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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
General Description
The LM2575 /LM2575HV 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, 12V, and an
adjustable output version.
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation†, and a fixed-frequency oscillator.
The LM2575 /LM2575HV series operates at a switching
frequency of150 kHz thus allowing smaller sized filter compontents 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-lesd TO-262 surface mount package.
A standard series of inductors are available from
several different manufacturers optimized for use with the
LM2575series. This feature greatly simplifies the
design of switch-mode power supplies.
Other features include a guaranteed ± 4% tolerance on output voltage under specified input voltage and output
load conditions, and ± 15% on the oscillator frequency.
External shutdown is included, featuring typically 80 µA
standby cur-rent. Self protection features include a two
stage frequency reducing current limit for the output
switch and an over temperature shutdown for complete
protection under fault conditions.
Typical Application
Features
n 3.3V, 5V, 12V, and adjustable output versions
n Adjustable version output voltage range, 1.2V to 37V
(57V for HV version) ± 4% max over line and load conditions
n Available in TO-220 and TO-263,TO-220B packages
n Guaranteed 2A output load current
n Input voltage range up to 40V
n Requires only 4 external components
n Excellent line and load regulation specifications
n 150 kHz fixed frequency internal oscillator
n TTL shutdown capability
n Low power standby mode, IQ typically 80 µA
n High efficiency
n Uses readily available standard inductors
n Thermal shutdown and current limit protection
Applications
n Simple high-efficiency step-down (buck) regulator
n On-card switching regulators
n Positive to negative converter
Note: †Patent Number 5,382,918.
(Fixed Output Voltage
Versions)
LM2575
/LM2575HV
5.0
Package Types
TO220B-5L
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TO220-5L
1
TO263-5L
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Pin Assignments
Pin Descriptions
5 ON/OFF
4 Feedback
3 Gnd
2 Output
1 Vin
TO220B-5L/TO220-5L
5 ON/OFF
4 Feedback
3 Gnd
2 Output
1 Vin
Name
Description
Vin
Input supply voltage
Output
Switching output
Gnd
Ground
Feedback
Output voltage feedback
ON/OFF
ON/OFF shutdown
Active is “Low” or floating
TO263-5L
Ordering information
Temperature
Range
-40°C ≤ TA
≤ 125°C
Output Voltage, V
3.3
5.0
12
Package Type
ADJ
LM2575HVS - 3.3 LM2575HVS -5.0 LM2575HVS -12 LM2575HVS -ADJ
LM2575S -3.3
LM2575S - 5.0
LM2575S -12
LM2575HVT -3.3 LM2575HVT -5.0 LM2575HVT -12 LM2575HVT- ADJ
LM2575T -3.3
LM2575T - 5.0
LM2575T -12
TO-263
LM2575S -ADJ
TO-220
LM2575T -ADJ
Absolute Maximum Ratings
Maximum Supply Voltage
LM2575
LM2575HV
ON /OFF Pin Input Voltage
−0.3 ≤ V ≤+25V
Power Dissipation
Internally limited
ESD Susceptibility HumanBodyModel
S Package
−0.3 ≤ V ≤ +25V
Feedback Pin Voltage
Output Voltage to Ground
(Steady State)
StorageTemperatureRange
Lead Temperature
45V
57V
−1V
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
−65˚Cto+150˚C
Temperature Range
LM2575
LM2575HV
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−40˚C ≤ TJ ≤ +125˚C
Supply Voltage
2kV
2
40V
57V
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Electrical Characteristics LM2575- 3.3,LM2575HV -3.3
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range
Symbol
Parameter
LM2575 -3.3
LM2575HV-3.3
Conditions
Typ
Limit
Units
(Limits)
3.168/3.135
V(min)
3.432/3.465
V(max)
%
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT
η
Output Voltage
Efficiency
4.75V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 2A
V IN = 12V, I LOAD = 2A
3.3
V
73
Electrical Characteristics LM2575-5.0,LM2575HV-5.0
