LX1745

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 9

Triple Output Boost – LED Driver / LCD Bias

Copyright

© 2000

Rev. 1.2, 2006-02-27

TM

®

APPLICATION NOTE

negative voltage transition that is greater than the output

voltage.

The LX1745 can source up to 100mA of gate current.

An logic level N-channel MOSFET with a low turn on

required to optimize overall circuit performance.

Since the output of the LED Driver is a current mode

configuration, it may be desirable to protect the output from

an over-voltage condition in the event the load is removed

or not present.

The LX1745 includes an over voltage monitor that is

easily programmed with two external resistors (Figure 6).

This feature eliminates the need for a Zener Diode clamp on

the output.

REF

REF

OVP

2

_

OVP

1

_

OVP

V

V

-

V

R

R

=

eq. 9

This voltage should be selected to accommodate the

maximum forward voltage of all the LEDs, over

temperature, plus the maximum feedback voltage.

Conversely, it may also be selected according to the

Setting of the peak inductor current limit is an important

aspect of the PFM constant off-time architecture; it

determines the maximum output power capability and has a

marked effect on efficiency.

It is recommended that the peak inductor current be set

to approximately two times the expected maximum DC

input current. This setting will minimize the inductor size,

the input ripple current, and the output ripple voltage. Care

should be taken to use inductors that will not saturate at the

peak inductor current level. The desired peak inductor

current can be estimated by the following equation:

IN

OUT

PK

V

P

2

I

⋅

η

⋅

=

eq. 10

can be chosen from the following equation:

6

PK

CS

10

30

185

.

0

I

R

−

⋅

−

≅

eq. 11

which is taken from the following graph (Figure 7).

0

200

400

600

800

1000

0

5

10

15

20

Figure 7 –

Peak Current Programming Resistor

This graph characterizes the relationship between peak

programming resistor.

An inductor value of 47

μH has been show to yield very

good results. Choosing a lower value emphasizes peak

current overshoot, effectively raises the switching

frequency, and increases the dissipative losses due to

increased currents.

Output voltage ripple is a function of the several

parameters: inductor value, output capacitance value, peak

switch current, load current, input voltage, and the output

voltage. All of these factors can be summarized by the

following equation:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

+

⋅

+

+

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

⋅

≅

IN

F

OUT

OUT

PK

L

SW

IN

OUT

OUT

PK

RIPPLE

V

V

V

I

I

)

V

V

(

V

1

C

I

I

L

V

eq. 12