View LM2574_988386.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

LM2574 LM2574HV Series SIMPLE SWITCHER 0 5A Step-Down Voltage Regulator
March 1995
LM2574 LM2574HV Series SIMPLE SWITCHER TM 0 5A Step-Down Voltage Regulator
General Description
The LM2574 series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator capable of driving a 0 5A load with excellent line and load regulation These devices are available in fixed output voltages of 3 3V 5V 12V 15V 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 LM2574 series offers a high-efficiency replacement for popular three-terminal linear regulators Because of its high efficiency the copper traces on the printed circuit board are normally the only heat sinking needed A standard series of inductors optimized for use with the LM2574 are available from several different manufacturers This feature greatly simplifies the design of switch-mode power supplies Other features include a guaranteed g4% tolerance on output voltage within specified input voltages and output load conditions and g10% on the oscillator frequency External shutdown is included featuring 50 mA (typical) standby current The output switch includes cycle-by-cycle current limiting as well as thermal shutdown for full protection under fault conditions
Features
Y Y
Y Y
Y Y Y Y Y Y
3 3V 5V 12V 15V and adjustable output versions Adjustable version output voltage range 1 23V to 37V (57V for HV version) g4% max over line and load conditions Guaranteed 0 5A output current Wide input voltage range 40V up to 60V for HV version Requires only 4 external components 52 kHz fixed frequency internal oscillator TTL shutdown capability low power standby mode High efficiency Uses readily available standard inductors Thermal shutdown and current limit protection
Applications
Y Y Y Y
Simple high-efficiency step-down (buck) regulator Efficient pre-regulator for linear regulators On-card switching regulators Positive to negative converter (Buck-Boost)
Typical Application (Fixed Output Voltage Versions)
TL H 11394 - 1
Note Pin numbers are for 8-pin DIP package
Connection Diagrams
8-Lead DIP (N)
No internal connection but should be soldered to PC board for best heat transfer
TL H 11394 - 2
14-Lead Wide Surface Mount (WM)
Top View Order Number LM2574-3 3HVN LM2574HVN-5 0 LM2574HVN-12 LM2574HVN-15 LM2574HVN-ADJ LM2574N-3 3 LM2574N-5 0 LM2574N-12 LM2574N-15 or LM2574N-ADJ See NS Package Number N08A
TL H 11394 - 3
Top View Order Number LM2574HVM-3 3 LM2574HVM-5 0 LM2574HVM-12 LM2574HVM-15 LM2574HVM-ADJ LM2574M-3 3 LM2574M-5 0 LM2574M-12 LM2574M-15 or LM2574M-ADJ See NS Package Number M14B
RRD-B30M75 Printed in U S A
Patent Pending SIMPLE SWITCHERTM is a trademark of National Semiconductor Corporation C1995 National Semiconductor Corporation
TL H 11394
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 LM2574 LM2574HV ON OFF Pin Input Voltage Output Voltage to Ground (Steady State) Power Dissipation Storage Temperature Range 45V 63V
b 0 3V s V s a VIN b 1V
Minimum ESD Rating (C e 100 pF R e 1 5 kX) Lead Temperature (Soldering 10 seconds) Maximum Junction Temperature
2 kV 260 C 150 C
Operating Ratings
Temperature Range LM2574 LM2574HV Supply Voltage LM2574 LM2574HV
b 40 C s TJ s a 125 C
Internally Limited b 65 C to a 150 C
40V 60V
LM2574-3 3 LM2574HV-3 3 Electrical Characteristics Specifications with standard type face are for TJ e 25 C and those with boldface type apply over full Operating Temperature Range
Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN e 12V ILOAD e 100 mA 33 3 234 3 366 VOUT Output Voltage LM2574 Output Voltage LM2574HV Efficiency 4 75V s VIN s 40V 0 1A s ILOAD s 0 5A 33 3 168 3 135 3 432 3 465 4 75V s VIN s 60V 0 1A s ILOAD s 0 5A 33 3 168 3 135 3 450 3 482 VIN e 12V ILOAD e 0 5A 72 V(Min) V(Max) % V V(Min) V(Max) V V(Min) V(Max) LM2574-3 3 LM2574HV-3 3 Limit (Note 2) Units (Limits)
VOUT
h
LM2574-5 0 LM2574HV-5 0 Electrical Characteristics Specifications with standard type face are for TJ e 25 C and those with boldface type apply over full Operating Temperature Range
Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN e 12V ILOAD e 100 mA 5 4 900 5 100 VOUT Output Voltage LM2574 Output Voltage LM2574HV Efficiency 7V s VIN s 40V 0 1A s ILOAD s 0 5A 5 4 800 4 750 5 200 5 250 7V s VIN s 60V 0 1A s ILOAD s 0 5A 5 4 800 4 750 5 225 5 275 VIN e 12V ILOAD e 0 5A 77 V(Min) V(Max) % V V(Min) V(Max) V V(Min) V(Max) LM2574-5 0 LM2574HV-5 0 Limit (Note 2) Units (Limits)
VOUT
h
2
LM2574-12 LM2574HV-12 Electrical Characteristics Specifications with standard type face are for TJ e 25 C and those with boldface type apply over full Operating Temperature Range
Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN e 25V ILOAD e 100 mA 10 11 76 12 24 VOUT Output Voltage LM2574 Output Voltage LM2574HV Efficiency 15V s VIN s 40V 0 1A s ILOAD s 0 5A 12 11 52 11 40 12 48 12 60 15V s VIN s 60V 0 1A s ILOAD s 0 5A 12 11 52 11 40 12 54 12 66 VIN e 15V ILOAD e 0 5A 88 V(Min) V(Max) % V V(Min) V(Max) V V(Min) V(Max) LM2574-12 LM2574HV-12 Limit (Note 2) Units (Limits)
VOUT
h
LM2574-15 LM2574HV-15 Electrical Characteristics Specifications with standard type face are for TJ e 25 C and those with boldface type apply over full Operating Temperature Range
Symbol Parameter Conditions Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VOUT Output Voltage VIN e 30V ILOAD e 100 mA 15 14 70 15 30 VOUT Output Voltage LM2574 Output Voltage LM2574HV Efficiency 18V s VIN s 40V 0 1A s ILOAD s 0 5A 15 14 40 14 25 15 60 15 75 18V s VIN s 60V 0 1A s ILOAD s 0 5A 15 14 40 14 25 15 68 15 83 VIN e 18V ILOAD e 0 5A 88 V(Min) V(Max) % V V(Min) V(Max) V V(Min) V(Max) LM2574-15 LM2574HV-15 Limit (Note 2) Units (Limits)
VOUT
h
3
