ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

31 of 42

www.diodes.com

September 2015

© Diodes Incorporated

ZXLD1371Q

Input Capacitor

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 ZXLD1371Q 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 or Analog Dimming

The ZXLD1371Q 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









PP

IN

SW

PP

L

IN

V

x

f

x

8

I

C











PP

IN

V

x

SW

f

LED

I

x

D

IN

C







)

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)

Led Current and Switching Frequency vs.

Switching

Frequency

T =25°C

V

=V =12V

2 LEDs

L=33µH

R =300m

A

AUX

IN

S