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range
Symbol
Parameter
LM2575 -5.0
LM2575HV-5.0
Conditions
Typ
Limit
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT
η
Output Voltage
Efficiency
7V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 2A
VIN = 12V, ILOAD = 2A
5.0
V
4.800/4.750
V(min)
5.200/5.250
V(max)
80
%
Electrical Characteristics LM2575-12,LM2596HV-12
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range
Symbol
Parameter
LM2575 -12
LM2575HV -12
Conditions
Typ
Limit
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT
η
Output Voltage
Efficiency
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15V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 2A
VIN = 25V, ILOAD = 2A
12.0
90
3
V
11.52/11.40
V(min)
12.48/12.60
V(max)
%
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Electrical Characteristics LM2575-ADJ,LM2575HV-ADJ
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range
Symbol
Parameter
LM2575 -ADJ
LM2575HV-ADJ
Typ
Limit
Conditions
Units
(Limits)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VFB
Feedback Voltage
4.5V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 2A
1.230
V
VOUT programmed for 3V. Circuit of Figure 1
η
Efficiency
VIN = 12V, VOUT = 3V, ILOAD = 2A
1.193/1.180
V(min)
1.267/1.280
V(max)
73
%
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, VIN = 12V for the 3.3V, 5V, and Adjustable version and VIN = 24V for the 12V version. ILOAD = 500 mA
Symbol
Parameter
LM2575-XX
LM2575HV-XX
Conditions
Typ
Limit
Units
(Limits)
DEVICE PARAMETERS
Ib
fO
Feedback Bias Current
Oscillator Frequency
Adjustable Version Only, VFB = 1.3V
(Note 6)
10
nA
50/100
nA (max)
127/110
kHz(min)
173/173
kHz(max)
150
VSAT
Saturation Voltage
IOUT = 2A (Notes 7, 8)
1.16
DC
Max Duty Cycle (ON)
(Note 8)
100
Min Duty Cycle (OFF)
(Note 9)
0
Current Limit
Peak Current (Notes 7, 8)
4.5
kHz
V
1.4/1.5
ICL
%
A
3.6/3.4
IL
IQ
ISTBY
θJC
Output Leakage Current
Quiescent Current
Standby Quiescent Current
Thermal Resistance
Output = 0V (Notes 7, 9)
Output = −1V (Note 10)
2
(Note 9)
5
ON/OFF pin = 5V (OFF)
(Note 10)
V(max)
A(min)
6.9/7.5
A(max)
50
µA(max)
mA
30
mA(max)
10
mA(max)
200/250
µA(max)
mA
80
µA
TO-220 or TO-263 Package, Junction to Case
2
θJA
TO-220 Package, Junction to Ambient (Note 11)
50
˚C/W
˚C/W
θJA
TO-263 Package, Junction to Ambient (Note 12)
50
˚C/W
θJA
TO-263 Package, Junction to Ambient (Note 13)
30
˚C/W
θJA
TO-263 Package, Junction to Ambient (Note 14)
20
˚C/W
ON/OFF CONTROL Test Circuit Figure 1
ON /OFF Pin Logic Input
VIH
Threshold Voltage
VIL
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1.3
V
Low (Regulator ON)
0.6
V(max)
High (Regulator OFF)
2.0
V(min)
4
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
All Output Voltage Versions
Electrical Characteristics
(Continued)
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 and VIN = 24V for the 12V version. ILOAD = 500 mA
Symbol
Parameter
Conditions
LM2575-XX
LM2575HV -XX
Typ
IH
ON /OFF Pin Input Current
IL
VLOGIC = 2.5V (Regulator OFF)
VLOGIC = 0.5V (Regulator ON)
Limit
5
Units
(Limits)
µA
15
µA(max)
5
µA(max)
0.02
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, and voltage programming resistors can affect switching regulator
system performance. When the LM2575 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.
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, and 15V for the 12V version, to force the output transistor
switch OFF.
Note 10: VIN = 40V.
Note 11: Junction to ambient thermal resistance (no external heat sink) for the 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 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 LM2596S side of the board, and approximately 16 in2 of copper on the other side of the p-c board. See Application Information in this data sheet and the thermal
model in Switchers Made Simple™ version 4.3 software.