LM2574-ADJ LM2574HV-ADJ Electrical Characteristics Specifications with standard type face are for TJ e 25 C and those with boldface type apply over full Operating Temperature Range Unless otherwise specified VIN e 12V ILOAD e 100 mA
Symbol Parameter Conditions LM2574-ADJ LM2574HV-ADJ Typ SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2 VFB Feedback Voltage VIN e 12V ILOAD e 100 mA 1 230 1 217 1 243 VFB Feedback Voltage LM2574 Feedback Voltage LM2574HV Efficiency 7V s VIN s 40V 0 1A s ILOAD s 0 5A VOUT Programmed for 5V Circuit of Figure 2 7V s VIN s 60V 0 1A s ILOAD s 0 5A VOUT Programmed for 5V Circuit of Figure 2 VIN e 12V VOUT e 5V ILOAD e 0 5A 1 230 1 193 1 180 1 267 1 280 1 230 1 193 1 180 1 273 1 286 77 V(Min) V(Max) % V V(Min) V(Max) V V(Min) V(Max) Limit (Note 2) Units (Limits)
VFB
h
All Output Voltage Versions Electrical Characteristics Specifications with standard type face are for TJ e 25 C and those with boldface type apply over full Operating Temperature Range Unless otherwise specified VIN e 12V for the 3 3V 5V and Adjustable version VIN e 25V for the 12V version and VIN e 30V for the 15V version ILOAD e 100 mA
Symbol Parameter Conditions LM2574-XX LM2574HV-XX Typ DEVICE PARAMETERS Ib fO Feedback Bias Current Oscillator Frequency Adjustable Version Only VOUT e 5V (see Note 10) 50 52 47 42 58 63 VSAT DC ICL Saturation Voltage Max Duty Cycle (ON) Current Limit IOUT e 0 5A (Note 4) (Note 5) Peak Current (Notes 4 10) 09 12 14 98 93 10 0 7 0 65 16 18 IL Output Leakage Current (Notes 6 7) Output e 0V Output e b1V Output e b1V 2 75 30 5 10 ISTBY iJA iJA iJA iJA VIH VIL IH IIL Standby Quiescent Current Thermal Resistance ON OFF Pin e 5V (OFF) N Package N Package M Package M Package Junction to Ambient (Note 8) Junction to Ambient (Note 9) Junction to Ambient (Note 8) Junction to Ambient (Note 9) 50 200 92 72 102 78 14 12 12 30 ON OFF Pin e 0V (ON) 0 10 22 24 10 08 100 500 nA kHz kHz(Min) kHz(Max) V V(max) % %(Min) A A(Min) A(Max) mA(Max) mA mA(Max) mA mA(Max) mA mA(Max) CW Limit (Note 2) Units (Limits)
IQ
Quiescent Current
(Note 6)
ON OFF CONTROL Test Circuit Figure 2 ON OFF Pin Logic Input Level ON OFF Pin Input Current VOUT e 0V VOUT e Nominal Output Voltage ON OFF Pin e 5V (OFF) V(Min) V(Max) mA mA(Max) mA mA(Max)
4
Electrical Characteristics (Continued)
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 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 Note 3 External components such as the catch diode inductor input and output capacitors can affect switching regulator system performance When the LM2574 is used as shown in the Figure 2 test circuit system performance will be as shown in system parameters section of Electrical Characteristics Note 4 Output pin sourcing current No diode inductor or capacitor connected to output pin Note 5 Feedback pin removed from output and connected to 0V Note 6 Feedback pin removed from output and connected to a 12V for the Adjustable 3 3V and 5V versions and a 25V for the 12V and 15V versions to force the output transistor OFF Note 7 VIN e 40V (60V for high voltage version) Note 8 Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads Additional copper area will lower thermal resistance further See application hints in this data sheet and the thermal model in Switchers Made Simple software Note 9 Junction to ambient thermal resistance with approximately 4 square inches of 1 oz (0 0014 in thick) printed circuit board copper surrounding the leads Additional copper area will lower thermal resistance further (See Note 8 ) Note 10 The oscillator frequency reduces to approximately 18 kHz in the event of an output short or an overload which causes the regulated output voltage to drop approximately 40% from the nominal output voltage This self protection feature lowers the average power dissipation of the IC by lowering the minimum duty cycle from 5% down to approximately 2%
Typical Performance Characteristics (Circuit of Figure 2 )
Normalized Output Voltage Line Regulation Dropout Voltage
Current Limit
Supply Current
Standby Quiescent Current
TL H 11394 - 17
5
Typical Performance Characteristics (Circuit of Figure 2 ) (Continued)
Oscillator Frequency Switch Saturation Voltage Efficiency
Minimum Operating Voltage
Supply Current vs Duty Cycle
Feedback Voltage vs Duty Cycle
TL H 11394 - 4
Feedback Pin Current
Junction to Ambient Thermal Resistance
TL H 11394 - 5
6
Typical Performance Characteristics (Circuit of Figure 2 ) (Continued)
Continuous Mode Switching Waveforms VOUT e 5V 500 mA Load Current L e 330 mH Discontinuous Mode Switching Waveforms VOUT e 5V 100 mA Load Current L e 100 mH
TL H 11394 - 6
TL H 11394 - 7
A Output Pin Voltage 10V div B Inductor Current 0 2 A div C Output Ripple Voltage 20 mV div AC-Coupled Horizontal Time Base 5ms div
A Output Pin Voltage 10V div B Inductor Current 0 2 A div C Output Ripple Voltage 20 mV div AC-Coupled Horizontal Time Base 5 ms div
500 mA Load Transient Response for Continuous Mode Operation L e 330 mH COUT e 300 mF
250 mA Load Transient Response for Discontinuous Mode Operation L e 68 mH COUT e 470 mF
TL H 11394 - 8
A Output Voltage 50 mV div AC Coupled B 100 mA to 500 mA Load Pulse Horizontal Time Base 200 ms div
TL H 11394 - 9
A Output Voltage 50 mV div AC Coupled B 50 mA to 250 mA Load Pulse Horizontal Time Base 200 ms div
Block Diagram
R1 e 1k 3 3V R2 e 1 7k 5V R2 e 3 1k 12V R2 e 8 84k 15V R2 e 11 3k For Adj Version R1 e Open R2 e 0X
Note Pin numbers are for the 8-pin DIP package
FIGURE 1
TL H 11394 - 10
7
Test Circuit and Layout Guidelines
Fixed Output Voltage Versions CIN COUT D1 L1 22 mF 75V Aluminum Electrolytic 220 mF 25V Aluminum Electrolytic Schottky 11DQ06 330 mH 52627 (for 5V in 3 3V out use 100 mH RL-1284-100) 2k 0 1% 6 12k 0 1%
R1 R2
TL H 11394 - 11
Adjustable Output Voltage Version
