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LT1424-5

sn14245 14245fs

Isolated Flyback

Switching Regulator

with 5V Output

■

No Transformer “Third Winding” or Optoisolator

Required

■

Designed for Use with 1:1 Ratio Transformers

■

Fixed, Application Specific 5V Output Voltage

■

Regulation Maintained Well into Discontinuous

Mode (Light Load)

■

Load Compensation Provides Excellent

Load Regulation

■

Available in 8-Pin PDIP and SO Packages

■

Operating Frequency: 285kHz

The LT

1424-5 is a monolithic high power switching

regulator specifically designed for the isolated flyback

topology. No “third winding” or optoisolator is required;

the integrated circuit senses the isolated output voltage

directly from the primary side flyback waveform. A high

current, high efficiency switch is included on the die along

with all oscillator, control and protection circuitry.

The LT1424-5 operates with input supply voltages from

3V to 20V and draws only 7mA quiescent current. It can

deliver up to 400mA at 5V with no external power devices.

By utilizing current mode switching techniques, it pro-

vides excellent AC and DC line regulation.

The LT1424-5 has a number of features not found on other

switching regulator ICs. Its unique control circuitry can

maintain regulation well into discontinuous mode. Load

compensation circuitry allows for improved load regula-

tion. An externally activated shutdown mode reduces total

supply current to 20µA typical for standby operation.

, LTC and LT are registered trademarks of Linear Technology Corporation.

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Isolated Communication Supplies

■

Industrial Automation

■

Instrumentation Systems

5V Output Isolated Power Supply

SHDN

LT1424-5

SYNC

SGND

1N5248

ISOLATION

BARRIER

MBRS130LT3

•

47Ω

1424-5 TA01

1.8k

MBR0540T4

0.1µF

C1, C2: AVX TPS D107M010R0080

T1: DALE LPE-4841-A307

330pF

0.1µF

100µF

10V

1000pF

INPUT

COM

CCOMP

PGND

•

100µF

10V

400mA

OUT

COM

OUTPUT CURRENT (mA)

OUTPUT VOLTAGE (V)

4.75

5.00

5.25

100 200

1424-5 TA02

300 400

Load Regulation

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (16 pages)

