RT7306

RT7306
Primary-Side Regulation Dimmable LED Driver Controller
with Active-PFC
General Description
Features
The RT7306 is a constant current LED driver with active
power factor correction. It supports high power factor
across a wide range of line voltages, and it drives the
converter in the Quasi-Resonant (QR) mode to achieve
higher efficiency. By using Primary Side Regulation
(PSR), the RT7306 controls the output current
accurately without a shunt regulator and an opto-coupler
at the secondary side, reducing the external component

count, the cost, and the volume of the driver board.

RT7306 is compatible with analog dimming. The output
current can be modulated by the DIM pin. An in-house
design high voltage (HV) start-up device is integrated in
the RT7306 to minimize the power loss and shorten the
start-up time.










Tight LED Current Regulation
No Opto-Coupler and TL431 Required
Power Factor Correction (PFC)
Compatible with Analog Dimming
Built-in HV Start-up Device
Quasi-Resonant
Maximum/Minimum Switching Frequency
Clamping
Maximum/Minimum on-Time Limitation
Wide VDD Range (up to 34V)
THD Optimization
Input-Voltage Feed-Forward Compensation
Multiple Protection Features

LED Open-Circuit Protection
LED Short-Circuit Protection
Output Diode Short-Circuit Protection
VDD Under-Voltage Lockout
VDD Over-Voltage Protection
Over-Temperature Protection

Cycle-by-Cycle Current Limitation


The RT7306 embeds comprehensive protection
functions for robust designs, including LED open-circuit
protection, LED short-circuit protection, output diode
short-circuit protection, VDD Under-Voltage Lockout



(UVLO), VDD Over-Voltage Protection (VDD OVP),
Over-Temperature Protection (OTP), and cycle-by-cycle
current limitation.
Applications

AC/DC LED Lighting Driver
Simplified Application Circuit
Buck-Boost Application Circuit
Flyback Application Circuit
TX1
BD
Line
RHV
CIN
CVDD
CCOMP
DOUT
HV
VOUT+
CCOMP
RPC
RCS
GND
Analog
Dimming
Signal
Q1
RT7306 GD
CVDD
COMP
CS
DIM
ZCD
COUT
VDD
Neutral
Q1
GD
RHV
CIN
VOUT-
RT7306
VOUT-
Line
COUT
COMP
Analog
Dimming
Signal
VOUT+
HV
VDD
Neutral
TX1
BD
DOUT
CS
DIM
ZCD
RPC
RCS
GND
DAUX
DAUX
RZCD1
RZCD1
RZCD2
RZCD2
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS7306-00
July 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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RT7306
Ordering Information
RT7306
Package Type
S : SOP-8
Lead Plating System
G : Green (Halogen Free and Pb Free)
Marking Information
RT7306GS : Product Number
YMDNN : Date Code
RT7306
GSYMDNN
Pin Configurations
(TOP VIEW)
Note :
Richtek products are :

RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.

8
HV
VDD
GND
2
7
GD
COMP
3
6
CS
DIM
4
5
ZCD
Suitable for use in SnPb or Pb-free soldering processes.
SOP-8
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
HV
High Voltage Input for Startup.
2
GND
Ground of the Controller.
3
COMP
Compensation Node. Output of the internal trans-conductance amplifier.
4
DIM
Analog Dimming Signal Input. LED driving current can be adjusted by an analog
voltage.
5
ZCD
Zero Current Detection Input. This pin is used to sense the voltage at auxiliary winding
of the transformer.
6
CS
Current Sense Input. Connect this pin to the current sense resistor.
7
GD
Gate Driver Output for External Power MOSFET.
8
VDD
Supply Voltage (VDD) Input. The controller will be enabled when VDD exceeds VTH_ON
and disabled when VDD is lower than VTH_OFF.
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RT7306
Function Block Diagram
HV
Valley Signal
Valley
Detector
ZCD
Clamping
Circuit
THD
Optimizer
Feed-Forward
Compensator
Ramp
Generator
Starter
Circuit
ICS
Under-Voltage
Lockout
(16V/9V)
UVLO
+
VDD OVP
Constant on-Time
Comparator
Constant
Current
Control
HV Start-Up
Control
PWM
Control
Logic
VDD
Over-Voltage
Protection
VCLAMP 13V
PWM
Output
Over-Voltage
Protection
VCS_CL
1.2V
Leading
Edge
Blanking
CS
+
VDD
GD
Gate
Driver
Current-Limit
Comparator
RGD
Output Diode
Short-Circuit
Protection
GND
Dimming
Control
OverTemperature
Protection
OTP
Output OVP
COMP
DIM
Operation
Critical-Conduction Mode (CRM) with Constant
On-Time Control
Figure 1 shows a typical flyback converter with input
voltage (VIN). When main switch Q1 is turned on with
a fixed on-time (tON), the peak current (IL_PK) of the
magnetic inductor (Lm) can be calculated by the
following equation :
If the input voltage is the output voltage of the
full-bridge rectifier with sinusoidal input voltage
(VIN_PKsin()), the inductor peak current (IL_PK) can be
expressed as the following equation :
IL_PK 
VIN_PK  sin(θ)  tON
Lm
When the converter operates in CRM with constant
on-time control, the envelope of the peak inductor
current will follow the input voltage waveform with
V
IL_PK  IN  tON
Lm
in-phase. Thus, high power factor can be achieved, as
shown in Figure 2.
TX1
NP:NS DOUT
IL
+
COUT
Lm
VIN
VOUT
-
IOUT
ROUT
Q1
Figure 1. Typical Flyback Converter
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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RT7306
Voltage Clamping Circuit
VIN
Input Voltage
IL_PK
Peak Inductor Current
IQ1_DS
MOSFET Current
Iin_avg
Average Input Current
IDo
Output Diode Current
VQ1_GS
MOSFET Gate Voltage
Figure 2. Inductor Current of CRM with Constant
On-Time Control
RT7306 needs no shunt regulator and opto-coupler at
the secondary side to achieve the output current
regulation. Figure 3 shows several key waveforms of a
conventional flyback converter in Quasi-Resonant (QR)
mode, in which VAUX is the voltage on the auxiliary
winding of the transformer.
VDS
VIN
0
GD
(VGS)
VAUX
(VOUT + Vf) x Naux / NS
0
VIN x Naux / NP
Clamped by Controller
The RT7306 provides a voltage clamping circuit at ZCD
pin since the voltage on the auxiliary winding is
negative when the main switch is turned on. The lowest
voltage on ZCD pin is clamped near zero to prevent the
IC from being damaged by the negative voltage.
Meanwhile, the sourcing ZCD current (IZCD_SH),
flowing through the upper resistor (RZCD1), is sampled
and held to be a line-voltage-related signal for
propagation delay compensation. The RT7306 embeds
the programmable propagation delay compensation
through CS pin. A sourcing current ICS (equal to
IZCD_SH x KPC) applies a voltage offset (ICS x RPC)
which is proportional to line voltage on CS to
compensate the propagation delay effect. Thus, the
output current can be equal at high and low line
voltage.
Quasi-Resonant Operation
Figure 4 illustrates how valley signal triggers PWM.
If
no valley signal detected for a long time, the next PWM
is triggered by a starter circuit at end of the interval
(tSTART, 130s typ.) which starts at the rising edge of
the previous PWM signal. A blanking time (tS(MIN),
8.5μs typ.), which starts at the rising edge of the
previous PWM signal, limits minimum switching period.
When the tS(MIN) interval is on-going, all of valley
signals are not allowed to trigger the next PWM signal.
After the end of the tS(MIN) interval, the coming valley
will trigger the next PWM signal. If one or more valley
signals are detected during the tS(MIN) interval and no
valley is detected after the end of the tS(MIN) interval,
the next PWM signal will be triggered automatically at
end of the tS(MIN) + 5s (typ.).
IQ1
IDOUT
Figure 3. Key Waveforms of a Flyback Converter
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RT7306
KCC (V)
～
～
Valley
Signal
～
～
PWM
KCC(MAX)
tSTART
0 VDIM_LOW V
DIM_HIGH
Valley
Signal
VDIM (V)
Figure 5. Dimming Curve
Protections
PWM
tS(MIN)
LED Open-Circuit Protection
In an event of output open circuit, the converter will be
shut down to prevent being damaged, and it will be
auto-restarted when the output is recovered. Once the
Valley
Signal
PWM
LED is open-circuit, the output voltage keeps rising,
causing the voltage on ZCD pin VZCD rising accordingly.
tS(MIN)
Valley
Signal
PWM
tS(MIN)
5μs
When the sample-and-hold ZCD voltage (VZCD_SH)
exceeds its OV threshold (VZCD_OVP, 3.2V typ.), output
OVP will be activated and the PWM output (GD pin) will
be forced low to turn off the main switch. If the output is
still open-circuit when the converter restarts, the
converter will be shut down again.
Figure 4. PWM Triggered Method
Output Diode Short-Circuit Protection
HV Start-up Device
An in-house design 500V start-up device is integrated
in RT7306 to minimize the power loss and shorten the
start-up time. The HV start-up device will be turned on
during start-up period and be turned off during normal
operation. It provides fast start-up time and no power
loss in this path during normal operation. A 10k
resistor is recommended to be connected in series with
HV pin.
Dimming Function
An analog dimming function is embedded in RT7306.
When the voltage on the DIM pin (VDIM) is within
VDIM_LOW and VDIM_HIGH, the regulation factor of
constant current control (KCC) is linearly proportional to
VDIM, as shown in Figure 5.
When the output diode is damaged as short-circuit, the
transformer will be led to magnetic saturation and the
main switch will suffer from a high current stress. To
avoid the above situation, an output diode short-circuit
protection is built-in. When CS voltage VCS exceeds
the threshold (VCS_SD 1.7 typ.) of the output diode
short-circuit protection, RT7306 will shut down the
PWM output (GD pin) in few cycles to prevent the
converter from damage. It will be auto-restarted when
the failure condition is recovered.
VDD Under-Voltage Lockout (UVLO) and
Over-Voltage Protection (VDD OVP)
RT7306 will be enabled when VDD voltage (VDD)
exceeds rising UVLO threshold (VTH_ON, 17V typ.) and
disabled when VDD is lower than falling UVLO
threshold (VTH_OFF, 8.5V typ.).
When VDD exceeds its over-voltage threshold (VOVP,
37.4V typ.), the PWM output of RT7306 is shut down. It
will be auto-restarted when the VDD is recovered to a
normal level.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS7306-00
July 2015
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RT7306
Over-Temperature Protection (OTP)
The RT7306 provides an internal OTP function to
protect the controller itself from suffering thermal stress
and permanent damage. It's not suggested to use the
function as precise control of over temperature. Once
the junction temperature is higher than the OTP
threshold (TSD, 150C typ.), the controller will shut
down until the temperature cools down by 30C (typ.).
Meanwhile, if VDD reaches falling UVLO threshold
voltage (VTH_OFF), the controller will hiccup till the over
temperature condition is removed.
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RT7306
Absolute Maximum Ratings
(Note 1)