VOUT e VREF R2 e R1
where VREF e 1 23V R1 between 1k 5k
VOUT b1 VREF
1
a
R2 R1
J
J
TL H 11394 - 12
FIGURE 2 As in any switching regulator layout is very important Rapidly switching currents associated with wiring inductance generate voltage transients which can cause problems For minimal inductance and ground loops the length of the leads indicated by heavy lines should be kept as short as possible Single-point grounding (as indicated) or ground plane construction should be used for best results When using the Adjustable version physically locate the programming resistors near the regulator to keep the sensitive feedback wiring short Inductor Value 68 mH 100 mH 150 mH 220 mH 330 mH 470 mH 680 mH 1000 mH 1500 mH 2200 mH Pulse Eng (Note 1) Renco (Note 2) RL-1284-68 RL-1284-100 RL-1284-150 RL-1284-220 RL-1284-330 RL-1284-470 RL-1283-680 RL-1283-1000 RL-1283-1500 RL-1283-2200 NPI (Note 3) NP5915 NP5916 NP5917 NP5918 5919 NP5920 5921 NP5922 NP5923
52625 52626 52627 52628 52629 52631
FIGURE 3 Inductor Selection by Manufacturer's Part Number U S Source Note 1 Pulse Engineering (619) 674-8100 P O Box 12236 San Diego CA 92112 Note 2 Renco Electronics Inc (516) 586-5566 60 Jeffryn Blvd East Deer Park NY 11729 Contact Manufacturer European Source
a 44 (0) 634 290588 Note 3 NPI APC 47 Riverside Medway City Estate Strood Rochester Kent ME2 4DP UK
Contact Manufacturer
8
LM2574 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions) Given VOUT e Regulated Output Voltage (3 3V 5V 12V or 15V) VIN(Max) e Maximum Input Voltage ILOAD(Max) e Maximum Load Current 1 Inductor Selection (L1) A Select the correct Inductor value selection guide from Figures 4 5 6 or 7 (Output voltages of 3 3V 5V 12V or 15V respectively) For other output voltages see the design procedure for the adjustable version B From the inductor value selection guide identify the inductance region intersected by VIN(Max) and ILOAD(Max) C Select an appropriate inductor from the table shown in Figure 3 Part numbers are listed for three inductor manufacturers The inductor chosen must be rated for operation at the LM2574 switching frequency (52 kHz) and for a current rating of 1 5 c ILOAD For additional inductor information see the inductor section in the Application Hints section of this data sheet 2 Output Capacitor Selection (COUT) A The value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop For stable operation and an acceptable output ripple voltage (approximately 1% of the output voltage) a value between 100 mF and 470 mF is recommended B The capacitor's voltage rating should be at least 1 5 times greater than the output voltage For a 5V regulator a rating of at least 8V is appropriate and a 10V or 15V rating is recommended Higher voltage electrolytic capacitors generally have lower ESR numbers and for this reasion it may be necessary to select a capacitor rated for a higher voltage than would normally be needed 3 Catch Diode Selection (D1) A The catch-diode current rating must be at least 1 5 times greater than the maximum load current Also if the power supply design must withstand a continuous output short the diode should have a current rating equal to the maximum current limit of the LM2574 The most stressful condition for this diode is an overload or shorted output condition B The reverse voltage rating of the diode should be at least 1 25 times the maximum input voltage 4 Input Capacitor (CIN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation EXAMPLE (Fixed Output Voltage Versions) Given VOUT e 5V VIN(Max) e 15V ILOAD(Max) e 0 4A 1 Inductor Selection (L1) A Use the selection guide shown in Figure 5 B From the selection guide the inductance area intersected by the 15V line and 0 4A line is 330 C Inductor value required is 330 mH From the table in Figure 3 choose Pulse Engineering PE-52627 Renco RL-1284-330 or NPI NP5920 5921
2
Output Capacitor Selection (COUT) A COUT e 100 mF to 470 mF standard aluminum electrolytic B Capacitor voltage rating e 20V
3
Catch Diode Selection (D1) A For this example a 1A current rating is adequate B Use a 20V 1N5817 or SR102 Schottky diode or any of the suggested fast-recovery diodes shown in Figure 9
4
Input Capacitor (CIN) A 22 mF aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing
9
LM2574 Series Buck Regulator Design Procedure (Continued)
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
TL H 11394-26
FIGURE 4 LM2574HV-3 3 Inductor Selection Guide
TL H 11394 - 13
FIGURE 5 LM2574HV-5 0 Inductor Selection Guide
TL H 11394-14
TL H 11394 - 15
FIGURE 6 LM2574HV-12 Inductor Selection Guide
FIGURE 7 LM2574HV-15 Inductor Selection Guide
TL H 11394 - 16
FIGURE 8 LM2574HV-ADJ Inductor Selection Guide
10
LM2574 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions) Given VOUT e Regulated Output Voltage VIN(Max) e Maximum Input Voltage ILOAD(Max) e Maximum Load Current F e Switching Frequency (Fixed at 52 kHz) 1 Programming Output Voltage (Selecting R1 and R2 as shown in Figure 2) Use the following formula to select the appropriate resistor values R2 VOUT e VREF 1 a where VREF e 1 23V R1 R1 can be between 1k and 5k (For best temperature coefficient and stability with time use 1% metal film resistors) VOUT b1 R2 e R1 VREF EXAMPLE (Adjustable Output Voltage Versions) Given VOUT e 24V VIN(Max) e 40V ILOAD(Max) e 0 4A F e 52 kHz 1 Programming Output Voltage (Selecting R1 and R2) VOUT e 1 23 1 a R2 e R1
J
V
VOUT
REF
R2 R1
b1
J
J
Select R1 e 1k
e 1k
1 23V 1 J
24V
b
R2 e 1k (19 51 b 1) e 18 51k closest 1% value is 18 7k
J
2
