LV5028TT

No.A1999-9/16

(for Isolation circuit)

Using the circuit diagram below, the wave form of the current that flows to Np and Ns is as follows.

Current waveform flows to primary side and secondary.

[Inductance Lp of primary side and sense resistor Rs]

If a peak current flow to transformer is represented as Ipk_p, the power (Pin) charged to the transformer on primary side

can be represented as:

Pin =

1

2

ƒosc (11)

Ipk_p =

Vac

Lp

× Ton_p (12)

Lp =

Vac

2

ƒosc

2

× Pin

=

Vac

2

2

× Pin×

ƒosc

(13)

(Don_p =

Ton_p

T

= Ton_p

×

ƒosc),

To substitute the following to the formula below,

...η = Pout

Pin

(14)

∴Lp =

Vac

2

ƒosc × η

2

× Pout

=

Vac

2

η

2

× Pout ×

ƒosc

(15)

RESET

REF_IN

CLK

Q

CS

VREF

(0.605V typ)

+

-

-

OUT

Vac

LP

(Np)

Ls

(Ns)

Rcs

R1

a blockdiagram in outline

R2

Vzd

Ip

Vac

T

T

Ipk_p = Vac/Lp

× Ton_p

Ipk_s = Vf/Ls

× Ton_s

Iout

(Ton_s)

ON

OFF

FET

REF_IN

VREF (0.605V)typ

(Primary side current)

Primary

side

Secondary

side

Is

Ipk_p

Ipk_s

(Secondary side current)

Ip (primary side)

Is (Secondary side current)

Ip slope is proportion to

Vac voltage (REF pin voltage)

T(1cycle)

FET_OFF

FET_ON

(Ton_p)

Is = Vf/Ls

× Ton_s

Ip = Vac/Lp

× Ton_p