HV Pin------------------------------------------------------------------------------------------------------------------- 0.3V to 500V

Supply Voltage, VDD ------------------------------------------------------------------------------------------------- 0.3V to 40V

Gate Driver Output, GD --------------------------------------------------------------------------------------------- 0.3V to 20V

Other Pins -------------------------------------------------------------------------------------------------------------- 0.3V to 6V

Power Dissipation, PD @ TA = 25C
SOP-8 -------------------------------------------------------------------------------------------------------------------- 0.48W

Package Thermal Resistance
(Note 2)
SOP-8, JA -------------------------------------------------------------------------------------------------------------- 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) (Except HV pin)-------------------------------------------------------------------- 2kV
MM (Machine Model) ------------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
(Note 4)

Supply Input Voltage, VDD ----------------------------------------------------------------------------------------- 11V to 34V

COMP Voltage, VCOMP --------------------------------------------------------------------------------------------- 0.7V to 4.3V

Junction Temperature Range -------------------------------------------------------------------------------------- 40C to 125C
Electrical Characteristics
(VDD = 15V, TA = 25C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
VDD < VTH_ON, VHV = 100V
VDD = VTH_ON + 1V, VHV =
500V
1
--
--
mA
--
--
30
A
35.4
37.4
39.4
V
16
17
18
V
7.5
8.5
9.5
V
HV Section
HV Start-up Average Current
IHV_ST
Off State Leakage Current
VDD Section
VDD OVP Threshold Voltage
VOVP
Rising UVLO Threshold Voltage
VTH_ON
VDD Rising
Falling UVLO Threshold Voltage VTH_OFF
Fault Released Voltage
VTH_FR
--
6
--
V
VDD Holdup Mode Entry Point
VDD_ET
--
10
--
V
VDD Holdup Mode Ending Point VDD_ED
--
10.5
--
V
VDD = 15V, IZCD = 0, GD open
--
2
3
mA
VDD = VTH_OFF
--
60
--
A
IVDD_ST
VDD = VTH_ON  1V
--
15
30
A
Lower Clamp Voltage
VZCDL
IZCD = 0 to 2.5mA
50
0
60
mV
ZCD OVP Threshold Voltage
VZCD_OVP
3.04
3.2
3.36
V
Operating Current
IDD_OP
Operating Current at Shutdown
Start-up Current
ZCD Section
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RT7306
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
250
300
350
mV
--
2.8
--
V
0.5
1
2
A
246.25
250
253.75
mV
Dimming Control Section
Analog Dimming Low Threshold
Voltage
VDIM_LOW
Analog Dimming High Threshold
VDIM_HIGH
Voltage
DIM Sourcing Current
Constant Current Control Section
Maximum Regulated factor for
constant-current control
KCC(MAX)
Maximum Comp Voltage
VCOMP(MAX)
5
5.5
--
V
Minimum Comp Voltage
VCOMP(MIN)
--
0.5
--
V
Maximum Sourcing Current
ICOMP(MAX)
--
100
--
A
240
400
570
ns
Peak Current Shutdown Voltage
VCS_SD
Threshold
1.53
1.7
1.87
V
Peak Current Limitation at
Normal Operation
VCS_CL
1.08
1.2
1.32
V
Propagation Delay
Compensation Factor
KPC
ICS = KPC x IZCD, IZCD =
150A
--
0.042
--
A/A
Rising Time
tR
VDD = 15V, CL = 1nF
--
250
350
ns
Falling Time
tF
VDD = 15V, CL = 1nF
--
40
70
ns
Gate Output Clamping Voltage
VCLAMP
VDD = 15V, CL = 1nF
10.8
12
13.2
V
Internal Pull Low Resistor
RGD
--
40
--
k
1
1.25
1.6
s
VDIM = 3V
During start-up period
Current Sense Section
Leading Edge Blanking Time
tLEB
Gate Driver Section
Timing Control Section
IZCD = 150A
Minimum on-Time
tON(MIN)
Minimum Switching Period
tS(MIN)
7
8.5
10
s
Duration of Starter at Normal
Operation
tSTART
75
130
300
s
Maximum on-Time
tON(MAX)
29
47
65
s
Over-Temperature Protection (OTP) Section
OTP Temperature Threshold
TOTP
(Note 5)
--
150
--
C
OTP Temperature Hysteresis
TOTP-HYS
(Note 5)
--
30
--
C