Inductor Selection (L1) A Calculate the inductor Volt microsecond constant E T (V ms) from the following formula VOUT 1000 (V ms) E T e (VIN b VOUT) VIN F (in kHz) 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 8 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 and note the inductor value for that region E Select an appropriate inductor from the table shown in Figure 3 Part numbers are listed for three inductor manufacturers The inductor chosen must be rated for operation at the LM2574 switching frequency (52 kHz) and for a current rating of 1 5 c ILOAD For additional inductor information see the inductor section in the application hints section of this data sheet Output Capacitor Selection (COUT) A The value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop For stable operation the capacitor must satisfy the following requirement VIN(Max) (mF) COUT t 13 300 VOUT L(mH) The above formula yields capacitor values between 5 mF and 1000 mF that will satisfy the loop requirements for stable operation But to achieve an acceptable output ripple voltage (approximately 1% of the output voltage) and transient response the output capacitor may need to be several times larger than the above formula yields B The capacitor's voltage rating should be at last 1 5 times greater than the output voltage For a 24V regulator a rating of at least 35V is recommended Higher voltage electrolytic capacitors generally have lower ESR numbers and for this reasion it may be necessary to select a capacitor rate for a higher voltage than would normally be needed
2
Inductor Selection (L1) A Calculate E T (V ms) E T e (40 b 24) 24 1000 e 185 V ms 40 52
B E T e 185 V ms C ILOAD(Max) e 0 4A D Inductance Region e 1000 E Inductor Value e 1000 mH Choose from Pulse Engineering Part PE-52631 or Renco Part RL-1283-1000
3
3
Output Capacitor Selection (COUT) 40 e 22 2 mF A COUT l 13 300 24 1000 However for acceptable output ripple voltage select COUT t 100 mF COUT e 100 mF electrolytic capacitor
11
LM2574 Series Buck Regulator Design Procedure (Continued)
PROCEDURE (Adjustable Output Voltage Versions) 4 Catch Diode Selection (D1) A The catch-diode current rating must be at least 1 5 times greater than the maximum load current Also if the power supply design must withstand a continuous output short the diode should have a current rating equal to the maximum current limit of the LM2574 The most stressful condition for this diode is an overload or shorted output condition Suitable diodes are shown in the selection guide of Figure 9 B The reverse voltage rating of the diode should be at least 1 25 times the maximum input voltage Input Capacitor (CIN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation 4 EXAMPLE (Adjustable Output Voltage Versions) Catch Diode Selection (D1) A For this example a 1A current rating is adequate B Use a 50V MBR150 or 11DQ05 Schottky diode or any of the suggested fast-recovery diodes in Figure 9
5
5
Input Capacitor (CIN) A 22 mF aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing VR 1 Amp Diodes Schottky 1N5817 SR102 MBR120P 1N5818 SR103 11DQ03 MBR130P 10JQ030 1N5819 SR104 11DQ04 11JQ04 MBR140P MBR150 SR105 11DQ05 11JQ05 MBR160 SR106 11DQ06 11JQ06 11DQ09 FIGURE 9 Diode Selection Guide Fast Recovery
20V
30V
40V
The following diodes are all rated to 100V 11DF1 10JF1 MUR110 HER102
50V
60V
90V
To further simplify the buck regulator design procedure National Semiconductor is making available computer design software to be used with the Simple Switcher line of switching regulators Switchers Made Simple (version 3 3) is available on a (3 ) diskette for IBM compatible computers from a National Semiconductor sales office in your area
12
Application Hints
INPUT CAPACITOR (CIN) To maintain stability the regulator input pin must be bypassed with at least a 22 mF electrolytic capacitor The capacitor's leads must be kept short and located near the regulator If the operating temperature range includes temperatures below b25 C the input capacitor value may need to be larger With most electrolytic capacitors the capacitance value decreases and the ESR increases with lower temperatures and age Paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures For maximum capacitor operating lifetime the capacitor's RMS ripple current rating should be greater than 12c constant As the load current rises or falls the entire sawtooth current waveform also rises or falls The average DC value of this waveform is equal to the DC load current (in the buck regulator configuration) If the load current drops to a low enough level the bottom of the sawtooth current waveform will reach zero and the switcher will change to a discontinuous mode of operation This is a perfectly acceptable mode of operation Any buck switching regulator (no matter how large the inductor value is) will be forced to run discontinuous if the load current is light enough The curve shown in Figure 10 illustrates how the peak-topeak inductor ripple current (DIIND) is allowed to change as different maximum load currents are selected and also how it changes as the operating point varies from the upper border to the lower border within an inductance region (see Inductor Selection guides)
TJ
tON
c ILOAD
where
tON V e OUT for a buck regulator T VIN
and
tON lVOUTl for a buck-boost regulator e T lVOUTl a VIN
