RT8494

13

DS8494-01 November 2015

www.richtek.com

©

Copyright 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Over-Temperature Protection

The RT8494 provides an 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 145

°C. The chip will automatically start to switch

again when the die junction temperature cools off.

Inductor Selection

The converter operates in discontinuous conduction mode

operates in Continuous Conduction Mode (CCM). The

For Buck application :





 





OUT

IN

OUT

BCM

OUT

IN

VV

V

L

2I

f

V

For Boost application :

For Buck-Boost application :

where

f = operating frequency.

Choose an inductance based on the operating frequency,

input voltage and output voltage to provide a current mode

ramp signal during the MOSFET on period for PWM control

loop regulation. The inductance also determines the

inductor ripple current. Operating the converter in CCM is

recommended, which will have the smaller inductor ripple

current and hence the less conduction losses from all

converter components.

As a design example, to design the peak to peak inductor

ripple to be

±30% of the output current, the following

equations can be used to estimate the size of the needed

inductance :





 







OUT

IN

OUT

OUT

IN

VV

V

L =

20.3 I

f

V

For Buck application :

For Boost application :

For Buck-Boost application :

The inductor must also be selected with a saturation

current rating greater than the maximum inductor current

during normal operation. The maximum inductor current

can be calculated by the following equations.

For Buck application :





 





OUT

IN

OUT

PEAK

OUT

IN

VV

V

I= I

+

2L f

V

For Boost application :







 

 



OUT

OUT

IN

OUT

IN

PEAK

IN

OUT

VI

V

V

V

I=

+

V2 L f

V

For Buck-Boost application :











 

 





IN

OUT

OUT

IN

OUT

PEAK

IN

IN

OUT

VV

I

VV

I=

+

V2 L f

V

V

where

η is the efficiency of the power converter.

Power MOSFET Selection

For applications operating at high input or output voltages,

the power N-MOS FET switch is typically chosen for drain

voltage VDS rating and low gate charge. Consideration of

because switching losses dominate power loss. The

GBIAS regulator on the RT8494 has a fixed current limit

to protect the IC from excessive power dissipation at high

VIN, so the N-MOSFET should be chosen so that the

exceed the GBIAS current limit.

ISW Sense Resistor Selection

MOSFET and GNDshould be selected to provide adequate

2

IN

OUT

IN

2

OUT

OUT

VV

V

L =

20.3 I

f

V





 











2

IN

OUT

2

OUT

IN

OUT

VV

L =

20.3 I

f

VV









2

IN

OUT

IN

BCM

2

OUT

OUT

VV

V

L

2I

f

V





 









2

IN

OUT

BCM

2

OUT

IN

OUT

VV

L

2I

f

VV