Typical Performance Characteristics
Normalized
Output Voltage
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(Circuit of Figure 1)
Line Regulation
5
Efficiency
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Typical Performance Characteristics
(Circuit of Figure 1) (Continued)
Switch Saturation
Voltage
Switch Current Limit
Dropout Voltage
Operating
Shutdown
Minimum Operating
Quiescent Current
Quiescent Current
Supply Voltage
ON /OFF Threshold
Voltage
ON /OFF Pin
Current (Sinking)
Switching Frequency
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
LM2575/LM2575HV SeriesBuckRegulatorDesignProcedure(FixedOutput)(Continued)
Conditions
Inductor
Output Capacitor
Through Hole Electrolytic
Surface Mount Tantalum
Output
Load
Max Input
Inductance
Inductor
Panasonic
Nichicon
AVX TPS
Sprague
Voltage
Current
Voltage
(µH)
(#)
HFQ Series
PL Series
Series
595D Series
(V)
(A)
(V)
(µF/V)
(µF/V)
(µF/V)
(µF/V)
3.3
2
470/25
560/16
330/6.3
390/6.3
5
2
5
2
2
12
2
2
22 L41
7
22 L41
560/35
560/35
330/6.3
390/6.3
10
22 L41
680/35
680/35
330/6.3
390/6.3
40
33 L40
560/35
470/35
330/6.3
390/6.3
6
22 L33
470/25
470/35
330/6.3
390/6.3
10
33 L32
330/35
330/35
330/6.3
390/6.3
40
47 L39
330/35
270/50
220/10
330/10
8
22 L41
470/25
560/16
220/10
330/10
10
22 L41
560/25
560/25
220/10
330/10
15
33 L40
330/35
330/35
220/10
330/10
40
47 L39
330/35
270/35
220/10
330/10
9
22 L33
470/25
560/16
220/10
330/10
20
68 L38
180/35
180/35
100/10
270/10
40
68 L38
180/35
180/35
100/10
270/10
15
22 L41
470/25
470/25
100/16
180/16
18
33 L40
330/25
330/25
100/16
180/16
30
68 L44
180/25
180/25
100/16
120/20
40
68 L44
180/35
180/35
100/16
120/20
15
33 L32
330/25
330/25
100/16
180/16
20
68 L38
180/25
180/25
100/16
120/20
40
150 L42
82/25
82/25
68/20
68/25
FIGURE 2. LM2575 Fixed Voltage Quick Design Component Selection Table
LM2575/LM2575HV SeriesBuckRegulatorDesignProcedure(AdjustableOutput)
Output
Voltage
(V)
Through Hole Output Capacitor
Surface Mount Output Capacitor
Panasonic
Nichicon PL
Feedforward
AVX TPS
Sprague
Feedforward
HFQ Series
Series
Capacitor
Series
595D Series
Capacitor
(µF/V)
(µF/V)
(µF/V)
(µF/V)
2
820/35
820/35
33 nF
330/6.3
470/4
4
560/35
470/35
10 nF
330/6.3
390/6.3
10 nF
6
470/25
470/25
3.3 nF
220/10
330/10
3.3 nF
9
330/25
330/25
1.5 nF
100/16
180/16
1.5 nF
12
330/25
330/25
1 nF
100/16
180/16
1 nF
15
220/35
220/35
680 pF
68/20
120/20
680 pF
24
220/35
150/35
560 pF
33/25
33/25
220 pF
28
100/50
100/50
390 pF
10/35
15/50
220 pF
33 nF
FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
LM2575/LM2575HV SeriesBuckRegulatorDesignProcedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
01258326
01258324
FIGURE6.LM2575/LM2575HV-12
FIGURE4.LM2575/LM2575HV-3.3
01258327
01258325
FIGURE7.LM2575/LM2575HV-ADJ
FIGURE5.LM2575/LM2575HV-5.0
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Test Circuit and Layout Guidelines
(Continued)
Adjustable Output Voltage Versions
LM2575/
LM2575HV
where VREF = 1.23V
Select R1 to be approximately 1 kΩ, use a 1% resistor for best stability.
CIN — 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”
COUT
— 220 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”
D1
— 5A, 40V Schottky Rectifier, 1N5825
L1
— 68 µH, L38
R1
— 1 kΩ, 1%
CFF
— See Application Information Section
FIGURE 1. Standard Test Circuits and Layout Guides
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, 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 COUT wiring can cause problems.
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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. (See application section for more
information.)
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
LM2575 /LM2575HV Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
Given:
VOUT = Regulated Output Voltage
VIN(max) = Maximum Input Voltage
EXAMPLE (Adjustable Output Voltage Version)
Given:
VOUT = 20V
VIN(max) = 28V
ILOAD(max) = Maximum Load Current
ILOAD(max) = 2A
F = Switching Frequency (Fixed at a nominal 150 kHz).
F = Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 1 )
Use the following formula to select the appropriate resistor
values.
1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 1 )
Select R1 to be 1 kΩ, 1%. Solve for R2.
Select a value for R1 between 240Ω and 1.5 kΩ. The lower
resistor values minimize noise pickup in the sensitive feedback pin. (For the lowest temperature coefficient and the best
stability with time, use 1% metal film resistors.)
R2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
R2 = 15.4 kΩ.
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant E • T (V
• µs), from the following formula:
2. Inductor Selection (L1)
A. Calculate the inductor Volt • microsecond constant
(E • T),
where VSAT = internal switch saturation voltage = 1.16V
and VD = diode forward voltage drop = 0.5V
B. Use the E • T value from the previous formula and match it
with the E • T number on the vertical axis of the Inductor Value
Selection Guide shown in Figure 7.
C. on the horizontal axis, select the maximum load current.
D. Identify the inductance region intersected by the E • T value
and the Maximum Load Current value. Each region is identified by an inductance value and an inductor code (LXX).
E. Select an appropriate inductor from the four manufacturer’s
part numbers listed in Figure 8.
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B. E • T = 34.2 (V • µs)
C. ILOAD(max) = 2A
D. From the inductor value selection guide shown in Figure 7,
the inductance region intersected by the 34 (V • µs) horizontal
L39.