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. JA is measured at TA = 25C on a low effective two-layer thermal conductivity test board of JEDEC 51-3.
Note 3. Devices are ESD sensitive. Handling precaution recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Note 5. Guarantee by design.
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RT7306
Typical Application Circuit
Flyback Application Circuit
RSN3
F1
TX1
BD
CSN2
DOUT
+
Line
COUT
CIN
RHV
CSN1
～
～ VOUT
RSN1
Neutral
-
DSN
Analog
Dimming
Signal
1
HV
4 DIM
8
GD
RG
7
RGP
RT7306
VDD
CVDD
3
CCOMP
5
Q1
RPC
6
CS
COMP
ZCD
RCS
2
GND
DAUX
RAUX
RZCD1
CZCD
RZCD2
Buck-Boost Application Circuit
F1
TX1
BD
-
Line
DOUT
RHV
CIN
COUT
～ VOUT
～
+
Neutral
Analog
Dimming
Signal
4
8
1
HV
5
Q1
RGP
RT7306
VDD
CS
3
7 RG
DIM
CVDD
CCOMP
GD
6
RCS
COMP
ZCD
RAUX
RPC
GND
2
DAUX
RZCD1
CZCD
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RT7306
Typical Operating Characteristics
IHV_ST vs. Junction Temperature
VOVP vs. Junction Temperature
36.7
5
36.6
4
VOVP (V)
I HV_ST (mA)
36.5
3
2
36.4
36.3
36.2
1
36.1
36.0
0
-50
-25
0
25
50
75
100
-50
125
-25
Junction Temperature (°C)
25
50
75
100
125
VTH_OFF vs. Junction Temperature
VTH_ON vs. Junction Temperature
8.60
17.15
17.10
8.56
17.05
VTH_OFF (V)
VTH_ON (V)
0
Junction Temperature (°C)
17.00
16.95
8.52
8.48
8.44
16.90
8.40
16.85
-50
-25
0
25
50
75
100
-50
125
-25
Junction Temperature (°C)
0
25
50
75
100
125
Junction Temperature (°C)
VZCDL vs. Junction Temperature
IDD_OP vs. Junction Temperature
25
2.00
1.95
20
VZCDL (mV)
I DD_OP (mA)
1.90
1.85
1.80
15
10
1.75
5
1.70
0
1.65
-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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10
-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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RT7306
KCC vs. Junction Temperature
260
380
258
KCC (mV)
VDIM_LOW (V)
VDIM_LOW vs. Junction Temperature
400
360
256
254
340
252
320
-50
-25
0
25
50
75
100
-50
125
-25
25
50
75
100
125
ICOMP(MAX) vs. Junction Temperature
VCOMP(MAX) vs. Junction Temperature
5.26
140
5.24
120
5.22
100
I COMP(MAX) (A)
VCOMP(MAX) (V)
0
Junction Temperature (°C)
Junction Temperature (°C)
5.20
5.18
80
60
5.16
40
5.14
20
0
5.12
-50
-25
0
25
50
75
100
-50
125
-25
Junction Temperature (°C)
0
25
50
75
100
125
Junction Temperature (°C)
VCS_SD vs. Junction Temperature
tLEB vs. Junction Temperature
500
1.850
480
VCS_SD (V)
tLEB (ns)
1.825
460
440
1.800
1.775
420
400
1.750
-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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RT7306
VCS_CL vs. Junction Temperature
KPC vs. Junction Temperature
0.045
1.30
0.044
1.26
KPC (A/A)
VCS_CL (V)
1.28
1.24
0.043
0.042
1.22
0.041
1.20
0.040
1.18
-50
-25
0
25
50
75
100
-50
125
-25
Junction Temperature (°C)
260
51
250
50
240
49
tf (ns)
tr (ns)
25
50
75
100
125
tf vs. Junction Temperature
tr vs. Junction Temperature
230
48
220
47
210
46
45
200
-50
-25
0
25
50
75
100
-50
125
-25
0
25
50
75
100
125
Junction Temperature (°C)
Junction Temperature (°C)
tON(MIN) vs. Junction Temperature
tSTART vs. Junction Temperature
1.30
150
1.25
140
130
tSTART (μs)
tON(MIN) (μs)
0
Junction Temperature (°C)
1.20
1.15
1.10
120
110
100
1.05
90
-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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RT7306
tON(MAX) vs. Junction Temperature
60
tON(MAX) (μs)
50
40
30
20
10
0
-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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RT7306
Application Information
Output Current Setting
Considering the conversion efficiency, the programmed
DC level of the average output current (IOUT(t)) can be
derived as :
t
VCOMP = IZCD_SH   ON   KRAMP  tON + VD
 tS 
IOUT_CC  1  NP  KCC  CTRTX1
2 NS RCS
IZCD_SH can be expressed as :
IZCD_SH 
CTRTX1 
ISEC_PK NS

