ZXLD1374

ZXLD1374

Document number: DS35032 Rev. 1 - 2

Page 23 of 35

www.diodes.com

October 2010

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Boost and Buck-boost

MAX

MAX

LED

RMS

COUTPUT

D

1

D

I

I

−

=

−

Ceramic capacitors with X7R dielectric are the best choice due to their high ripple current rating, long lifetime, and

performance over the voltage and temperature ranges.

BOOTSTRAP CIRCUIT

In Boost and Buck-boost modes with input voltages below 12V to fully enhance the internal power switch it is required to use a

bootstrap network as shown in figure 40.

Figure 40: Bootstrap circuit for low voltage operations

The bootstrap circuit is realized by adding a reservoir capacitor, C8, current limiting resistor R13 (=100

Ω) and a blocking

diode D2 (DFSL160). During the power switch turn-on C8 needs to be able to supply approximately 10mA current.

A capacitor of 1uF (C8) provides a reasonable trade-off between VAUX supply needs and LED current accuracy. At start-up

the VAUX pin requires only a few mA of current from the LED current. In normal operation the current taken from the LED

current to supply VAUX will be negligible.

INPUT CAPACITOR

The input capacitor and minimum RMS current for the output capacitor can be calculated knowing the input voltage ripple

Input capacitor

Minimum RMS current

Buck

PP

IN

SW

LED

IN

V

x

f

I

x

)

D

1

(

x

D

C

−

Δ

−

=

)

D

1

(

Dx

x

I

I

LED

RMS

CIN

−

=

−

use D=0.5 as worst case

use D=0.5 as worst case

Boost

PP

IN

SW

PP

COIL

IN

V

x

f

x

8

I

C

−

−

Δ

Δ

=

12

I

I

PP

L

RMS

CIN

−

−

=

Buck-boost

PP

IN

SW

LED

IN

V

x

f

I

x

D

C

−

Δ

=

)

D

1

(

D

x

I

I

LED

RMS

CIN

−

=

−