PAGE 1
LT1424-5 Isolated Flyback Switching Regulator with 5V Output U FEATURES ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT ®1424-5 is a monolithic high power switching regulator specifically designed for the isolated flyback topology. No “third winding” or optoisolator is required; the integrated circuit senses the isolated output voltage directly from the primary side flyback waveform. A high current, high efficiency switch is included on the die along with all oscillator, control and protection circuitry.
PAGE 2
LT1424-5 W U U U W W W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) Supply Voltage (VIN) ................................................ 20V Switch Voltage (VSW) .............................................. 35V SHDN, SYNC Pin Voltage ........................................... 7V Operating Junction Temperature Range Commercial .......................................... 0°C to 125°C Industrial ......................................... – 40°C to 125°C Storage Temperature Range ....
PAGE 3
LT1424-5 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VSW Open, VC = 1.4V, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 260 240 285 285 300 320 kHz kHz 170 200 260 ns Timing f Switching Frequency ● tON Minimum Switch ON Time tED Flyback Enable Delay Time 150 ns tEN Minimum Flyback Enable Time 180 ns 90 % 0.
PAGE 4
LT1424-5 U W TYPICAL PERFOR A CE CHARACTERISTICS Switch Saturation Voltage vs Switch Current Switch Current Limit vs Duty Cycle Minimum Input Voltage vs Temperature 2.0 1.2 3.1 3.0 125°C 0.8 25°C 0.6 –55°C 0.4 1.5 INPUT VOLTAGE (V) 1.0 SWITCH CURRENT LIMIT (A) SWITCH SATURATION VOLTAGE (V) TA = 25°C 1.0 0.5 0.2 0 0 0.2 0.4 0.6 0.8 1.0 SWITCH CURRENT (A) 1.2 0 1.4 2.7 2.6 0 5.32 5.30 4.28 4.26 50 25 75 0 TEMPERATURE (°C) 100 125 60 40 0 –20 –40 –60 1.
PAGE 5
LT1424-5 U W TYPICAL PERFOR A CE CHARACTERISTICS Flyback Enable Delay Time vs Temperature Minimum Flyback Enable Time vs Temperature 250 275 250 225 250 200 225 225 200 175 150 125 ENABLE TIME (ns) 275 ENABLE DELAY TIME (ns) SWITCH ON TIME (ns) Minimum Switch On Time vs Temperature 175 150 125 100 100 – 50 – 25 50 25 75 0 TEMPERATURE (°C) 100 125 75 – 50 – 25 200 175 150 125 50 25 75 0 TEMPERATURE (°C) 1424-5 G10 100 125 1424-5 G11 100 – 50 – 25 50 25 75 0 TEMPERATURE (°C) 100
PAGE 6
LT1424-5 W BLOCK DIAGRAM VIN RFB 2.
PAGE 7
LT1424-5 WU W TI I G DIAGRA VSW VOLTAGE COLLAPSE DETECT VFLBK 0.
PAGE 8
LT1424-5 U OPERATION The LT1424-5 is a current mode switching regulator IC that has been designed specifically for the isolated flyback topology. The special problem normally encountered in such circuits is that information relating to the output voltage on the isolated secondary side of the transformer must be communicated to the primary side in order to maintain regulation. Historically, this has been done with optoisolators or extra transformer windings.
PAGE 9
LT1424-5 U OPERATION Combination with the previous VFLBK expression yields an expression for VOUT, in terms of the internal reference, programming resistors, transformer turns ratio and diode forward voltage drop: VOUT = VBG ) )) ) RFB RREF NSP α – VF – ISEC (ESR) Additionally, it includes the effect of nonzero secondary output impedance. See Load Compensation for details. The practical aspects of applying this equation for VOUT are found in the Applications Information section.
PAGE 10
LT1424-5 U OPERATION time. Certain parameters of flyback amp behavior will then be directly affected by the variable enable period. These include effective transconductance and VC node slew rate. LOAD COMPENSATION THEORY The LT1424-5 uses the flyback pulse to obtain information about the isolated output voltage. A potential error source is caused by transformer secondary current flow through the real life nonzero impedances of the output rectifier, transformer secondary and output capacitor.
PAGE 11
LT1424-5 U W U U APPLICATIONS INFORMATION The LT1424-X is an application-specific 8-pin part which implements an isolated flyback switcher/controller. Three on-chip thin-film resistors are used to “program” the part for a specific application including mainly desired output voltage, transformer turns ratio and secondary circuit ESR behavior. As of Initial Release, two versions of the LT1424 are available.
PAGE 12
LT1424-5 U U W U APPLICATIONS INFORMATION pulse extends beyond the flyback pulse, loss of regulation will occur. The onset of this condition can be determined by setting the width of the flyback pulse equal to the sum of the flyback enable delay, tED, plus the minimum enable time, tEN.
PAGE 13
LT1424-5 U W U U APPLICATIONS INFORMATION power paths (primary and secondary). B field (magnetic) radiation is minimized by keeping output diode, switch pin and output bypass capacitor leads as short as possible. E field radiation is kept low by minimizing the length and area of all traces connected to the switch pin. A ground plane should always be used under the switcher circuitry to prevent interplane coupling.
PAGE 14
LT1424-5 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.400* (10.160) MAX 8 7 6 5 1 2 3 4 0.255 ± 0.015* (6.477 ± 0.381) 0.300 – 0.325 (7.620 – 8.255) 0.009 – 0.015 (0.229 – 0.381) ( +0.035 0.325 –0.015 8.255 +0.889 –0.381 ) 0.045 – 0.065 (1.143 – 1.651) 0.130 ± 0.005 (3.302 ± 0.127) 0.065 (1.651) TYP 0.100 ± 0.010 (2.540 ± 0.254) 0.125 (3.175) 0.020 MIN (0.508) MIN 0.018 ± 0.003 (0.
PAGE 15
LT1424-5 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. S8 Package 8-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.189 – 0.197* (4.801 – 5.004) 8 7 6 5 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 0.053 – 0.069 (1.346 – 1.752) 0°– 8° TYP 0.016 – 0.050 0.406 – 1.270 0.014 – 0.019 (0.355 – 0.483) 2 3 4 0.004 – 0.010 (0.101 – 0.254) 0.050 (1.
PAGE 16
LT1424-5 U TYPICAL APPLICATION Triple Isolated 5V Supply MBR0530 5V T1 + 22µF 35V 6 0.1µF 1 2 1nF 3 4 8 SHDN RCCOMP 7 VC VIN LT1424-5 6 VSW SYNC 5 PGND SGND • • 12 3 + 9 100µF 10V 1k 1N752A 5V 0.12A MBR0530 0.1µF INPUT COM 5 • • 10 2 T1: COILTRONICS CTX02-13835 ER11/5 N = 1:1:1:1:1:1 LP = 27.4µH + 7 100µF 10V 1k 1N752A 5V 0.12A MBR0530 4 1 • + 8 • 11 100µF 10V 1k 1N752A 5V 0.