ZXLD1371

ZXLD1371

Document number: DS35436 Rev. 1 - 2

32 of 42

www.diodes.com

February 2012

© Diodes Incorporated

A Product Line of

Diodes Incorporated

Input Capacitor

The input capacitor can be calculated knowing the input voltage ripple

ΔV

Buck

Use D = 0.5 as worst case

Boost

Buck-boost

The minimum RMS current for the output capacitor is calculated as follows:

Buck

use D=0.5 as worst case

Boost

Buck-boost

LED Current Dimming

The ZXLD1371 has 3 dimming methods for reducing the average LED current

1.

DC dimming using the ADJ pin

2.

PWM dimming using the PWM pin

3.

DC dimming for thermal protection using the TADJ pin.

DC Dimming

The ZXLD1371 has a clamp on the ADJ pin to prevent over-driving of the LED current which results in the maximum

voltage being applied to internal circuitry is the reference voltage. This provides a 10:1 dynamic range of dc LED current

adjustment.

The equation for DC dimming of the LED current is approximately:

Where

is

the

dimmed

LED

current

One consequence of DC dimming is that as the ADJ pin

voltage is reduced the sense voltage will also be reduced

which has an impact on accuracy and switching frequency

especially at lower ADJ pin voltages.

Figure 9. LED Current and switching frequency vs.

ADJ Voltage

PP

IN

SW

LED

IN

V

x

f

I

x

)

D

1

(

x

D

C

−

Δ

−

=

PP

IN

SW

PP

L

IN

V

x

f

x

8

I

C

−

−

Δ

Δ

=

PP

IN

V

x

SW

f

LED

I

x

D

IN

C

−

Δ

=

)

D

1

(

Dx

x

I

I

LED

RMS

CIN

−

=

−

12

I

I

PP

L

RMS

CIN

−

−

=

)

D

1

(

D

x

I

I

LED

RMS

CIN

−

=

−

0

150

300

450

600

750

0

150

300

450

600

750

0

0.25

0.5

0.75

1

1.25

ADJ VOLTAGE (V)

Switching

Frequency

T=25°C

V=V =12V

2 LEDs

L=33µH

R=300m

A

AUX

IN

S

Ω