RT8498

9

DS8498-01 April 2017

www.richtek.com

©

Copyright 2017 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT8498 is specifically designed to be operated in

Buck converter applications. This device uses a fixed

frequency, current-mode control scheme to provide

excellent line and load regulation.

LED Current Setting

The LED current can be calculated by the following

equation :





VCC

ISN

LED(MAX)

SENSE

VV

I

=

R



ISN pins (100mV typ. if ACTL dimming is not applied) and

Current Limit

The RT8498 can limit the peak switch current with its

internal over-current protection feature. In normal operation,

the power switch is turned off when the switch current

hits the loop-set value. The over-current protection function

will turn off the power switch independent of the loop control

when the peak switch current reaches around 2.5A (type.).

Over-Temperature Protection

The RT8498 has over-temperature protection (OTP)

function to prevent the excessive power dissipation from

overheating. The OTP function will shut down switching

operation when the die junction temperature exceeds

150

°C. The chip will automatically start to switch again

when the die junction temperature cools off.

Inductor Selection

Choose an inductor that can handle the necessary peak

power losses. Inductor manufacturers specify the

maximum current rating as the current where the

inductance falls to certain percentage of its nominal value,

typically 65%. In Multiple-Topology application where the

transition between discontinuous and continuous modes

OUT

OUT

LIN(MAX)

VV

L =

1

fI

V



 







 

OUT

OUT

L

IN

L

PEAK

OUT

VV

I =

1

fL

V

I

I

= I

+

2

 



 

 





 





The ripple current

calculated :

For Boost application :

The ripple current

calculated :

IN

IN

L

OUT

OUT

OUT

L

PEAK

IN

VV

I =

1

fL

V

IV

I

I

=

+

V2







 







 













IN

IN

LOUT

VV

L =

1

fI

V



 







 

occurs, the value of the required output inductor, L, can

be approximated by the following equation :

For Buck application :

For Buck-Boost application :

OUT

OUT

LIN

OUT

VV

L =

1

If

V + V

 





 





 



The ripple current

calculated :



OUT

OUT

L

IN

OUT

IN

OUT

OUT

L

PEAK

IN

VV

I =

1

Lf

V + V

V + V

I

I

I

=

+

V2



 



 

 





 

















where,

f = switching frequency.