LV5028TT

No.A1999-7/16

LED current and inductance setting

It is available to use both no-isolation and isolation applications.

(For non-isolation application)

The output current value is the average of the current value that flows during one cycle. The current value that flows into

coil is a triangular wave shown in the figure below. Make sure to set IL_PK so that (average of current value at one cycle)

is equal to (LED current value).

Given that the period when current flows into coil is

DutyI =

T_c+T_d

T

Ipk

×

1

2

× (Duty × T)/T = ILED

Ipk

×

2

× ILED

DutyI

(1) since

Ipk

×

VREF_IN

Rcs

Rcs

×

VFEF_IN

Ipk

=

DutyI

× VFEF_IN

2ILED

(2)

Since formula for LED current is different between on period and off period as shown above,

Ipk

×

Vac-V

ƒ

L

× T_c =

V

ƒ

L

× T_d (3)

Since T_c + T_d = DutyI

× T, T_c = DutyI × T - T_d (4)

Based on the result of (3) and (4), T_d = DutyI

× T ×

Vac-V

ƒ

Vac

(5)

To obtain L from the equation (1), (3), (5),

L

×

V

ƒ × DutyI

2

× ILED × DutyI × T =

Vac - V

ƒ

Vac

=

V

ƒ

2

× ILED ×

1

ƒosc ×

Vac - V

ƒ

Vac

Since LED and inductor are connected in serial in non-isolation mode, LED current flows only when AC voltage exceed

VF.

Ipk: peak inductor current

Vf: LED forward voltage drop

Vac: effective value(R.M.S value)

VREF: Built-in reference voltage (0.605V)

VREF_IN: REF_IN voltage (6 pin)

Rs: External sense resistor

Vzd: Zener diode voltage (REF_IN pin)

RESET

REF_IN

CLK

Q

CS

VREF

(0.605V typ)

Rcs

a blockdiagram in outline

+

-

-

OUT

Vac

L

LED

R1

R2

Vzd

Inductor current

Inductor

current

FET

Vac

T

ON

OFF

Ipk

Ipk = (Vac-Vf)/L

× T_c

= Vf/L

× T_d

IL = Vac/L

× T

T_c

T_d

FET_on

FET_off

T (1cycle)

IL = Vf/L

× T

REF_IN

VREF (0.605V typ)/built-in reference

ILED slope is proportion to

Vac voltage (REF pin voltage)