®

RT8409

Green Mode Buck Converter

General Description Features
The RT8409 integrates a power MOSFET controller. It is  Built-In 600V Power MOSFET
used for step down converters by well controlling the  Programmable Constant Output Voltage
internal MOSFET and regulating a constant output voltage.  Extremely Low Quiescent Current Consumption and
μA Shutdown Current
1μ
The RT8409 features few system component counts and
 Low System BOM Cost for Economical Step Down
simple system design. Especially, the RT8409 can use a
Converter Solution
cheap simple drum core inductor in the system instead
 Universal Off-Line Input Voltage operation Range
of an EE core while maintaining the high efficiency.
 Built-In Over-Temperature Protection
The RT8409 is housed in a SOP-8 package. The  Built-In Over-Voltage Protection
components of the whole system can be made very  Output Open Protection
compact.  Output Short Protection
 Output Over-Current Protection
Ordering Information  SOP-8 Package
RT8409
Package Type Applications
S : SOP-8
 Home Appliance
Lead Plating System  Standby Power
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
Pin Configuration
(TOP VIEW)
 RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020. SGND 8 VCC
 Suitable for use in SnPb or Pb-free soldering processes. COMP 2 7 NC
FB 3 6 DRAIN
SENSE 4 5 DRAIN

Marking Information SOP-8

RT8409GS : Product Number
RT8409 YMDNN : Date Code
GSYMDNN

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RT8409
Typical Application Circuit

F1
D1 D2
CIN
RSTART
D3 D4

R3

5, 6 D8
R2A DRAIN X
3 FB VCC 8
RT8409
2 COMP SENSE 4
D7
C3 Bootstrap diode
R1 1 SGND R2
C1 R2B C5 L1
VOUT
C2

+
C4 R4
D5

Functional Pin Description

Pin No. Pin Name Pin Function
1 SGND Signal ground of the chip.
2 COMP Close loop compensation node.
3 FB Error amplifier input.
4 SENSE Current sense input.
5, 6 DRAIN Internal MOSFET drain.
7 NC No internal connection.
Supply voltage input of the chip. For good bypass, a ceramic capacitor near the VCC pin is
8 VCC
required.

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 RT8409
Functional Block Diagram

VCC

Regulator

DRAIN
FB +
EA State Machine A
-
+ 1.75V
- SGND

COMP SENSE

Operation
The RT8409 senses the output voltage via the bootstrap Burst Mode
loop. The output voltage feedback signal is compared to For the no load power saving demand, burst mode is a
an internal reference voltage for the output voltage way to save power and maintain output regulation. High
regulation. The COMP pin, which is the operational efficiency is achieved at light loads when Burst Mode
amplifier output node, is used for the control loop operation is entered. The typical burst mode trigger levels
compensation to obtain stable response. To stabilize the are defined as follows. The MOSFET will stop switching
system properly select the compensation network is when VC voltage goes lower than 200mV (typ.). The
required. MOSFET will resume switching again once the VC voltage
The FB pin is the voltage loop input for the system goes higher than 300mV (typ.).
regulation. The above COMP pin related compensation In this mode the output ripple has a variable frequency
will be determined by specified system demand and be component that depends upon load current. Burst Mode
adapted to various applications. operation ripple can be reduced slightly by using more
output capacitance.

VC

VC.high

VC.low

VDS

IOUT

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RT8409
Absolute Maximum Ratings (Note 1)
 Supply Input Voltage, VCC -------------------------------------------------------------------------------------------- 40V
 Power Dissipation, PD @ TA = 25°C
SOP-8 ---------------------------------------------------------------------------------------------------------------------- 0.48W
 Package Thermal Resistance (Note 2)
SOP-8, θJA ----------------------------------------------------------------------------------------------------------------- 206.9°C/W
 Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------------------- 260°C
 Junction Temperature --------------------------------------------------------------------------------------------------- 150°C
 Storage Temperature Range ------------------------------------------------------------------------------------------- −65°C to 150°C
 ESD Susceptibility (Note 3)
HBM (Human Body Model) -------------------------------------------------------------------------------------------- 2kV

