PPMIC BU

www.belling.com.cn

BL9582 Rev 1.2

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2/2010

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13

BL9582

32V, 1.4A Step-down

High Brightness LED Driver with Internal Switch

The inductor value should be chosen to

maintain operating duty cycle and switch

'on'/'off' times within the specified limits

over the supply voltage and load current

range.

The following equations can be used as a

guide.

LX Switch 'On' time

()

ON

IN

LED

AVG

S

L

LX

LI

T

VV

I

R

R

R

× Δ

=

−−

×

+

+

LX Switch 'Off' time

()

OFF

LED

D

AVG

S

L

LI

T

VV

I

R

R

× Δ

=

++

+

Where:

L is the coil inductance (H)

ΔI is the coil peak-peak ripple current (A)

nominal)

required load current (V)

Diode selection

For maximum efficiency and performance,

the rectifier (D1) should be a fast low

capacitance Schottky diode with low

reverse leakage at the maximum operating

voltage

and

temperature.

They

also

provide better efficiency than silicon diodes,

due to a combination of lower forward

voltage and reduced recovery time. It is

important to select parts with a peak

current rating above the peak coil current

and a continuous current rating higher than

the maximum output load current. It is very

important to consider the reverse leakage

of the diode when operating above 85°C.

Excess leakage will increase the power

dissipation in the device and if close to the

load may create a thermal runaway

condition. The higher forward voltage and

overshoot due to reverse recovery time in

silicon diodes will increase the peak

voltage on the LX output. If a silicon diode

is used, care should be taken to ensure

that the total voltage appearing on the LX

pin including supply ripple, does not

exceed the specified maximum value. The

following web sites are useful when finding

alternatives:

www.onsemi.com

Reducing output ripple

Peak to peak ripple current in the LED(s)

can be reduced, if required, by shunting a

below:

U1

BL9582

ADJ

RS

L1

47uH

1

2

D1

1

2

CIN

4.7uF

VIN

CLED

1uF

A value of 1uF will reduce the supply ripple

current by a factor three (approx.).

Proportionally lower ripple can be achieved

with higher capacitor values. Note that the

capacitor will not affect operating fre-

quency or efficiency, but it will increase

start-up delay and reduce the frequency of