IPRI_PK NP
in which CTRTX1 is the current transfer ratio of the
transformer TX1, ISEC_PK is the peak current of the
secondary side, and IPRI_PK is the peak current of the
primary side. CTRTX1 can be estimated to be 0.9.
According to the above parameters, current sense
resistor RCS can be determined as the following
equation :
RCS  1  NP  KCC  CTRTX1
2 NS IOUT_CC
Propagation Delay Compensation Design
The VCS deviation (VCS) caused by propagation delay
effect can be derived as:
V  t R
VCS  IN D CS ,
Lm
in which tD is the delay period which includes the
propagation delay of RT7306 and the turn-off transition
of the main MOSFET. The sourcing current from CS pin
of RT7306 (ICS) can be expressed as :
N
ICS  KPC  VIN  A  1
NP RZCD1
where NA is the turns number of the auxiliary winding.
RPC can be designed by :
RPC 
VCS tD  RCS  RZCD1 NP


ICS
Lm  KPC
NA
In addition, RPC must be higher than 750.
Feed-Forward Compensation Design
The COMP voltage, VCOMP, is a function of the
sample-and-hold ZCD current (IZCD_SH) as following :
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VIN  NA
RZCD1  NP
KRAMP is a constant in the ramp generator, and its
typical value is 3.1109 V/(sec  A).
VD is the offset of the constant on-time comparator, and
its typical value is 0.63V. It is recommended to design
VCOMP = 2 to 3V. If the COMP voltage is over its
recommended operating range (0.7 to 4.3V), output
current regulation may be affected. Thus, the resistors
RZCD1 can be determined according to the above
parameters.
Minimum On-Time Setting
The RT7306 limits a minimum on-time (tON(MIN)) for
each switching cycle. The tON(MIN) can be derived from
the following equations.
tON(MIN)  IZCD_SH  187.5p  sec  A (typ.)
Thus, RZCD1 can be determined by:
RZCD1 
tON(MIN)  VIN NA

