD
ESR = 100 mΩ
OUT
10134720
Discontinuous Mode Switching Waveforms
V
= 20V, V
IN
L = 15 µH, C
OUT
= 5V, I
OUT
= 150 µF, C
LOAD
OUT
Horizontal Time Base: 2 µs/div.
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.25A/div.
C: Output Ripple Voltage, 100 mV/div.
= 250 mA
ESR=90mΩ
10134719
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Test Circuit and Layout Guidelines
LM2590HV
Fixed Output Voltage Versions
Component Values shown are for VIN= 15V,
10134724
V
= 5V, I
OUT
C
— 470 µF, 50V, Aluminum Electrolytic Nichicon “PM Series”
IN
C
— 220 µF, 25V Aluminum Electrolytic, Nichicon “PM Series”
OUT
LOAD
= 1A.
D1 — 2A, 60V Schottky Rectifier, 21DQ06 (International Rectifier)
L1 — 68 µH, See Inductor Selection Procedure
Adjustable Output Voltage Versions
Select R1to be approximately 1 kΩ, use a 1% resistor for best stability.
Component Values shown are for V
V
= 10V, I
OUT
C
: — 470 µF, 35V, Aluminum Electrolytic Nichicon “PM Series”
IN
C
: — 220 µF, 35V Aluminum Electrolytic, Nichicon “PM Series”
OUT
LOAD
= 1A.
IN
= 20V,
D1 — 2A, 60V Schottky Rectifier, 21DQ06 (International Rectifier)
L1 — 100 µH, See Inductor Selection Procedure
R
—1kΩ,1%
1
R
— 7.15k, 1%
2
C
— 3.3 nF
FF
Typical Values
CSS—0.1 µF
C
—0.1 µF
DELAY
R
†
††
drag feedback pin below ground). Required if V
— 4.7k (use 22k if V
PULL UP
OUT
is ≥ 45V)
Resistive divider is required to aviod exceeding maximum rating of 45V/3mA on/into flag pin.
Small signal Schottky diode to prevent damage to feedback pin by negative spike when output is shorted (CFFnot being able to discharge immediately will
>
40V
IN
10134725
FIGURE 1. Standard Test Circuits and Layout Guides
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Page 9
Block Diagram
LM2590HV
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
mately −0.5V, with a duty cycle of V
IN−VSAT
OUT/VIN
Error Flag (Pin 3)—Open collector output that goes active
low (≤ 1.0V) when the output of the switching regulator is out
of regulation (less than 95% of its nominal value). In this
state it can sink maximum 3mA. When not low, it can be
pulled high to signal that the output of the regulator is in
regulation (power good). During power-up, it can be programmed to go high after a certain delay as set by the Delay
pin (Pin 5). The maximum rating of this pin should not be
exceeded, so if the rail to which it will be pulled-up to is
higher than 45V, a resistive divider must be used instead of
a single pull-up resistor, as indicated in
Ground (Pin 4)—Circuit ground.
Delay (Pin 5)—This sets a programmable power-up delay
from the moment that the output reaches regulation, to the
high signal output (power good) on Pin 3. A capacitor on this
pin starts charging up by means on an internal () 3 µA)
current source when the regulated output rises to within 5%
of its nominal value. Pin 3 goes high (with an external
pull-up) when the voltage on the capacitor on Pin 5 exceeds
1.3V. The voltage on this pin is clamped internally to about
1.7V. If the regulated output drops out of regulation (less
than 95% of its nominal value), the capacitor on Pin 5 is
rapidly discharged internally and Pin 3 will be forced low in
about 1/1000
th
of the set power-up delay time.
) and approxi-
.
Figure 1
.
10134730
Feedback (Pin 6)—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.
Shutdown /Soft-start (Pin 7)—The regulator is in shutdown mode, drawing about 90 µA, when this pin is driven to
a low level (≤ 0.6V), and is in normal operation when this Pin
is left floating (internal-pullup) or driven to a high level (≥
2.0V). The typical value of the threshold is 1.3V and the pin
is internally clamped to a maximum of about 7V. If it is driven
higher than the clamp voltage, it must be ensured by means
of an external resistor that the current into the pin does not
exceed 1mA. The duty cycle is minimum (0%) if this Pin is
below 1.8V, and increases as the voltage on the pin is
increased. The maximum duty cycle (100%) occurs when
this pin is at 2.8V or higher. So adding a capacitor to this pin
produces a softstart feature. An internal current source will
charge the capacitor from zero to its internally clamped
value. The charging current is about 5 µA when the pin is
below 1.3V but is reduced to only 1.6 µA above 1.3V, so as
to allow the use of smaller softstart capacitors.
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PIN FUNCTIONS (Continued)
Note If any of the above three features (Shutdown
/Soft-start, Error Flag, or Delay) are not used, the respective
LM2590HV
pins can be left open.
FIGURE 2. Soft-Start, Delay, Error Output
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10134731
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FIGURE 3. Timing Diagram for 5V Output
LM2590HV
10134732
INDUCTOR VALUE SELECTION GUIDES
(For Continuous Mode Operation)
FIGURE 4. LM2590HV-3.3
10134726
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INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation) (Continued)
LM2590HV
10134727
FIGURE 5. LM2590HV-5.0
10134729
FIGURE 6. LM2590HV-ADJ
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INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation) (Continued)
10134765
FIGURE 7. Current Ripple Ratio
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
LM2590HV
FIGURE 8. Contact Information for Suggested Inductor Manufacturers
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Page 14
Application Information
INDUCTOR SELECTION PROCEDURE
LM2590HV
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
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
2. The Energy under steady operation is
Figure 4toFigure 6
. To widen the choice of the
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
of the device, and this current
CLIM
could be much higher than the rated load, I
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
. But if the DC input voltage
CLIM
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
without saturating,
CLIM
irrespective of the type of core structure/material
LOAD
. This
.
consider the rather wide tolerance on the nominal inductance of commercial inductors.
