ZLED7001 Datasheet

© 2016 Integrated Device Technology, Inc.

10

April 20, 2016

Resulting in a supply resistor of

Ω

≈

−

=

−

=

k

mA

V

V

I

V

V

R

IN

IN

INDC

IN

195

4

.

1

7

280

An 180k

Ω type can be chosen with a power rating of ≥0.5W. C

IN is chosen to be a 10

μF/ ≥10V capacitor.

TOFF Time Capacitor (COFF)

The selection of the operation frequency is based on a tradeoff between higher frequencies allowing for smaller

and less expensive inductors and lower frequencies incurring lower losses in the power switch.

An estimation of the duty cycle D is based on the ratio of output voltage over input voltage:

15

.

0

280

43

_

_

_

≈

=

Σ

≈

+

=

V

V

V

V

T

T

T

D

INDC

F

TIME

OFF

TIME

ON

TIME

ON

A timing capacitor of 220pF yields

s

pF

pF

s

pF

C

s

T

OFF

TIME

OFF

µ

µ

µ

7

.

11

10

220

1

51

.

0

10

1

51

.

0

_

=













+

⋅

=













+

⋅

=

Resulting in an operation frequency of

kHz

s

T

D

f

TIME

OFF

O

72

7

.

11

15

.

0

1

1

_

≈

−

=

−

=

µ

Inductor (L1)

The inductance L1 is determined by the LED string’s forward voltage, the off-time, and the acceptable current

ripple. Assuming a ripple of 30% of the average current yields

mH

mA

s

V

I

T

V

L

Ripple

TIME

OFF

F

8

.

4

350

%

30

7

.

11

43

_

≈

⋅

⋅

=

⋅

Σ

=

µ

Lower ripple at the same average current will increase the lifetime of the LEDs but requires a more expensive

higher value inductor and increased thermal losses since the inductor’s DC resistance will increase as well. The

DC resistance of the inductor is an important design parameter, too. A capacitor placed in parallel with the array

of LEDs can be used to reduce the LED current ripple while keeping the same average current.

The inductor chosen must have a saturation current higher than the peak output current and a continuous current

rating above the required mean output current. The circuit design must also consider the decrease of inductance

and saturation current with rising temperature.

Current Sense Resistor (RCS)

With peak current-mode control, the output is switched off when the LED current reaches its maximum value

summing up the average LED current and half of the defined current ripple, yielding for the given example

mA

mA

mA

I

I

I

Ripple

AVG

LED

PEAK

O

5

.

402

2

350

%

30

350

2

)

(

_

=

⋅

+

=

+

=