DS8409 Datasheet, PDF (8/11 Pages) Richtek Technology Corporation – Green Mode Buck Converter

RT8409
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 :
V_OUT
=
R_2A + R_2B
R_2B
1.75V
+
V_{F_D7}
- V_{F_D5}
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
  ^{2 90V}
R_START
where 90V is assumed the minimum line input voltage.
The typical required minimum start-up current is 60μA.
The recommended total start-up resistance R_START for
universal inputs should be no more than 2MΩ.
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8
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}
where VAC(MAX) is the maximum input voltage (RMS)
and the parameter 1.2 is used for safety margin.
For this example :
    ^{VBR = 1.2 2  VACMAX = 1.2 2  264 = 448V}
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 :
I_PEAK
=
V_IN  V_OUT
L
 t_ON
L
 VIN
 V_OUT    V_OUT + V_{F_D5} 
2  f_SW  I_OUT  V_IN
t_ON =
V_OUT
V_IN  V_OUT
 t_{OFF_1}
K_b +
t_{OFF_1} =
K_b2 + 4  K_a  K_c
2  K_a
K_a
=
L
V_OUT
  V_IN


V_IN
V_OUT 
K_b
=
V_OUT  I_OUT
V_IN  V_OUT
Kc = I_OUT  t_OFF
Where
V_OUT is output voltage.
V_IN is input voltage.
I_OUT is full load current.
is a registered trademark of Richtek Technology Corporation.
DS8409-00 September 2016