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14

LT1425

APPLICATIO

N

S I

N

FOR

M

ATIO

N

W

U

U

U

“collapse,” thereby supporting operation well into discon-
tinuous mode. Nevertheless, there still remain constraints
to ultimate low load operation. They relate to the minimum
switch ON time and the minimum enable time. Discontinu-
ous mode operation will be assumed in the following
theoretical derivations.

As outlined in the Operation section, the LT1425 utilizes a
minimum output switch ON time, t

ON

. This value can be

combined with expected V

IN

 and switching frequency to

yield an expression for minimum delivered power.

1
2

))

f

L

PRI

)

)

Min Power = 

(V

IN

 • t

ON

)

2

= (V

OUT

)(I

OUT

)

This expression then yields a minimum output current
constraint:

1
2

))

f

(L

PRI

)(V

OUT

)

)

)

I

OUT(MIN)

 = 

where,
f = Switching frequency (nominally 285kHz)
L

PRI

 = Transformer primary side inductance

V

IN

 = Input voltage

V

OUT

 = Output voltage

t

ON

 = Output switch minimum ON time

(V

IN

 • t

ON

)

2

An additional constraint has to do with the minimum
enable time. The LT1425 derives its output voltage infor-
mation from the flyback pulse. If the internal minimum
enable time 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, t

ED

, plus the

minimum enable time, t

EN

. Minimum power delivered to

the load is then:

1
2

))

f

L

SEC

)

)

Min Power = 

[V

OUT

 • (t

EN

 + t

ED

)]

2

= (V

OUT

)(I

OUT

)

which yields a minimum output constraint:

V

F

 of this diode should therefore be included in R

FB

calculations. Lot-to-lot and ambient temperature varia-
tions will show up as output voltage shift/drift.

Secondary Leakage Inductance

Leakage inductance on the transformer secondary
reduces the effective primary-to-secondary turns ratio
(N

P

/N

S

) from its ideal value. This will increase the output

voltage target by a similar percentage. To the extent that
secondary leakage inductance is constant from part-to-
part, this can be accommodated by adjusting the R

FB

 to

R

REF

 resistor ratio.

Output Impedance Error

An additional error source is caused by transformer sec-
ondary current flow through the real life nonzero imped-
ances of the output rectifier, transformer secondary and
output capacitor. Because the secondary current only
flows during the off portion of the duty cycle, the effective
output impedance equals the “DC” lumped secondary
impedance times the inverse of the off duty cycle. If the
output load current remains relatively constant, or, in less
critical applications, the error may be judged acceptable
and the R

FB

 value adjusted for nominal expected error. In

more demanding applications, output impedance error
may be minimized by the use of the load compensation
function (see Load Compensation).

V

IN

 Sense Error

The LT1425 determines the size of the flyback pulse by
comparing the V

SW

 signal to V

IN

, through R

FB

. This

comparison is not perfect, in the sense that an offset exists
between the sensing mechanism and the actual V

IN

. This

is expressed in the data sheet as V

IN

 sense error. This error

is fixed in absolute millivolt terms relative to V

OUT

 (with the

exception that it is reflected to V

OUT

 by any nonunity

secondary-to-primary turns ratio).

MINIMUM LOAD CONSIDERATIONS

The LT1425 generally provides better low load perfor-
mance than previous generation switcher/controllers
utilizing indirect output voltage sensing techniques.
Specifically, it contains circuitry to detect flyback pulse

Содержание LT1425

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

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

Страница 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...

Страница 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...

Страница 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...

Страница 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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