E. From the table in Figure 8, locate line L39, and select an
bers.
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Block Diagram
01258321
FIGURE 12.
EXTERNAL COMPONENTS
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
Application Information
PIN FUNCTIONS
+VIN — 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.
Ground — Circuit ground.
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 RMS current rating of a capacitor could be viewed as a
capacitor’s power rating. The RMS current flowing through
the capacitors internal ESR produces power which causes
the internal temperature of the capacitor to rise. The RMS
current rating of a capacitor is determined by the amount of
current required to raise the internal temperature approximately 10˚C above an ambient temperature of 105˚C. The
ability of the capacitor to dissipate this heat to the surrounding air will determine the amount of current the capacitor can
safely sustain. Capacitors that are physically large and have
a large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage
capacitor, and thus be able to dissipate more heat to the
surrounding air, and therefore will have a higher RMS current rating.
Output — Internal switch. The voltage at this pin switches
between (+VIN − VSAT) and approximately −0.5V, with a duty
cycle of approximately VOUT/VIN. To minimize coupling to
sensitive circuitry, the PC board copper area connected to
this pin should be kept to a minimum.
Feedback — Senses the regulated output voltage to complete the feedback loop.
ON /OFF — Allows the switching regulator circuit to be shut
down using logic level signals thus dropping the total input
supply current to approximately 80 µA. Pulling this pin below
a threshold voltage of approximately 1.3V turns the regulator
on, and pulling this pin above 1.3V (up to a maximum of 25V)
shuts the regulator down. If this shutdown feature is not
needed, the ON /OFF pin can be wired to the ground pin or
it can be left open, in either case the regulator will be in the
ON condition.
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Application Information
(Continued)
LM2575
/LM2575HV
5.0
LM2575
/LM2575HV
5.0
01258336
01258337
FIGURE 21. Delayed Startup
FIGURE 22. Undervoltage Lockout
for Buck Regulator
This circuit has an ON/OFF threshold of approximately 13V.
LM2575
/LM2575HV
5.0
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 22, while Figure 23 and 24 applies the same
feature to an inverting circuit. The circuit in Figure 23 fea01258338
tures a constant threshold voltage for turn on and turn off
(zener voltage plus approximately one volt). If hysteresis is
needed, the circuit in Figure 24 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.
for Inverting Regulator
FIGURE 23. Undervoltage Lockout
DELAYED STARTUP
The circuit in Figure 21 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.
INVERTING REGULATOR
The circuit in Figure 25 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,
This example uses the LM2575/LM2575HV-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. The curve shown in Figure 26 provides a
guide as to the amount of output load current possible for the
different input and output voltage conditions.
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.
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Application Information
(Continued)
LM2575
/LM2575HV
5.0
This circuit has hysteresis
Regulator starts switching at VIN = 13V
Regulator stops switching at VIN = 8V
FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator
LM2575
/LM2575HV
5.0
CIN
— 68 µF/25V Tant. Sprague 595D
470 µF/50V Elec. Panasonic HFQ
COUT
— 47 µF/20V Tant. Sprague 595D
220 µF/25V Elec. Panasonic HFQ
FIGURE 25. Inverting −5V Regulator with Delayed Startup
be narrowed down to just a few values. Using the values
shown in Figure 25 will provide good results in the majority of
inverting designs.
This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light
loads. Input currents as high as the LM2575 /LM2575HV
current limit (approx 4.5A) are needed for at least 2 ms or more,
until the output reaches its nominal output voltage.
The adual time depends on the output voltage and
the size of the outputcapacitor. 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
tolology the delayed startup feature (C1, R1 and R2) shown the
in Figure 25 is recommended. By delaying the regulator
startup, the input capacitor is allowed to charge up to a higher votage before the switcher begins operating. A portion
of the high nput 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.
FIGURE 26. Inverting Regulator Typical Load Current
Because of differences in the operation of the inverting
regulator, the standard design procedure is not used to
select the inductor value. In the majority of designs, a 33 µH,
3.5A inductor is the best choice. Capacitor selection can also
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
Application Information
(Continued)
INVERTING 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 27 and 28.
LM2575
/LM2575HV
5.0
01258342
FIGURE 27. Inverting Regulator Ground Referenced Shutdown
LM2575
/LM2575HV
5.0
01258343
FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device
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友台半导体有限公司
UMW
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UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
PACKAGES DIMENSION : TO220B-5L
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友台半导体有限公司
UMW
R
UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
PACKAGES DIMENSION : TO220-5L
www.umw-ic.com
16
友台半导体有限公司
UMW
R
UMW LM2575/LM2575HV
2A Step-Down Voltage Regulator
PACKAGES DIMENSION : TO263-5L
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