Recommended Operating Conditions (Note 4)

 Supply Input Voltage Range, VCC ----------------------------------------------------------------------------------- 11V to 22V
 Junction Temperature Range ------------------------------------------------------------------------------------------ −40°C to 125°C
 Ambient Temperature Range ------------------------------------------------------------------------------------------ −40°C to 85°C

Electrical Characteristics
(VCC = 15V, TA = 25°C, unless otherwise specified)
Parameter Symbol Test Conditions Min Typ Max Unit
VCC UVLO ON VUVLO_ON 17 18 19 V
VCC UVLO OFF VUVLO_OFF 6.5 7 8 V
VCC Shutdown Current ISD VCC = VUVLO_ON  3V -- 1.5 3 A
VCC Quiescent Current IQC Gate stands still -- 0.5 2 mA
VCC Operating Current ICC By CGATE = 1nF, Freq.= 20kHz -- 1 2 mA
VCC OVP Level VCC_OVP 23.75 25 26.25 V
Current Sense Threshold VSENSE 0.97 1.04 1.11 V
Sense Pin Leakage Current ISENSE VSENSE = 3V -- 1 5 A
FB Pin Threshold VFB 1.7 1.75 1.8 V
FB Over Voltage Protection VFB_OVP 1.82 1.96 2.1 V
FB Pin Leakage Current IFB VFB = 5V -- 1 3 A
Switch Off Time tOFF 18 25 32 s
Static Drain-Source
RDS(ON) VGD = 12V, ID = 50mA -- 6 -- 
On-Resistance
Drain-Source Leakage
IDSS VDS = 600V -- -- 10 A
Current

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 RT8409
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect
device reliability.
Note 2. θJA is measured under natural convection (still air) at TA = 25°C with the component mounted on a low effective-thermal-
conductivity two-layer test board on a JEDEC thermal measurement standard.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.

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RT8409
Typical Operating Characteristics
Standby Power Operating Current vs. Supply Voltage
100 1.1
90
1.0

Operating Current (mA)

80
Input Power (mW)

70 0.9

60
0.8
50
0.7
40
30 0.6
20
0.5
10
VOUT = 15V, no Load, start-up resistor = 2MΩ
0 0.4
90 110 130 150 170 190 210 230 250 270 8 10 12 14 16 18 20 22
Input AC Voltage (VAC) Supply Voltage (V)

Operating Current vs. Temperature UVLO vs. Temperature

1.3 20

1.2 18
UVLO_ON
Operating Current (mA)

16
1.1
14
1.0
UVLO (V)

12
0.9 10

0.8 8
6 UVLO_OFF
0.7
4
0.6
2
VCC = 15V
0.5 0
-50 -25 0 25 50 75 100 125 -50 -25 0 25 50 75 100 125
Temperature (°C) Temperature (°C)

VCC OVP vs. Temperature Sense Threshold vs. Supply Votatge

26 1.2

1.2
25
Sense Threshold (V)

1.1
24
VCC OVP (V)

1.1

23 1.0

1.0
22
0.9
21
0.9

20 0.8
-50 -25 0 25 50 75 100 125 10 12 14 16 18 20 22
Temperature (°C) Supply Voltage (V)

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 RT8409

Sense Threshold vs. Temperature FB Voltage vs. Supply Voltage

1.20 1.80

1.15
1.78
1.10
Sense Voltage (V)

FB Voltage (V)
1.05
1.76
1.00

0.95 1.74

0.90
1.72
0.85
VCC = 15V
0.80 1.70
-50 -25 0 25 50 75 100 125 10 12 14 16 18 20 22
Temperature (°C) Supply Voltage (V)

FB Voltage vs. Temperature FB OVP vs. Supply Voltage

1.86 2.10
1.84 2.08

1.82 2.06
2.04
1.80
FB Voltage (V)

FB OVP (V)

