RT8409GS datasheet(8/11 Pages) RICHTEK

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RT8409

DS8409-00

September 2016

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Applications Information

The RT8409 is a constant off time control, high efficiency

constant voltage high side converter with internal MOSFET.

It can be used in buck down solution, to provide a constant

output voltage to the load. It contains high side bootstrap

voltage sense circuitry, while minimizing external

component count. The SOP8 package keeps application

footprint small, and makes the RT8409 a cost effective

solution for off-line buck converter.

The RT8409 can achieve high accuracy output voltage via

FB pin, which is the average output voltage sense feedback

loop control pin. The internal FB reference voltage (1.75V

typ.) is used to set the average output voltage by the

external resistor, R2A, R2B. The sense voltage from sense

pin is used for Over Current Protection (OCP) function.

Under-Voltage Lockout (UVLO)

The RT8409 includes a UVLO function with 11V hysteresis.

For system start up, the VCC must rise over 18V (typ.) to

turn on the internal MOSFET.

The internal MOSFET will be turned off if VCC falls below

7V (typ.)

Setting Average Output Voltage

The output voltage that provides to the output load is set

by external resistors, R2A, R2B, which is connected

between the cathode of D7 and SGND pins. The

relationship between output voltage, and R2A, R2B is

shown below :

Start-Up Resistor

The start-up resistor should be chosen to set the start up

current exceeds certain minimum value. Otherwise, the

RT8409 may latch off and the system will never start. The

start-up current equals

where 90V is assumed the minimum line input voltage.

The typical required minimum start-up current is 60

μA.

The recommended total start-up resistance RSTART for

universal inputs should be no more than 2M

Ω.

OUT

F_D7

F_D5

+ R

1.75V + V

- V







START

290V



Input Diode Bridge Rectifier Selection

The voltage rating of the input bridge rectifier, VBR,

dependent on the input voltage. Thus, the VBR rating is

calculated as below :



VBR = 1.2

2 VAC MAX





 

VBR = 1.2

2 VAC MAX

= 1.2

2 264 = 448V





where VAC(MAX) is the maximum input voltage (RMS)

and the parameter 1.2 is used for safety margin.

For this example :

If the input source is universal, VBR will reach 448V. In

this case, a 600V, 0.5A bridge rectifier can be chosen.

Inductor Selection

For best efficiency, the RT8409 should be operated near

boundary conduction mode. Based on this recommendation,

the required inductor value is related to the input voltage,

output voltages, the min. on-time and the max. off time.

The inductor saturation current will be related to the over

current limit set by the sense resistor between the sense

pin and ground pin. The over current limit design information

can be found in the later section.

The peak current of inductor is showed as below :

OUT

PEAK











OUT

F_D5

OUT

+ V











OFF_1

OUT

OFF

K +

+ 4

K =

Kc = I













Where

VOUT is output voltage.

VIN is input voltage.

IOUT is full load current.

OUT

OFF_1

OUT





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