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Additional Applications

(Continued)

UNDERVOLTAGE LOCKOUT

In some applications it is desirable to keep the regulator off
until the input voltage reaches a certain threshold. An und-
ervoltage lockout circuit which accomplishes this task is
shown in

Figure 12

while

Figure 13

shows the same circuit

applied to a buck-boost configuration. These circuits keep
the regulator off until the input voltage reaches a predeter-
mined level.

V

TH

V

Z1

+ 2V

BE

(Q1)

DELAYED STARTUP

The ON /OFF pin can be used to provide a delayed startup
feature as shown in

Figure 14

With an input voltage of 20V

and for the part values shown, the circuit provides approxi-
mately 10 ms of delay time before the circuit begins switch-
ing. 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 1A power supply that features an adjustable output voltage
is shown in

Figure 15

An additional L-C filter that reduces

the output ripple by a factor of 10 or more is included in this
circuit.

01147516

Typical Load Current

200 mA for V

IN

= −5.2V

500 mA for V

IN

= −7V

Note:

Pin numbers are for TO-220 package.

FIGURE 11. Negative Boost

01147517

Note:

Complete circuit not shown.

Note:

Pin numbers are for the TO-220 package.

FIGURE 12. Undervoltage Lockout for Buck Circuit

01147518

Note:

Complete circuit not shown (see

Figure 10

).

Note:

Pin numbers are for the TO-220 package.

FIGURE 13. Undervoltage Lockout

for Buck-Boost Circuit

01147519

Note:

Complete circuit not shown.

Note:

Pin numbers are for the TO-220 package.

FIGURE 14. Delayed Startup

LM1575/LM2575/LM2575HV

www.national.com

21

Summary of Contents for LM1575 Series

Page 1: ...d 10 on the oscillator frequency External shutdown is included featuring 50 A typical standby cur rent The output switch includes cycle by cycle current limit ing as well as thermal shutdown for full...

Page 2: ...red Leads 5 Lead TO 220 T 01147522 Top View LM2575T XX or LM2575HVT XX See NS Package Number T05A 01147523 Top View 01147524 Side View LM2575T XX Flow LB03 or LM2575HVT XX Flow LB03 See NS Package Num...

Page 3: ...Side View LM2575S XX or LM2575HVS XX See NS Package Number TS5B Ordering Information Package NSC Standard High Temperature Type Package Voltage Rating Voltage Rating Range Number 40V 60V 5 Lead TO 22...

Page 4: ...LM2575HVN ADJ 24 Pin M24B LM2575M 5 0 LM2575HVM 5 0 Surface Mount LM2575M 12 LM2575HVM 12 LM2575M 15 LM2575HVM 15 LM2575M ADJ LM2575HVM ADJ 5 Lead TO 263 TS5B LM2575S 3 3 LM2575HVS 3 3 Surface Mount L...

Page 5: ...UT Output Voltage VIN 12V ILOAD 0 2A 3 3 V Circuit of Figure 2 3 267 3 234 V Min 3 333 3 366 V Max VOUT Output Voltage 4 75V VIN 40V 0 2A ILOAD 1A 3 3 V LM1575 LM2575 Circuit of Figure 2 3 200 3 168 3...

Page 6: ...4 11 52 11 52 11 40 V Min Circuit of Figure 2 12 36 12 48 12 48 12 60 V Max VOUT Output Voltage 0 2A ILOAD 1A 12 V LM2575HV 15V VIN 60V 11 64 11 52 11 52 11 40 V Min Circuit of Figure 2 12 42 12 54 12...

Page 7: ...rical Characteristics Specifications with standard type face are for TJ 25 C and those with boldface type apply over full Operating Tempera ture Range Unless otherwise specified VIN 12V for the 3 3V 5...

Page 8: ...tor input and output capacitors can affect switching regulator system performance When the LM1575 LM2575 is used as shown in the Figure 2 test circuit system performance will be as shown in system par...