2.02
1.78
2.00
1.76
1.98
1.74
1.96
1.72
1.94
1.70 1.92
1.68 1.90
VCC = 15V
1.66 1.88
-50 -25 0 25 50 75 100 125 10 12 14 16 18 20 22
Temperature (°C) Supply Voltage (V)

FB OVP vs. Temperature

2.20
2.16
2.12
2.08
FB OVP (V)

2.04
2.00
1.96
1.92
1.88
1.84
VCC = 15V
1.80
-50 -25 0 25 50 75 100 125
Temperature (°C)

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RT8409
Applications Information
The RT8409 is a constant off time control, high efficiency Input Diode Bridge Rectifier Selection
constant voltage high side converter with internal MOSFET. The voltage rating of the input bridge rectifier, VBR,
It can be used in buck down solution, to provide a constant dependent on the input voltage. Thus, the VBR rating is
output voltage to the load. It contains high side bootstrap calculated as below :
voltage sense circuitry, while minimizing external
component count. The SOP8 package keeps application
VBR = 1.2   2  VAC MAX  
where VAC(MAX) is the maximum input voltage (RMS)
footprint small, and makes the RT8409 a cost effective
and the parameter 1.2 is used for safety margin.
solution for off-line buck converter.
For this example :
The RT8409 can achieve high accuracy output voltage via
FB pin, which is the average output voltage sense feedback VBR = 1.2    
2  VAC MAX  = 1.2  2  264 = 448V 
loop control pin. The internal FB reference voltage (1.75V If the input source is universal, VBR will reach 448V. In
typ.) is used to set the average output voltage by the this case, a 600V, 0.5A bridge rectifier can be chosen.
external resistor, R2A, R2B. The sense voltage from sense
pin is used for Over Current Protection (OCP) function. Inductor Selection
For best efficiency, the RT8409 should be operated near
Under-Voltage Lockout (UVLO) boundary conduction mode. Based on this recommendation,
The RT8409 includes a UVLO function with 11V hysteresis. the required inductor value is related to the input voltage,
For system start up, the VCC must rise over 18V (typ.) to output voltages, the min. on-time and the max. off time.
turn on the internal MOSFET. The inductor saturation current will be related to the over
current limit set by the sense resistor between the sense
The internal MOSFET will be turned off if VCC falls below
pin and ground pin. The over current limit design information
7V (typ.)
can be found in the later section.
Setting Average Output Voltage The peak current of inductor is showed as below :
The output voltage that provides to the output load is set VIN  VOUT
IPEAK =  tON
by external resistors, R2A, R2B, which is connected L
between the cathode of D7 and SGND pins. The
 VIN  VOUT    VOUT + VF_D5 
relationship between output voltage, and R2A, R2B is L 
2  fSW  IOUT  VIN
shown below :
VOUT
R2A + R2B t ON =  tOFF_1
VOUT =  1.75V + VF_D7 - VF_D5 VIN  VOUT
R 2B
Start-Up Resistor K b + Kb2 + 4  K a  K c
t OFF_1 =
The start-up resistor should be chosen to set the start up 2  Ka
current exceeds certain minimum value. Otherwise, the VOUT  VIN
Ka =
RT8409 may latch off and the system will never start. The L   VIN  VOUT 
start-up current equals VOUT  IOUT
Kb =
VIN  VOUT
 2  90V  Kc = IOUT  tOFF
RSTART Where
where 90V is assumed the minimum line input voltage. VOUT is output voltage.
The typical required minimum start-up current is 60μA. VIN is input voltage.
The recommended total start-up resistance RSTART for
IOUT is full load current.
universal inputs should be no more than 2MΩ.
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 RT8409
fSW is switching frequency from 30kHz to 55kHz. Thermal Considerations
tOFF is constant off time (25μs, Typ.). The junction temperature should never exceed the
absolute maximum junction temperature TJ(MAX), listed
Forward Diode Selection under Absolute Maximum Ratings, to avoid permanent
When the power switch turns off, the path for the current damage to the device. The maximum allowable power
is through the diode connected between the switch output dissipation depends on the thermal resistance of the IC
and ground. This forward biased diode must have minimum package, the PCB layout, the rate of surrounding airflow,
voltage drop and recovery time. The reverse voltage rating and the difference between the junction and ambient
of the diode should be greater than the maximum input temperatures. The maximum power dissipation can be
voltage and the current rating should be greater than the calculated using the following formula :
maximum load current. PD(MAX) = (TJ(MAX) − TA) / θJA
The peak voltage stress of diode is : where TJ(MAX) is the maximum junction temperature, TA is
VD = 1.2    
2  VAC MAX  = 1.2  2  264 = 448V  the ambient temperature, and θJA is the junction-to-ambient
thermal resistance.
The input source is universal (VIN = 90V to 264V), VD will
For continuous operation, the maximum operating junction
reach 448V.
temperature indicated under Recommended Operating
Conditions is 125°C. The junction-to-ambient thermal
Bootstrap Diode Selection
resistance, θJA, is highly package dependent. For a
The bootstrap diode is connected the switch output to
SOP-8 package, the thermal resistance, θ JA , is
provide supply voltage for VCC, and output voltage for FB
206.9°C/W on a standard JEDEC low effective-thermal-
divided resistors. The reverse voltage rating of the diode
conductivity two-layer test board. The maximum power
should be greater than maximum input voltage. A fast
dissipation at TA = 25°C can be calculated as below :
diode can be used, such as ES1J or FR107.
PD(MAX) = (125°C − 25°C) / (206.9°C/W) = 0.48W for a
Sense Resistor Selection SOP-8 package.
The resistor, R5, between the Source of the external N- The maximum power dissipation depends on the operating
MOSFET and SGND should be selected to provide ambient temperature for the fixed TJ(MAX) and the thermal
adequate switch current to drive the application without resistance, θJA. The derating curves in Figure 1 allows
exceeding the current limit threshold set by the SENSE the designer to see the effect of rising ambient temperature
pin sense threshold of the RT8409. The Sense resistor on the maximum power dissipation.
value can be calculated according to the formula below :
VCLT
R2 
IOCP