INDUCTOR SELECTION All switching regulators have two basic modes of operation continuous and discontinuous The difference between the two types relates to the inductor current whether it is flowing continuously or if it drops to zero for a period of time in the normal switching cycle Each mode has distinctively different operating characteristics which can affect the regulator performance and requirements The LM2574 (or any of the Simple Switcher family) can be used for both continuous and discontinuous modes of operation In many cases the preferred mode of operation is in the continuous mode It offers better load regulation lower peak switch inductor and diode currents and can have lower output ripple voltage But it does require relatively large inductor values to keep the inductor current flowing continuously especially at low output load currents To simplify the inductor selection process an inductor selection guide (nomograph) was designed (see Figures 4 through 8 ) This guide assumes continuous mode operation and selects an inductor that will allow a peak-to-peak inductor ripple current (DIIND) to be a certain percentage of the maximum design load current In the LM2574 SIMPLE SWITCHER the peak-to-peak inductor ripple current percentage (of load current) is allowed to change as different design load currents are selected By allowing the percentage of inductor ripple current to increase for lower current applications the inductor size and value can be kept relatively low INDUCTOR RIPPLE CURRENT When the switcher is operating in the continuous mode the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage) For a given input voltage and output voltage the peakto-peak amplitude of this inductor current waveform remains
TL H 11394 - 18
FIGURE 10 Inductor Ripple Current (DIIND) Range Based on Selection Guides from Figures 4 -8 Consider the following example VOUT e 5V 0 4A VIN e 10V minimum up to 20V maximum The selection guide in Figure 5 shows that for a 0 4A load current and an input voltage range between 10V and 20V the inductance region selected by the guide is 330 mH This value of inductance will allow a peak-to-peak inductor ripple current (DIIND) to flow that will be a percentage of the maximum load current For this inductor value the DIIND will also vary depending on the input voltage As the input voltage increases to 20V it approaches the upper border of the inductance region and the inductor ripple current increases Referring to the curve in Figure 10 it can be seen that at the 0 4A load current level and operating near the upper border of the 330 mH inductance region the DIIND will be 53% of 0 4A or 212 mA p-p This DIIND is important because from this number the peak inductor current rating can be determined the minimum load current required before the circuit goes to discontinuous operation and also knowing the ESR of the output capacitor the output ripple voltage can be calculated or conversely measuring the output ripple voltage and knowing the DIIND the ESR can be calculated
13
Application Hints (Continued)
From the previous example the Peak-to-peak Inductor Ripple Current (DIIND) e 212 mA p-p Once the DIND value is known the following three formulas can be used to calculate additional information about the switching regulator circuit 1 Peak Inductor or peak switch current
e
I
LOAD a
DIIND 2
J 0 4A
e
a
212 2
J
e 506 mA
(DIIND) See the section on inductor ripple current in Application Hints The lower capacitor values (100 mF- 330 mF) will allow typically 50 mV to 150 mV of output ripple voltage while largervalue capacitors will reduce the ripple to approximately 20 mV to 50 mV Output Ripple Voltage e (DIIND) (ESR of COUT) To further reduce the output ripple voltage several standard electrolytic capacitors may be paralleled or a higher-grade capacitor may be used Such capacitors are often called ``high-frequency '' ``low-inductance '' or ``low-ESR '' These will reduce the output ripple to 10 mV or 20 mV However when operating in the continuous mode reducing the ESR below 0 03X can cause instability in the regulator Tantalum capacitors can have a very low ESR and should be carefully evaluated if it is the only output capacitor Because of their good low temperature characteristics a tantalum can be used in parallel with aluminum electrolytics with the tantalum making up 10% or 20% of the total capacitance The capacitor's ripple current rating at 52 kHz should be at least 50% higher than the peak-to-peak inductor ripple current CATCH DIODE Buck regulators require a diode to provide a return path for the inductor current when the switch is off This diode should be located close to the LM2574 using short leads and short printed circuit traces Because of their fast switching speed and low forward voltage drop Schottky diodes provide the best efficiency especially in low output voltage switching regulators (less than 5V) Fast-Recovery High-Efficiency or Ultra-Fast Recovery diodes are also suitable but some types with an abrupt turnoff characteristic may cause instability and EMI problems A fast-recovery diode with soft recovery characteristics is a better choice Standard 60 Hz diodes (e g 1N4001 or 1N5400 etc ) are also not suitable See Figure 9 for Schottky and ``soft'' fast-recovery diode selection guide OUTPUT VOLTAGE RIPPLE AND TRANSIENTS The output voltage of a switching power supply will contain a sawtooth ripple voltage at the switcher frequency typically about 1% of the output voltage and may also contain short voltage spikes at the peaks of the sawtooth waveform The output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by the ESR of the output capacitor (See the inductor selection in the application hints ) The voltage spikes are present because of the the fast switching action of the output switch and the parasitic inductance of the output filter capacitor To minimize these voltage spikes special low inductance capacitors can be used and their lead lengths must be kept short Wiring inductance stray capacitance as well as the scope probe used to evaluate these transients all contribute to the amplitude of these spikes An additional small LC filter (20 mH 100 mF) can be added to the output (as shown in Figure 16 ) to further reduce the amount of output ripple and transients A 10 c reduction in output ripple voltage and transients is possible with this filter