5.
Figure 6
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
@
5V
0.8A
1. A first pass inductor selection is based upon
and rated max load current
Inductance value indicated by the nomograph (
≤ 40V) LM2590HV-5.0, VIN= 24V, Output
IN
Inductance
. We choose an inductor with the
Figure 5
) and
a current rating equal to the maximum load current. We
therefore quick-select a 100µH/0.8 A inductor (designed for
150 kHz operation) for this application.
2. We should confirm that it is rated to handle 50 µJ (see
Figure 5
) 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
@
5V
1A
1. A first pass inductor selection is based upon
and the switch currrent limit
Inductance value indicated by the nomograph (
a current rating equal to I
>
40V) LM2590HV-5.0, VIN= 48V, Output
IN
. We choose an inductor with the
Figure 5
. We therefore quick-select a
CLIM
Inductance
) and
100µH/3A 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:
where L is in µH and I
waveform with the regulator delivering I
energy values shown in the nomographs. See
is the peak of the inductor current
PEAK
. These are the
LOAD
Example 1
below.
3. The Energy under overload is
>
If V
instead of the steady energy values. The worst case I
40V, the inductor should be sized to handle e
IN
CLIM
CLIM
for
the LM2590HV is 3A. 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 7
). This
was done to permit the use of smaller inductors at light
loads.
Figure 7
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
Example 3: (VIN≤ 40V) LM2590HV-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 6
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
the drop across the switch ()1.5V). So
SAT
And the switch ON time is
where f is the switching frequency in Hz. So
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Page 15
Application Information (Continued)
LM2590HV
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.
Therefore, looking at
Figure 4
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 6
) 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).
Note that we have taken V
as 1.5V which includes an
SAT
estimated resistive drop across the inductor.
This completes the simplified inductor selection procedure.
For more general applications and better optimization, the
designer should refer to AN-1197.
Figure 8
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
- A Feedforward Capacitor CFF, shown across R2 in
FF
Figure 1
or when C
is used when the output voltage is greater than 10V
has a very low ESR. This capacitor adds lead
OUT
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
, in series with the feedforward capacitor, ap-
FF
proximately 3 times R1, will attenuate the ripple voltage at
the feedback pin.
INPUT CAPACITOR
C
—Alow ESR aluminum or tantalum bypass capacitor is
IN
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
OUTPUT CAPACITOR
C
—An output capacitor is required to filter the output
OUT
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 LM2590HV
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).
SHUTDOWN /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 startup 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.
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Page 16
Application Information (Continued)
LM2590HV
FIGURE 9. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
10134742
10134743
FIGURE 10. Inverting −5V Regulator With Shutdown and Soft-start
lNVERTING REGULATOR
The circuit in
Figure 10
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 LM2590HV-5 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
www.national.com 16
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
While checking for this, take I
is limited by the requirement that I
LOAD
CLIM
≤ I
PEAK
CLIM
to be the lowest possible
current limit value (min across tolerance and temperature is
1.2A for the LM2590HV). 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-
.
Page 17
Application Information (Continued)
suming 100% efficiency, which is never so. Therefore expect
I
to be an additional 10-20% higher than calculated from
PEAK
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. In this example, when
converting +20V to −5V, the regulator would see 25V between the input pin and ground pin. The LM2590HV has a
maximum input voltage rating of 60V.
An additional diode is required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the C
or no load conditions. Also, this diode isolation changes the
topology to closely 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 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,
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 LM2590HV current limit
(approximately 3.0A) 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
Also shown in
Figure 10
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.
capacitor to the output, under light
IN
Figure 10
is recommended.
are several shutdown methods for
10134745
FIGURE 11. Undervoltage Lockout for a Buck
Regulator
Figure 12
inverting circuit.
and
Figure 13
Figure 12
apply the same feature to an
features a constant threshold
voltage for turn on and turn off (zener voltage plus approximately one volt). If hysteresis is needed, the circuit in
13
has a turn ON voltage which is different than the turn OFF
Figure
voltage. The amount of hysteresis is approximately equal to
the value of the output voltage. Since the SD /SS pin has an
internal 7V zener clamp, R2 is needed to limit the current into
this pin to approximately 1 mA when Q1 is on.
10134747
FIGURE 12. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
LM2590HV
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage.
Figure 11
contains a undervoltage lockout circuit for a buck configuration, while
Figure 12
and
Figure 13
are for the inverting types
(only the circuitry pertaining to the undervoltage lockout is
shown).
Figure 11
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 1.5V greater than the zener voltage.
10134746
FIGURE 13. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
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
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Page 18
Application Information (Continued)
possible. For best results, external components should be
located as close to the switcher lC as possible using ground
LM2590HV
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
wiring can cause problems.
OUT
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.
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Page 19
Physical Dimensions inches (millimeters)
unless otherwise noted
LM2590HV
7-Lead TO-220 Bent and Staggered Package
Order Number LM2590HVT-3.3, LM2590HVT-5.0 or LM2590HVT-ADJ
NS Package Number TA07B
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Page 20
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
7-Lead TO-263 Bent and Formed Package
Order Number LM2590HVS-3.3, LM2590HVS-5.0 or LM2590HVS-ADJ
NS Package Number TS7B
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