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.

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RT8409
0.6 Layout Consideration
Maximum Power Dissipation (W)1

Two-Layer PCB
For best performance of the RT8409, the following layout
0.5
guidelines should be strictly followed.
0.4 The hold up capacitor, C1, must be placed as close as
possible to the VCC pin.
0.3
 The compensation component C2 and R1, must be
0.2 placed as close as possible to the COMP pin.
The IC SOURCE pin are high frequency switching nodes.
0.1
The traces must be as wide and short as possible.
0.0
 Keep the main traces with switching current as short
0 25 50 75 100 125
and wide as possible.
Ambient Temperature (°C)
Place CIN, L1, R2, COUT, and D5 as close to each other
Figure 1. Derating Curve of Maximum Power Dissipation
as possible.

Place the capacitor C1 as close as

possible to the VCC pin

VMAIN

RSTART
DRAIN
DRAIN
VCC
NC

R3
VCC
8

C1
FB

CIN D8
R2B R2A
2

4
SGND
COMP
FB
SENSE

Analog GND C3 D7
R2 L1
VOUT
Analog GND
D5
C2 R1
R4 COUT

Power GND Power GND

Place the compensation

Place the Diode D5 and the resistor R2 as Narrow trace from main circuit
Components C2 and R1 as
close as possible to the SOURCE pin to the IC to avoid the switching
close as possible to the IC
noise

Figure 2. PCB Layout Guide

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 RT8409
Outline Dimension

H
A

J B

C
I
D

Dimensions In Millimeters Dimensions In Inches

Symbol
Min Max Min Max
A 4.801 5.004 0.189 0.197
B 3.810 3.988 0.150 0.157
C 1.346 1.753 0.053 0.069
D 0.330 0.508 0.013 0.020
F 1.194 1.346 0.047 0.053
H 0.170 0.254 0.007 0.010
I 0.050 0.254 0.002 0.010
J 5.791 6.200 0.228 0.244
M 0.400 1.270 0.016 0.050

8-Lead SOP Plastic Package

Richtek Technology Corporation

14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789

Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.

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