2 Mimimum load current before the circuit becomes discontinuous DIIND 212 e e 106 mA 2 2 3 Output Ripple Voltage e (DIIND) c (ESR of COUT) The selection guide chooses inductor values suitable for continuous mode operation but if the inductor value chosen is prohibitively high the designer should investigate the possibility of discontinuous operation The computer design software Switchers Made Simple will provide all component values for discontinuous (as well as continuous) mode of operation Inductors are available in different styles such as pot core toroid E-frame bobbin core etc as well as different core materials such as ferrites and powdered iron The least expensive the bobbin core type consists of wire wrapped on a ferrite rod core This type of construction makes for an inexpensive inductor but since the magnetic flux is not completely contained within the core it generates more electromagnetic interference (EMI) This EMl can cause problems in sensitive circuits or can give incorrect scope readings because of induced voltages in the scope probe The inductors listed in the selection chart include powdered iron toroid for Pulse Engineering and ferrite bobbin core for Renco An inductor should not be operated beyond its maximum rated current because it may saturate When an inductor begins to saturate the inductance decreases rapidly and the inductor begins to look mainly resistive (the DC resistance of the winding) This can cause the inductor current to rise very rapidly and will affect the energy storage capabilities of the inductor and could cause inductor overheating Different inductor types have different saturation characteristics and this should be kept in mind when selecting an inductor The inductor manufacturers' data sheets include current and energy limits to avoid inductor saturation
e
OUTPUT CAPACITOR An output capacitor is required to filter the output voltage and is needed for loop stability The capacitor should be located near the LM2574 using short pc board traces Standard aluminum electrolytics are usually adequate but low ESR types are recommended for low output ripple voltage and good stability The ESR of a capacitor depends on many factors some which are the value the voltage rating physical size and the type of construction In general low value or low voltage (less than 12V) electrolytic capacitors usually have higher ESR numbers The amount of output ripple voltage is primarily a function of the ESR (Equivalent Series Resistance) of the output capacitor and the amplitude of the inductor ripple current
14
Application Hints (Continued)
FEEDBACK CONNECTION The LM2574 (fixed voltage versions) feedback pin must be wired to the output voltage point of the switching power supply When using the adjustable version physically locate both output voltage programming resistors near the LM2574 to avoid picking up unwanted noise Avoid using resistors greater than 100 kX because of the increased chance of noise pickup ON OFF INPUT For normal operation the ON OFF pin should be grounded or driven with a low-level TTL voltage (typically below 1 6V) To put the regulator into standby mode drive this pin with a high-level TTL or CMOS signal The ON OFF pin can be safely pulled up to a VIN without a resistor in series with it The ON OFF pin should not be left open GROUNDING The 8-pin molded DIP and the 14-pin surface mount package have separate power and signal ground pins Both ground pins should be soldered directly to wide printed circuit board copper traces to assure low inductance connections and good thermal properties THERMAL CONSIDERATIONS The 8-pin DIP (N) package and the 14-pin Surface Mount (M) package are molded plastic packages with solid copper lead frames The copper lead frame conducts the majority of the heat from the die through the leads to the printed circuit board copper which acts as the heat sink For best thermal performance wide copper traces should be used and all ground and unused pins should be soldered to generous amounts of printed circuit board copper such as a ground plane Large areas of copper provide the best transfer of heat (lower thermal resistance) to the surrounding air and even double-sided or multilayer boards provide better heat paths to the surrounding air Unless the power levels are small using a socket for the 8-pin package is not recommended because of the additional thermal resistance it introduces and the resultant higher junction temperature Because of the 0 5A current rating of the LM2574 the total package power dissipation for this switcher is quite low ranging from approximately 0 1W up to 0 75W under varying conditions In a carefully engineered printed circuit board both the N and the M package can easily dissipate up to 0 75W even at ambient temperatures of 60 C and still keep the maximum junction temperature below 125 C A curve displaying thermal resistance vs pc board area for the two packages is shown in the Typical Performance Characteristics curves section of this data sheet These thermal resistance numbers are approximate and there can be many factors that will affect the final thermal resistance Some of these factors include board size shape thickness position location and board temperature Other factors are the area of printed circuit copper copper thickness trace width multi-layer single- or double-sided and the amount of solder on the board The effectiveness of the pc board to dissipate heat also depends on the size number and spacing of other components on the board Furthermore some of these components such as the catch diode and inductor will generate some additional heat Also the thermal resistance decreases as the power level increases because of the increased