LT1105C datasheet(25/32 Pages) LINER

25 / 32 page

LT1103/LT1105

where N is the transformer turns ratio of secondary to

primary and DC is the duty cycle. This formula can be

rewritten in terms of duty cycle as:

DC =

VN • V

OUT

()

It is important to define the full range of input voltage, the

range of output loading conditions and the regulation

requirements for a design. Duty cycle should be calculated

for both minimum and maximum input voltage.

In many applications, N can vary over a wide range without

degrading performance. If maximum output power is

desired, N can be optimized:

VVf

V– V

– V

OPT

OUT

IN MAX

SNUB

()

where

Vf = Forward voltage of the output diode

VM = Maximum switch voltage

VSNUB = Snubber clamp level – primary flyback

voltage.

In the isolated flyback mode, the LT1103/LT1105 sense

and regulate the transformer primary voltage VPRI during

“switch off” time. The secondary output voltage will be

regulated if VPRI is regulated. VPRI is related to VOUT by:

VVf

PRI

OUT +

()

This allows duty cycle for an isolated flyback converter to

be rewritten as:

DC = Duty Cycle =

PRI

()

An important transformer parameter to be determined is

the primary inductance LPRI. The value of this inductance

is a trade-off between core size, regulation requirements,

leakage inductance effects and magnetizing current

∆I.

Magnetizing current is the difference between the primary

current at the start of “switch on” time and the current at

APPLICATIONS INFORMATION

the end of “switch on” time. If maximum output power is

needed, a reasonable starting value is found by assigning

∆I a value of 20% of the peak switch current (2A for the

LT1103 and set by the external FET rating used with the

LT1105). With this design approach, LPRI is defined as:

( I)(f) 1

PRI









∆

If maximum output power is not required, then

∆I can be

increased which results in lower primary inductance and

smaller magnetics. Maximum output power with an isolated

flyback converter is defined by the primary flyback voltage

and the peak allowed switch current and is limited to:

VI –

–Ip R E

OUT(MAX)

PRI

IN P

()







 ()













∆

where

R = Total “switch” on resistance

IP = Maximum switch current

E = Overall efficiency

≈ 75%

Peak primary current is used to determine core size for the

transformer and is found from:

PRI

OUT

PRI

= ()(

)

()

()( )

+ ∆

A second consideration on primary inductance is the

transition point from continuous mode to discontinuous

mode. At light loads, the flyback pulse across the primary

will drop to zero before the end of “switch off” time. The

load current at which this starts to occur can be calculated

from:

f L

OUT(TRANSITION)

PRI

OUT

PRI

()

() (

)( )( )

•

The forward converter as shown below is another

transformer-based topology that converts one voltage to

either a higher or a lower voltage.

LT1105C Datasheet(PDF) 25 Page - Linear Technology

LT1105C Datasheet(HTML) 25 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL