RT8494GS datasheet(14/17 Pages) RICHTEK

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RT8494

DS8494-01 November 2015

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

For LED applications, the equivalent resistance of the LED

is typically low and the output filter capacitor should be

sized to attenuate the current ripple. Use of X7R type

ceramic capacitors is recommended. Lower operating

frequencies will require proportionately higher capacitor

values.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TAis

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125

°C. The junction to

ambient thermal resistance,

θJA, is layout dependent. For

SOP-14 packages, the thermal resistance,

θJA, is

113.9

°C/W on a standard JEDEC 51-7 four-layer thermal

test board. The maximum power dissipation at TA = 25

°C

can be calculated by the following formula :

PD(MAX) = (125

°C − 25°C) / (113.9°C/W) = 0.87W for

SOP-14 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. The derating curves in Figure 6 allow the

designer to see the effect of rising ambient temperature

on the maximum power dissipation.







OUT

RIPPLE

Schottky Diode Selection

The Schottky diode, with their low forward voltage drop

and fast switching speed, is necessary for the RT8494

applications. In addition, power dissipation, reverse voltage

rating and pulsating peak current are the important

parameters for the Schottky diode selection. Choose a

suitable Schottky diode whose reverse voltage rating is

greater than maximum output voltage. The diode’s

average current rating must exceed the average output

current. The diode conducts current only when the power

switch is turned off (typically less than 50% duty cycle).

If using the PWM feature for dimming, it is important to

consider diode leakage, which increases with the

temperature, from the output during the PWM low interval.

Therefore, choose the Schottky diode with sufficiently low

leakage current.

Capacitor Selection

The input capacitor reduces current spikes from the input

supply and minimizes noise injection to the converter. For

most the RT8494 applications, a 10

μF ceramic capacitor

is sufficient. A value higher or lower may be used

depending on the noise level from the input supply and

the input current to the converter.

In Boost application, the output capacitor is typically a

ceramic capacitor and is selected based on the output

voltage ripple requirements. The minimum value of the

output capacitor COUT is approximately given by the

following equation :



OCP

Current Limlit Threshold Minimum Value

where IOCP is about 1.33 to 1.5 times of inductor peak

current IPEAK.

The placement of RSW should be close to the source of

the N-MOSFET and the IC GND of the RT8494. The ISW

pin input to RT8494 should be a Kelvin sense connection

to the positive terminal of RSW.

switch current to drive the application without exceeding

the current limit threshold set by the ISW pin sense

threshold of RT8494. The ISW sense resistor value can

be calculated according to the formula below :

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