air current activity at the higher power levels and the lower surface to air resistance coefficient at higher temperatures The data sheet thermal resistance curves and the thermal model in Switchers Made Simple software (version 3 3) can estimate the maximum junction temperature based on operating conditions ln addition the junction temperature can be estimated in actual circuit operation by using the following equation Tj e Tcu a (ij-cu c PD) With the switcher operating under worst case conditions and all other components on the board in the intended enclosure measure the copper temperature (Tcu ) near the IC This can be done by temporarily soldering a small thermocouple to the pc board copper near the IC or by holding a small thermocouple on the pc board copper using thermal grease for good thermal conduction The thermal resistance (ij-cu) for the two packages is ij-cu e 42 C W for the N-8 package ij-cu e 52 C W for the M-14 package The power dissipation (PD) for the IC could be measured or it can be estimated by using the formula PD e (VIN) (IS) a
Where IS is obtained from the typical supply current curve (adjustable version use the supply current vs duty cycle curve)
V J (I
VO
IN
LOAD) (VSAT)
Additional Applications
INVERTING REGULATOR Figure 11 shows a LM2574-12 in a buck-boost configuration to generate a negative 12V output from a positive input voltage This circuit bootstraps the regulator's ground pin to the negative output voltage then by grounding the feedback pin the regulator senses the inverted output voltage and regulates it to b12V
Note Pin numbers are for the 8-pin DIP package
TL H 11394 - 19
FIGURE 11 Inverting Buck-Boost Develops b12V 15
Additional Applications (Continued)
For an input voltage of 8V or more the maximum available output current in this configuration is approximately 100 mA At lighter loads the minimum input voltage required drops to approximately 4 7V The switch currents in this buck-boost configuration are higher than in the standard buck-mode design thus lowering the available output current Also the start-up input current of the buck-boost converter is higher than the standard buck-mode regulator and this may overload an input power source with a current limit less than 0 6A Using a delayed turn-on or an undervoltage lockout circuit (described in the next section) would allow the input voltage to rise to a high enough level before the switcher would be allowed to turn on Because of the structural differences between the buck and the buck-boost regulator topologies the buck regulator design procedure section can not be used to to select the inductor or the output capacitor The recommended range of inductor values for the buck-boost design is between 68 mH and 220 mH and the output capacitor values must be larger than what is normally required for buck designs Low input voltages or high output currents require a large value output capacitor (in the thousands of micro Farads) The peak inductor current which is the same as the peak switch current can be calculated from the following formula VV 1 ILOAD (VIN a lVOl) a IN l Ol c VIN VIN a lVOl 2L1 fosc Where fosc e 52 kHz Under normal continuous inductor current operating conditions the minimum VIN represents the worst case Select an inductor that is rated for the peak current anticipated Also the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage For a b 12V output the maximum input voltage for the LM2574 is a 28V or a 48V for the LM2574HV The Switchers Made Simple (version 3 3) design software can be used to determine the feasibility of regulator designs using different topologies different input-output parameters different components etc Ip NEGATIVE BOOST REGULATOR Another variation on the buck-boost topology is the negative boost configuration The circuit in Figure 12 accepts an input voltage ranging from b5V to b12V and provides a regulated b12V output Input voltages greater than b12V will cause the output to rise above b12V but will not damage the regulator
TL H 11394 - 22
Because of the boosting function of this type of regulator the switch current is relatively high especially at low input voltages Output load current limitations are a result of the maximum current rating of the switch Also boost regulators can not provide current limiting load protection in the event of a shorted load so some other means (such as a fuse) may be necessary UNDERVOLTAGE LOCKOUT In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold An undervoltage lockout circuit which accomplishes this task is shown in Figure 13 while Figure 14 shows the same circuit applied to a buck-boost configuration These circuits keep the regulator off until the input voltage reaches a predetermined level VTH VZ1 a 2VBE (Q1)
TL H 11394 - 21
Note Complete circuit not shown Note Pin numbers are for 8-pin DIP package
FIGURE 13 Undervoltage Lockout for Buck Circuit
Note Complete circuit not shown (see Figure 11) Note Pin numbers are for 8-pin DIP package
FIGURE 14 Undervoltage Lockout for Buck-Boost Circuit
TL H 11394-20
Note Pin numbers are for 8-pin DIP package
FIGURE 12 Negative Boost 16
Additional Applications (Continued)
DELAYED STARTUP The ON OFF pin can be used to provide a delayed startup feature as shown in Figure 15 With an input voltage of 20V and for the part values shown the circuit provides approximately 10 ms of delay time before the circuit begins switching Increasing the RC time constant can provide longer delay times But excessively large RC time constants can cause problems with input voltages that are high in 60 Hz or 120 Hz ripple by coupling the ripple into the ON OFF pin ADJUSTABLE OUTPUT LOW-RIPPLE POWER SUPPLY A 500 mA power supply that features an adjustable output voltage is shown in Figure 16 An additional L-C filter that reduces the output ripple by a factor of 10 or more is included in this circuit
