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5

LT1425

PI

N

 FU

N

CTIO

N

S

U

U

U

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.

R

FB

 (Pin 3): Input Pin for External “Feedback” Resistor

Connected to Transformer Primary (V

SW

). The ratio of this

resistor to the R

REF

 resistor, times the internal bandgap

(V

BG

) 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 400

µ

A. See Appli-

cations Information for more details.

V

C

 (Pin 4): Control Voltage. This pin is the output of the

feedback amplifier and the input of the current compara-
tor. Frequency compensation of the overall loop is effected
by placing a capacitor between this node and ground.

R

REF

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

“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 compat-
ible 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

REF

 and the V

C

compensation capacitor free of large ground currents.

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.

V

SW

 (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.

V

IN

 (Pin 12): Supply Voltage. Bypass input supply pin with

10

µ

F or more. The part goes into undervoltage lockout

when V

IN

 drops below 2.8V. Undervoltage lockout stops

switching and pulls the V

C

 pin low.

R

CCOMP

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

Load Compensation Function. A common 0.1

µ

F

ceramic capacitor will suffice for most applications. See
Applications Information for further details.

R

OCOMP

 (Pin 14): Input Pin for Optional External Load

Compensation Resistor. Use of this pin allows nominal
compensation for nonzero output impedance in the power
transformer secondary circuit, including secondary wind-
ing 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 V

IN

 input current to a few tens of

microamperes. The SHDN pin can be left floating when
unused.

Summary of Contents for LT1425

Page 1: ...6W with no external power devices Byutilizingcurrentmodeswitchingtechniques it provides excellent AC and DC line regulation The LT1425 has a number of features not found on other switching regulator I...

Page 2: ...tion 5V VIN 18V 0 01 0 04 V Voltage Gain Note 3 500 V V VIN Sense Error 10 25 mV 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 ILIM...

Page 3: ...Voltage vs Switch Current TEMPERATURE C 50 3 1 3 0 2 9 2 8 2 7 2 6 2 5 2 4 25 75 1425 G03 25 0 50 100 125 INPUT VOLTAGE V Switch Current Limit vs Duty Cycle Minimum Input Voltage vs Temperature SWITC...

Page 4: ...Temperature SHDN Pin Input Current vs Voltage Minimum Synchronization Voltage vs Temperature TEMPERATURE C 50 300 295 290 285 280 275 270 265 25 75 1425 G07 25 0 50 100 125 SWITCHING FREQUENCY kHz TEM...

Page 5: ...nd This pin is a clean ground The internal reference and feedback amplifier are referred to it Keep the ground path connection to RREF and the VC compensation capacitor free of large ground currents P...

Page 6: ...VSW VC CEXT RFB RFB RREF RREF VBG Q4 D2 Q1 Q2 Q3 VIN I IM IM IFXD ENABLE 1425 EA LOAD COMPENSATION CURRENT AMPLIFIER DRIVER LOGIC 285kHz OSCILLATOR 2 6V REGULATOR SHDN FLYBACK ERROR AMPLIFIER COMP RCC...

Page 7: ...tra transformer windings also exhibit defi ciencies The extra winding adds to the transformer s physical size and cost Dynamic response is often mediocre There is usually no method for maintaining loa...

Page 8: ...n in the overall loop will then cause the voltage at the RREF resistor to be nearly equal to the bandgap reference VBG VBG is not present in final output voltage setting equation See Applications Info...

Page 9: ...for fur ther details Enable Delay When the output switch shuts off the flyback pulse appears However it takes a finite time until the trans formerprimarysidevoltagewaveformapproximatelyrep resents th...

Page 10: ...voltage terms in a single variable IIN K1 IOUT where K1 VOUT VIN Eff Switch current is converted to voltage by a sense resistor and amplified by the current sense amplifier with associ ated gain G Thi...

Page 11: ...henewcompensationinplace Modify the original ROCOMP value if necessary to increase or decrease the effective compensation Once the proper load compensation resistor has been chosen it may be necessary...

Page 12: ...m So the user is generally advised to arrange the snubber circuit to clamp at as high a voltage as comfortably possible observing switch breakdown such that leakage spike duration is as short as possi...

Page 13: ...ble state whereby the top of the leakage spike is the control point and the trailing edge of the leakage spike triggers the collapse detect circuitry This will typically reduce the output volt age abr...

Page 14: ...kage Inductance Leakage inductance on the transformer secondary reduces the effective primary to secondary turns ratio NP NS from its ideal value This will increase the output voltage target by a simi...

Page 15: ...MIN where f Switching frequency nominally 285kHz LSEC Transformer secondary side inductance VOUT Output voltage tED Enable delay time tEN Minimum enable time tED tEN 2 Note that generally depending on...

Page 16: ...se but is then held during the subsequent switch ON portion of the nextcycle ThisactionnaturallyholdstheVC voltagestable duringthecurrentcomparatorsenseaction currentmode switching PCB LAYOUT CONSIDER...

Page 17: ...nce to discharge to 11V Feedback voltage is fed directly through a resistor divider to the RREF pin The load compensation circuitry is bypassed resulting in 5 load regulation Finally the 12V to 5V Iso...

Page 18: ...2 F 35V 15 F 35V 3k 15 F 35V 1000pF 0 1 F 130 330pF 9 MBR0540LT1 1425 TA06 BAV21 BAV21 MUR120 LT1425 5k 18 MBR745 10 4 7 8 T1 3 2 1 GND NC RFB VC RREF SYNC SGND GND GND SHDN ROCOMP RCCOMP VIN VSW PGND...

Page 19: ...rwise noted S Package 16 Lead Plastic Small Outline Narrow 0 150 LTC DWG 05 08 1610 0 016 0 050 0 406 1 270 0 010 0 020 0 254 0 508 45 0 8 TYP 0 008 0 010 0 203 0 254 1 2 3 4 5 6 7 8 0 150 0 157 3 810...

Page 20: ...Flyback Regulators Uses Ultrasmall Magnetics LT1424 Application Specific Isolated Regulator 8 Pin Fixed Voltage Version of LT1425 220 F 10V 1425 TA05 LT1425 MBRS340T3 2 5 1 4 6 3 10 7 11 8 12 9 GND NC...

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