Linear Technology LT1425 Datasheet

FEATURES

OUTPUT CURRENT (mA)

0

OUTPUT VOLTAGE (V)

8.5

9.0

8.9

8.8

8.7

8.6

9.5

9.4

9.3

9.2

9.1

50 100

1425 TA02

150 200

■

No Transformer “Third Winding” or Optoisolator
Required

■

±5% Accurate Output Voltage Without User Trims
(See Circuit Below)

■

Resistor Programmable Output Voltage

■

Regulation Maintained Well Into Discontinuous
Mode (Light Load)

■

Optional Load Compensation

■

Operating Frequency: 285kHz

■

Easily Synchronized to External Clock

■

Available in 16-Pin Narrow SO Package

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APPLICATIONS

■

Isolated Flyback Switching Regulators

■

Ethernet Isolated 5V to –9V Converters

■

Medical Instruments

■

Isolated Telecom Supplies

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

LT1425

Isolated Flyback

Switching Regulator

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DESCRIPTION

The LT®1425 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 LT1425 operates with input supply voltages from 3V
to 20V and draws only 7mA quiescent current. It can
deliver output power up to 6W with no external power
devices. By utilizing current mode switching techniques, it
provides excellent AC and DC line regulation.

The LT1425 has a number of features not found on other
switching regulator ICs. Its unique control circuitry can
maintain regulation well into discontinuous mode in most
applications. Optional load compensation circuitry allows
for improved load regulation. An externally activated shut­down mode reduces total supply current to 15µA for
standby operation.

TYPICAL APPLICATION

+

C1
100µF
10V

C3
1000pF

15

SHDN

6

SYNC

4

V

C

SGND PGND

5V to Isolated –9V

5V

12

V

IN

11

V

SW

LT1425

710

R

FB

R

REF

R

OCOMP

R

CCOMP

3

5
14
13

500V

ISOLATION BARRIER

T1*

D1

1N5819

••

R1

22.6k
1%

*DALE LPE 4841-330MB

R2

3.01k
1%

C4

0.1µF

R3
15k

U

OUT

C2
47µF

+

16V

1425 TA01

–

V

ISOLATED
–9V ±5% AT
20mA TO 200mA

F

Load Regulation

1

LT1425

WW

W

ABSOLUTE MAXIMUM RATINGS

U

U

W

PACKAGE/ORDER INFORMATION

(Note 1)

Supply Voltage ........................................................ 20V

Switch Voltage......................................................... 35V

SHDN, SYNC Pin Voltage........................................... 7V

RFB Pin Current....................................................... 2mA

Operating Junction Temperature Range

Commercial .......................................... 0°C to 100°C

Industrial ......................................... – 40°C to 100°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

1

GND

2

NC

3

R



FB

4

V



C

5

R



REF

6

SYNC

7

SGND

8

GND

16-LEAD PLASTIC SO

T

JMAX

TOP VIEW

16
15
14
13
12
11
10

9

S PACKAGE



= 145°C, θJA = 75°C/ W

GND
SHDN
R

OCOMP

R

CCOMP

V

IN

V

SW

PGND
GND








ORDER PART

NUMBER

LT1425CS
LT1425IS

Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Feedback Amplifier

I

REF

I

IN

g

m

I

SOURCE

V

CL

Output Switch

BV Output Switch Breakdown Voltage IC = 5mA ● 35 50 V
V(VSW) Output Switch ON Voltage ISW = 1A ● 0.55 0.85 V
I

LIM

Current Amplifier

Timing

f Switching Frequency 260 285 300 kHz

t

ON

t

ED

t

EN

Reference Current Measured at RFB Pin with R

R

Pin Input Current 500 nA

REF

Feedback Amplifier Transconductance ∆IC = ±10µA (Note 2) ● 400 1000 1600 µmho

, I

Feedback Amplifier Source or Sink Current ● 30 50 80 µA

SINK

Feedback Amplifier Clamp Voltage 1.9 V
Reference Voltage/Current Line Regulation 5V ≤ VIN ≤ 18V ● 0.01 0.04 %/V
Voltage Gain (Note 3) 500 V/V
VIN Sense Error ● 10 25 mV

Switch Current Limit Duty Cycle = 50%, 0°C ≤ TJ ≤ 100°C ● 1.35 1.60 1.9 A

Control Pin Threshold Duty Cycle = Minimum 0.95 1.2 1.3 V

Control Voltage to Switch Transconductance 2 A/V

Minimum Switch ON Time 170 210 260 ns
Flyback Enable Delay Time 150 ns
Minimum Flyback Enable Time 180 ns
Maximum Switch Duty Cycle ● 85 90 95 %

VIN = 5V, TJ = 25°C, VSW open, VC = 1.4V, unless otherwise specified.

= 3.000k 402 408 414 µA

REF

Duty Cycle = 50%, –40°C ≤ T
Duty Cycle = 80% 1.30 A

≤ 100°C ● 1.25 1.60 1.9 A

J

● 396 420 µA

● 0.85 1.4 V

● 240 320 kHz

U

2

LT1425

ELECTRICAL CHARACTERISTICS

VIN = 5V, TJ = 25°C, VSW Open, VC = 1.4V, unless otherwise specified.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Load Compensation

∆V

RCCOMP

/∆I

SW

0.45 Ω

SYNC Function

Minimum SYNC Amplitude ● 1.5 2.2 V
Synchronization Range 320 450 kHz
SYNC Pin Input Resistance 40 kΩ

Power Supply

V

IN(MIN)

I

CC

Minimum Input Voltage ● 2.8 3.1 V
Supply Current ● 7.0 9.5 mA
Shutdown Mode Supply Current ● 15 40 µA
Shutdown Mode Threshold ● 0.4 0.9 1.3 V

The ● denotes the specifications which apply over the full operating
temperature range.

Note 2: Feedback amplifier transconductance is R
Note 3: Voltage gain is R

referred.

REF

referred.

REF

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

UW

TYPICAL PERFORMANCE CHARACTERISTICS

Switch Saturation Voltage vs
Switch Current

1.2

1.0

0.8

0.6

0.4

0.2

SWITCH SATURATION VOLTAGE (V)

0

0

0.2 0.4
SWITCH CURRENT (A)

0.8 1.2 1.4

0.6 1.0

125°C

25°C

–55°C

1425 G01

Switch Current Limit vs
Duty Cycle

2.0
TA = 25°C

1.5

1.0

0.5

SWITCH CURRENT LIMIT (A)

0

102030

0

40

DUTY CYCLE (%)

50 60 70 80 90 100

1425 G02

Minimum Input Voltage vs
Temperature

3.1

3.0

2.9

2.8



2.7

INPUT VOLTAGE (V)

2.6

2.5

2.4
–50

–25 0

TEMPERATURE (°C)

50 100 125

25 75

1425 G03

3

LT1425

UW

TYPICAL PERFORMANCE CHARACTERISTICS

Feedback Amplifier Output

60

40

20

0



–20

–40

–60

FEEDBACK AMPLIFIER OUTPUT CURRENT (µA)

–80

1.05

Current vs R

1.10 1.15

REF

1.20 1.30

R

NODE VOLTAGE (V)

REF

Switching Frequency vs
Temperature

300

295

290

285



280

275

SWITCHING FREQUENCY (kHz)

270

Pin Voltage

25°C
125°C
–55°C

1.25 1.35 1.40

1425 G04

Error Amplifier Transconductance
vs Temperature (R

1400

1200

1000

800



600

400

TRANSCONDUCTANCE (µmho)

200

0

–50

–25 0

TEMPERATURE (°C)

REF

50 100 125

25 75

Minimum Synchronization
Voltage vs Temperature

)

2.50

P-P

2.25

2.00

1.75



1.50

1.25

1.00

Referred)

1425 G05

VC Pin Threshold and High Clamp
Voltage vs Temperature

2.50

2.25

2.00

1.75



1.50

PIN VOLTAGE (V)

C

V

1.25

1.00

0.75
–50

–25 0

VC HIGH CLAMP

VC THRESHOLD

50 100 125

25 75

TEMPERATURE (°C)

SHDN Pin Input Current vs
Voltage

1

TA = 25°C

0

–1

–2

–3

SHDN PIN INPUT CURRENT (µA)

1425 G06

265

–50

–25 0

25 75

TEMPERATURE (°C)

Minimum Switch ON Time vs
Temperature

300

275

250

225



200

175

SWITCH ON TIME (ns)

150

125

–50

–25 0

25 75

TEMPERATURE (°C)

4

50 100 125

1425 G07

50 100 125

1425 G10

MINIMUM SYNCHRONIZATION VOLTAGE (V

0.75
–50

–25 0

TEMPERATURE (°C)

50 100 125

25 75

Flyback Enable Delay Time vs
Temperature

250

225

200

175



150

125

ENABLE DELAY TIME (ns)

100

75

–50

–25 0

TEMPERATURE (°C)

50 100 125

25 75

1425 G08

1425 G11

–4

1

0

SHDN PIN VOLTAGE (V)

3

2

Minimum Flyback Enable Time vs
Temperature

275

250

225

200



175

ENABLE TIME (ns)

150

125

100

–50

–25 0

TEMPERATURE (°C)

50 100 125

25 75

4

5

1425 G09

1425 G12

PIN FUNCTIONS

LT1425

UUU

GND (Pins 1, 8, 9, 16): Ground. These pins connect to the
substrate of the die and are separate from the power
ground and signal ground. They should connect directly to
a good quality ground plane.

RFB (Pin 3): Input Pin for External “Feedback” Resistor
Connected to Transformer Primary (VSW). The ratio of this
resistor to the R
(VBG) reference, is the primary determinant of the output
voltage (plus the effect of any nonunity transformer turns
ratio). The average current through this resistor during the
flyback period should be approximately 400µ A. See Appli­cations Information for more details.

VC (Pin 4): Control Voltage. This pin is the output of the
feedback amplifier and the input of the current compara­tor. Frequency compensation of the overall loop is effected
by placing a capacitor between this node and ground.

R

(Pin 5): Input Pin for External Ground-Referred

REF

“Reference” Resistor. This resistor should be in the range
of 3k, but for convenience, need not be this value precisely.
See Applications Information for more details.

SYNC (Pin 6): Pin to Synchronize Internal Oscillator to
External Frequency Reference. It is directly logic compat­ible and can be driven with any signal between 10% and
90% duty cycle. If unused, this pin can be left floating;
however, for best noise immunity the pin should be
grounded.

SGND (Pin 7): Signal Ground. This pin is a clean ground.
The internal reference and feedback amplifier are referred
to it. Keep the ground path connection to R
compensation capacitor free of large ground currents.

resistor, times the internal bandgap

REF

and the V

REF

C

PGND (Pin 10): Power Ground. This pin is the emitter of
the power switch device and has large currents flowing
through it. It should be connected directly to a good quality
ground plane.

VSW (Pin 11): This is the collector node of the output
switch and has large currents flowing through it. Keep the
traces to the switching components as short as possible
to minimize electromagnetic radiation and voltage spikes.

VIN (Pin 12): Supply Voltage. Bypass input supply pin with
10µ F or more. The part goes into undervoltage lockout
when VIN drops below 2.8V. Undervoltage lockout stops
switching and pulls the VC pin low.

R

Load Compensation Function. A common 0.1µF
ceramic capacitor will suffice for most applications. See
Applications Information for further details.

R

Compensation Resistor. Use of this pin allows nominal
compensation for nonzero output impedance in the power
transformer secondary circuit, including secondary wind­ing impedance, output Schottky diode impedance and
output capacitor ESR. In less demanding applications this
resistor is not needed. See Applications Information for
more details.

SHDN (Pin 15): Shutdown. This pin is used to turn off the
regulator and reduce VIN input current to a few tens of
microamperes. The SHDN pin can be left floating when
unused.

(Pin 13): Pin for the External Filter Capacitor for

CCOMP

(Pin 14): Input Pin for Optional External Load

OCOMP

5

LT1425

BLOCK DIAGRAM

V

W

IN

R

FB

R

REF

SHDN

SYNC



2.6V

REGULATOR

285kHz

OSCILLATOR



COMP

SGND

GND IS OMITTED FOR CLARITY

WW

FLYBACK ERROR A PLIFIER DIAGRA

FLYBACK

ERROR

AMPLIFIER

COMPENSATION

V

C

LOAD

DRIVERLOGIC

R

OC0MP

R

CCOMP

CURRENT

AMPLIFIER

V

SW

+

R

SENSE

–

PGND

1425 BD

6

V

IN

V

SW

V

IN

D2

R

FB

R

FB

Q1

Q2 Q3

R

REF

I

R

REF

•

Q4

V

BG

D1

T1

•

+

C1

+

ISOLATED

V

OUT

–

I

M

I

FXD

V

C

ENABLE

C

EXT

I

M

1425 EA