RT8409GS datasheet(9/11 Pages) RICHTEK

9 / 11 page

RT8409

DS8409-00

September 2016

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

The input source is universal (VIN = 90V to 264V), VD will

reach 448V.

Bootstrap Diode Selection

The bootstrap diode is connected the switch output to

provide supply voltage for VCC, and output voltage for FB

divided resistors. The reverse voltage rating of the diode

should be greater than maximum input voltage. A fast

diode can be used, such as ES1J or FR107.

Sense Resistor Selection

The resistor, R5, between the Source of the external N-

MOSFET and SGND should be selected to provide

adequate switch current to drive the application without

exceeding the current limit threshold set by the SENSE

pin sense threshold of the RT8409. The Sense resistor

value can be calculated according to the formula below :



 

VD = 1.2

2 VAC MAX

= 1.2

2 264 = 448V





fSW is switching frequency from 30kHz to 55kHz.

tOFF is constant off time (25

μs, Typ.).

Forward Diode Selection

When the power switch turns off, the path for the current

is through the diode connected between the switch output

and ground. This forward biased diode must have minimum

voltage drop and recovery time. The reverse voltage rating

of the diode should be greater than the maximum input

voltage and the current rating should be greater than the

maximum load current.

The peak voltage stress of diode is :

CLT

OCP



where VCTL is the current limit threshold (0.97V, min.).

IOCP is about 1.2 to 1.5 times of the peak inductor current.

Thermal Protection (OTP)

A thermal protection feature is included to protect the

RT8409 from excessive heat damage. When the junction

temperature exceeds a threshold of 150

°C, the thermal

protection OTP will be triggered and the RT8409 will be

turned off.

Thermal Considerations

The junction temperature should never exceed the

absolute maximum junction temperature TJ(MAX), listed

under Absolute Maximum Ratings, to avoid permanent

damage to the device. The maximum allowable power

dissipation depends on the thermal resistance of the IC

package, the PCB layout, the rate of surrounding airflow,

and the difference between the junction and ambient

temperatures. The maximum power dissipation can be

calculated using the following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

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

the ambient temperature, and

θJA is the junction-to-ambient

thermal resistance.

For continuous operation, the maximum operating junction

temperature indicated under Recommended Operating

Conditions is 125

°C. The junction-to-ambient thermal

resistance,

θJA, is highly package dependent. For a

SOP-8 package, the thermal resistance,

θJA, is

206.9

°C/W on a standard JEDEC low effective-thermal-

conductivity two-layer test board. The maximum power

dissipation at TA = 25

°C can be calculated as below :

PD(MAX) = (125

°C − 25°C) / (206.9°C/W) = 0.48W for a

SOP-8 package.

The maximum power dissipation depends on the operating

ambient temperature for the fixed TJ(MAX) and the thermal

resistance,

θJA. The derating curves in Figure 1 allows

the designer to see the effect of rising ambient temperature

on the maximum power dissipation.

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