TL H 11394 - 23
Note Complete circuit not shown Note Pin numbers are for 8-pin DIP package
FIGURE 15 Delayed Startup
TL H 11394 - 24
Note Pin numbers are for 8-pin DIP package
FIGURE 16 1 2V to 55V Adjustable 500 mA Power Supply with Low Output Ripple
Definition of Terms
BUCK REGULATOR A switching regulator topology in which a higher voltage is converted to a lower voltage Also known as a step-down switching regulator BUCK-BOOST REGULATOR A switching regulator topology in which a positive voltage is converted to a negative voltage without a transformer DUTY CYCLE (D) Ratio of the output switch's on-time to the oscillator period
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR) The purely resistive component of a real capacitor's impedance (see Figure 17 ) It causes power loss resulting in capacitor heating which directly affects the capacitor's operating lifetime When used as a switching regulator output filter higher ESR values result in higher output ripple voltages
TL H 11394 - 25
for buck regulator
tON V e OUT De T VIN tON lVOl e De T lVOl a VIN
FIGURE 17 Simple Model of a Real Capacitor Most standard aluminum electrolytic capacitors in the 100 mF - 1000 mF range have 0 5X to 0 1X ESR Highergrade capacitors (``low-ESR'' ``high-frequency'' or ``low-inductance''') in the 100 mF - 1000 mF range generally have ESR of less than 0 15X EQUIVALENT SERIES INDUCTANCE (ESL) The pure inductance component of a capacitor (see Figure 17 ) The amount of inductance is determined to a large extent on the capacitor's construction In a buck regulator this unwanted inductance causes voltage spikes to appear on the output
for buck-boost regulator
CATCH DIODE OR CURRENT STEERING DIODE The diode which provides a return path for the load current when the LM2574 switch is OFF EFFICIENCY (h) The proportion of input power actually delivered to the load POUT POUT e he PIN POUT a PLOSS
17
Definition of Terms (Continued)
OUTPUT RIPPLE VOLTAGE The AC component of the switching regulator's output voltage It is usually dominated by the output capacitor's ESR multiplied by the inductor's ripple current (DIIND) The peakto-peak value of this sawtooth ripple current can be determined by reading the Inductor Ripple Current section of the Application hints CAPACITOR RIPPLE CURRENT RMS value of the maximum allowable alternating current at which a capacitor can be operated continuously at a specified temperature STANDBY QUIESCENT CURRENT (ISTBY) Supply current required by the LM2574 when in the standby mode (ON OFF pin is driven to TTL-high voltage thus turning the output switch OFF) INDUCTOR RIPPLE CURRENT (DIIND) The peak-to-peak value of the inductor current waveform typically a sawtooth waveform when the regulator is operating in the continuous mode (vs discontinuous mode) CONTINUOUS DISCONTINUOUS MODE OPERATION Relates to the inductor current In the continuous mode the inductor current is always flowing and never drops to zero vs the discontinuous mode where the inductor current drops to zero for a period of time in the normal switching cycle INDUCTOR SATURATION The condition which exists when an inductor cannot hold any more magnetic flux When an inductor saturates the inductor appears less inductive and the resistive component dominates Inductor current is then limited only by the DC resistance of the wire and the available source current OPERATING VOLT MICROSECOND CONSTANT (ETop) The product (in VoItms) of the voltage applied to the inductor and the time the voltage is applied This ETop constant is a measure of the energy handling capability of an inductor and is dependent upon the type of core the core area the number of turns and the duty cycle
18
Physical Dimensions inches (millimeters)
14-Lead Wide Surface Mount (WM) Order Number LM2574M-3 3 LM2574HVM-3 3 LM2574M-5 0 LM2574HVM-5 0 LM2574M-12 LM2574HVM-12 LM2574M-15 LM2574HVM-15 LM2574M-ADJ or LM2574HVM-ADJ NS Package Number M14B
19
LM2574 LM2574HV Series SIMPLE SWITCHER 0 5A Step-Down Voltage Regulator
Physical Dimensions inches (millimeters) (Continued)
8-Lead DIP (N) Order Number LM2574M-3 3 LM2574HVM-3 3 LM2574HVN-5 0 LM2574HVN-12 LM2574HVN-15 LM2574HVN-ADJ LM2574N-5 0 LM2574N-12 LM2574N-15 or LM2574N-ADJ NS Package Number N08A
LIFE SUPPORT POLICY NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems which (a) are intended for surgical implant into the body or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user
National Semiconductor Corporation 2900 Semiconductor Drive P O Box 58090 Santa Clara CA 95052-8090 Tel 1(800) 272-9959 TWX (910) 339-9240 National Semiconductor GmbH Livry-Gargan-Str 10 D-82256 F4urstenfeldbruck Germany Tel (81-41) 35-0 Telex 527649 Fax (81-41) 35-1 National Semiconductor Japan Ltd Sumitomo Chemical Engineering Center Bldg 7F 1-7-1 Nakase Mihama-Ku Chiba-City Ciba Prefecture 261 Tel (043) 299-2300 Fax (043) 299-2500
2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness
National Semiconductor Hong Kong Ltd 13th Floor Straight Block Ocean Centre 5 Canton Rd Tsimshatsui Kowloon Hong Kong Tel (852) 2737-1600 Fax (852) 2736-9960
National Semiconductores Do Brazil Ltda Rue Deputado Lacorda Franco 120-3A Sao Paulo-SP Brazil 05418-000 Tel (55-11) 212-5066 Telex 391-1131931 NSBR BR Fax (55-11) 212-1181
National Semiconductor (Australia) Pty Ltd Building 16 Business Park Drive Monash Business Park Nottinghill Melbourne Victoria 3168 Australia Tel (3) 558-9999 Fax (3) 558-9998
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

▲Up To Search▲

Price & Availability of LM2574

	To Download LM2574 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .