LX1995

Microsemi

Integrated Products Division

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

Page 6

Miniature LED Driver

Copyright

© 2000

Rev. 1.1, 2005-07-07

TM

®

TH EORY OF O PERAT ION

The LX1995-x is a PFM boost converter optimized for

driving a series of white or color LEDs. It operates in a

pseudo-hysteretic mode with a fixed, 300ns, switch “off

time”.

When the LX1995-x enables, the

LED current

decreases causing the FB voltage to decrease to a value less

than 320mV. The feedback comparator (See Simplified

Block Diagram) activates the control logic.

The control

logic turns on the DRV output circuit that connects to the

internal N-Channel MOSFET gate.

The switch output

(SW) is switched “on” and remains “on” until the inductor

current ramps up to the peak current level (typically 325mA

for LX1995-1).

The LED load is powered from energy stored in the

output capacitor during the inductor charging cycle. Once

the peak inductor current value is achieved, the output is

turned off and the energy stored in the inductor delivers to

the load. This causes the voltage to rise across the current

LX1995-x continues to switch until the voltage at the FB

pin exceeds 320mV.

dividing 320mV by the maximum series LED current. A

minimum value of 3.3

voltage at the FB pin is the product of the LED current

)

MAX

(

LED

1

I

mV

320

R

=

eq. 1

LX1995-x supports two dimming methods: PWM or DC

Voltage.

PWM mode: Connect system PWM logic signal to the

SHDN pin (See Figure 1). This turns the LX1995-X on and

off which pulses the LED current between zero and the

DC Voltage mode: The designer can apply an adjustable

DC voltage supply to the FB pin.

As the DC voltage

increases, the LED current decreases. The equation (see

Figure 3) is:

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

⋅

=

3

2

ADJ

3

3

2

1

LED

R

R

V

R

R

R

mV

320

R

1

I

eq. 2

Microsemi recommends the use inductors (for the

LX1995-1) in the range of 10µH to 47µH due to saturation

of peak inductor current.

By increasing the average

inductor current, the LX1995-x will extend the power

range. Smaller inductor values will reduce output voltage

ripple and are smaller in size.

Output voltage ripple is depended on the selection of the

voltage ripple is a function of the output droop (as the

error (i.e., typically 10mV), and the output overshoot

(energy stored in the inductor). When the switch is first

turn on, the total ripple voltage is:

10mV

V

V

V

OVERSHOOT

DROOP

RIPPLE

+

∆

+

∆

=

eq. 3

The initial droop can be estimated with the assumption

of 0.5V of voltage drop across the inductor and FET

()

5

.

0

V

I

I

C

L

V

IN

LED

PK

OUT

DROOP

−

×

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

∆

eq. 4

The output overshoot calculated with 0.5V as voltage

drop across the diode.

()

5

.

0

V

V

I

I

C

L

2

1

V

IN

OUT

2

LED

PK

OUT

OVERSHOOT

+

−

−

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

=

∆

eq. 5

Once the output voltage ripple is determined for the

selected components, the output capacitor can then be

adjusted to meet the target ripple voltage requirements.

The LX1995-x is targeted for LED driver applications;

output

voltage

ripple

is

not

a

critical

application

requirement.