Page 9: ...s Circuit of Figure 2 Normalized Output Voltage Line Regulation 01147532 01147533 Dropout Voltage Current Limit 01147534 01147535 Quiescent Current Standby Quiescent Current 01147536 01147537 LM1575 L...

Page 10: ...Continued Oscillator Frequency Switch Saturation Voltage 01147538 01147539 Efficiency Minimum Operating Voltage 01147540 01147541 Quiescent Current vs Duty Cycle Feedback Voltage vs Duty Cycle 011475...

Page 11: ...Layout Guidelines As in any switching regulator layout is very important Rap idly switching currents associated with wiring inductance generate voltage transients which can cause problems For minimal...

Page 12: ...tic COUT 330 F 25V Aluminum Electrolytic D1 Schottky 11DQ06 L1 330 H PE 52627 for 5V in 3 3V out use 100 H PE 92108 Adjustable Output Voltage Version 01147509 where VREF 1 23V R1 between 1k and 5k R1...

Page 13: ...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 F and 470 F is recommended...

Page 14: ...For Continuous Mode Operation 01147510 FIGURE 3 LM2575 HV 3 3 01147511 FIGURE 4 LM2575 HV 5 0 01147512 FIGURE 5 LM2575 HV 12 01147513 FIGURE 6 LM2575 HV 15 01147514 FIGURE 7 LM2575 HV ADJ LM1575 LM25...

Page 15: ...Inductor Value 470 H Choose from AIE part 430 0634 Pulse Engineering part PE 53118 or Renco part RL 1961 3 Output Capacitor Selection COUT A The value of the output capacitor together with the induct...

Page 16: ...Capacitor CIN A 100 F aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing To further simplify the buck regulator design procedure National Semiconducto...

Page 17: ...E 92108 RL2444 L150 150 H 67127010 PE 53113 RL1954 L220 220 H 67127020 PE 52626 RL1953 L330 330 H 67127030 PE 52627 RL1952 L470 470 H 67127040 PE 53114 RL1951 L680 680 H 67127050 PE 52629 RL1950 H150...

Page 18: ...eadings because of induced voltages in the scope probe The inductors listed in the selection chart include ferrite pot core construction for AIE powdered iron toroid for Pulse Engineering and ferrite...

Page 19: ...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...

Page 20: ...sing 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 Bec...

Page 21: ...cessively 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...

Page 22: ...less than 0 15 EQUIVALENT SERIES INDUCTANCE ESL The pure inductance component of a capacitor see Figure 16 The amount of inductance is determined to a large extent on the capacitor s construction In a...

Page 23: ...y more magnetic flux When an inductor saturates the induc tor appears less inductive and the resistive component domi nates Inductor current is then limited only by the DC resis tance of the wire and...

Page 24: ...5J 3 3 883 LM1575J 5 0 883 LM1575J 12 883 LM1575J 15 883 or LM1575J ADJ 883 NS Package Number J16A 14 Lead Wide Surface Mount WM Order Number LM2575M 5 0 LM2575HVM 5 0 LM2575M 12 LM2575HVM 12 LM2575M...

Page 25: ...5 0 LM2575N 12 LM2575HVN 12 LM2575N 15 LM2575HVN 15 LM2575N ADJ or LM2575HVN ADJ NS Package Number N16A 5 Lead TO 220 T Order Number LM2575T 3 3 LM2575HVT 3 3 LM2575T 5 0 LM2575HVT 5 0 LM2575T 12 LM25...

Page 26: ...oted Continued TO 263 Molded 5 Lead Surface Mount Order Number LM2575S 3 3 LM2575HVS 3 3 LM2575S 5 0 LM2575HVS 5 0 LM2575S 12 LM2575HVS 12 LM2575S 15 LM2575HVS 15 LM2575S ADJ or LM2575HVS ADJ NS Packa...

Page 27: ...easonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness BANNED SUBSTANCE COMPLIANCE National Semiconductor certifies that the products and...

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