(typ.)
187.5p
NP
In addition, the current flowing out of ZCD pin must be
lower than 2.5mA (typ.). Thus, the RZCD1 is also
determined by :
RZCD1 
2  VAC(MAX) NA

2.5m
NP
where the VAC(MAX) is maximum input AC voltage.
Output Over-Voltage Protection Setting
Output OVP is achieved by sensing the voltage on the
auxiliary winging. It is recommended that output OV
level (VOUT_OVP) is set at 120% of nominal output
voltage (VOUT). Thus, RZCD1 and RZCD2 can be
determined by the equation as :
RZCD2
N
VOUT  A 
 120%  3.2V(typ.)
NS RZCD1  RZCD2
is a registered trademark of Richtek Technology Corporation.
DS7306-00
July 2015
RT7306
When the MOSFET is turned off, the leakage
inductance of the transformer and parasitic capacitance
(COSS) of the MOSFET induce resonance waveform on
the ZCD pin. The resonance waveform may make the
controller false trigger the ZCD OVP, and it may cause
the controller operate in unstable condition. As load
increases, the resonance time also increases. It is
recommended to add a 10pF to 47pF bypass capacitor,
and it should be as close to ZCD pin as possible. The
larger bypass capacitor may cause phase shift on ZCD
waveform, so the MOSFET is not turned on at exact
valley point.
To avoid the above issue, the RT7306 provides
adaptive blanking time (tBK). It varies with the peak
voltage of the CS pin (VCS_PK), as shown by the
following formula :
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation.
0.6
Maximum Power Dissipation (W)1
Adaptive Blanking Time
Two-Layer PCB
0.5
0.4
0.3
0.2
0.1
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 6. Derating Curve of Maximum Power
Dissipation
tBK = 2s + VCS_PK x 2s/V (typ.)
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,
TA is the ambient temperature, and θJA is 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-8 packages, the thermal
resistance, θJA, is 206.9°C/W on a standard JEDEC
51-3 two-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) / (206.9°C/W) = 0.48W for
SOP-8 package.
The maximum power dissipation depends on the
operating ambient temperature for fixed TJ(MAX) and
thermal resistance, θJA. The derating curve in Figure 6
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS7306-00
July 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
15
RT7306
Layout Considerations

The path(5) from input capacitor to HV pin is a high
voltage loop. Keep a space from path(5) to other low
voltage traces.

It is good for reducing noise, output ripple and EMI
issue to separate ground traces of input capacitor(a),
MOSFET(b), auxiliary winding(c) and IC control
circuit(d). Finally, connect them together on input
capacitor ground(a). The areas of these ground
traces should be kept large.

To minimize parasitic trace inductance and EMI,
minimize the area of the loop connecting the
secondary winding, the output diode, and the output
filter capacitor. In addition, apply sufficient copper
A proper PCB layout can abate unknown noise
interference and EMI issue in the switching power
supply. Please refer to the guidelines when designing a
PCB layout for switching power supply :

The current path(1) from input capacitor, transformer,
MOSFET, RCS return to input capacitor is a high
frequency current loop. The path(2) from GD pin,
MOSFET, RCS return to the ground of the IC is also
a high frequency current loop. They must be as short
as possible to decrease noise coupling and kept a
space to other low voltage traces, such as IC control
circuit paths, especially. Besides, the path(3)
between MOSFET ground(b) and IC ground(d) is
recommended to be as short as possible, too.

area at the anode and cathode terminal of the diode
for heat-sinking. It is recommended to apply a larger
area at the quiet cathode terminal. A large anode
area will induce high-frequency radiated EMI.
The path(4) from RCD snubber circuit to MOSFET is
a high switching loop. Keep it as small as possible.
Line
～
～
CIN
Neutral
(4)
(5)
(a)
HV
GD
DM
RT7306
VDD
CVDD
CS
CCOMP
COMP
ZCD
(2)
(1)
GND
(d)
(3)
Input Capacitor
Ground (a)
(b)
Trace
Trace
IC
Ground (d)
Trace
Auxiliary
Ground (c)
MOSFET
Ground (b)
(c)
Figure 7. PCB Layout Guide
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is a registered trademark of Richtek Technology Corporation.
DS7306-00
July 2015
RT7306
Outline Dimension
Dimensions In Millimeters
Symbol
Dimensions In Inches
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.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS7306-00
July 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
17