RT7306D
Primary-Side Regulation Dimmable LED Driver Controller
with Active-PFC
General Description
Features
The RT7306D 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 RT7306D 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.
RT7306D 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 RT7306D 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 RT7306D 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
VDD
Neutral
CVDD
RT7306D
Analog
Dimming
Signal
RHV
Neutral
CVDD
Q1
GD
COUT
CCOMP
RPC
RCS
GND
Analog
Dimming
Signal
DOUT
HV
VDD
RT7306D GD
COMP
CS
DIM
ZCD
VOUT-
Line
CIN
VOUT-
COMP
CCOMP
VOUT+
COUT
HV
TX1
BD
DOUT
CS
DIM
ZCD
VOUT+
Q1
RPC
RCS
GND
DAUX
DAUX
RZCD1
RZCD1
RZCD2
RZCD2
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS7306D-01
September
2016
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RT7306D
Ordering Information
Marking Information
RT7306D
Package Type
S : SOP-8
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
RT7306DGS : Product Number
YMDNN : Date Code
RT7306D
GSYMDNN
Pin Configuration
(TOP VIEW)
Richtek products are :
RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
8
HV
VDD
GND
2
7
GD
COMP
3
6
CS
DIM
4
5
ZCD
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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is a registered trademark of Richtek Technology Corporation.
DS7306D-01
September
2016
RT7306D
Functional 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
VDD
Over-Voltage
Protection
VCLAMP 13V
PWM
Output
Over-Voltage
Protection
VCS_CL
1.2V
Leading
Edge
Blanking
CS
+
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_PKsin()), 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 © 2016 Richtek Technology Corporation. All rights reserved.
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2016
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RT7306D
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
The RT7306D 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 RT7306D 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 RT7306D
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, 130s 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) + 5s (typ.).
IQ1
IDOUT
Figure 3. Key Waveforms of a Flyback Converter
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DS7306D-01
September
2016
RT7306D
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 the RT7306D 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 the RT7306D.
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.
DIM pin sourcing a current (1A typ.) when VDD > VTH_ON,
and the sourcing current is shut down after 100ms (typ.)
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS7306D-01
September
2016
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, the RT7306D 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)
The RT7306D 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 the RT7306D is shut
down. It will be auto-restarted when the VDD is
recovered to a normal level.
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RT7306D
Over-Temperature Protection (OTP)
The RT7306D 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, 150C typ.), the controller will shut
down until the temperature cools down by 30C (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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is a registered trademark of Richtek Technology Corporation.
DS7306D-01
September
2016
RT7306D
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 = 25C
SOP-8 -------------------------------------------------------------------------------------------------------------------- 0.48W
Package Thermal Resistance
(Note 2)
SOP-8, JA -------------------------------------------------------------------------------------------------------------- 206.9C/W
Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------- 260C
Junction Temperature ------------------------------------------------------------------------------------------------ 150C
Storage Temperature Range --------------------------------------------------------------------------------------- 65C to 150C
ESD Susceptibility
(Note 3)
HBM (Human Body Model) (Except HV pin)-------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)
Supply Input Voltage, VDD ----------------------------------------------------------------------------------------- 11V to 34V
COMP Voltage, VCOMP --------------------------------------------------------------------------------------------- 0.7V to 4.3V
Junction Temperature Range -------------------------------------------------------------------------------------- 40C to 125C
Electrical Characteristics
(VDD = 15V, TA = 25C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
1
--
--
mA
--
--
30
A
35.4
37.4
39.4
V
16
7.5
17
8.5
18
9.5
V
V
VTH_FR
VDD_ET
---
6
10
---
V
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
HV Section
HV Start-Up Average Current
IHV_ST
Off State Leakage Current
VDD < VTH_ON, VHV = 100V
VDD = VTH_ON + 1V,
VHV = 500V
VDD Section
VDD OVP Threshold Voltage
VOVP
VDD Rising
Rising UVLO Threshold Voltage VTH_ON
Falling UVLO Threshold Voltage VTH_OFF
Fault Released Voltage
VDD Holdup Mode Entry Point
Operating Current
IDD_OP
Operating Current at Shutdown
Start-Up Current
ZCD Section
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2016
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RT7306D
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
5
5.5
--
V
VCOMP(MIN)
--
0.5
--
V
ICOMP(MAX) During start-up period
--
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 = 150A
--
0.042
--
A/A
Rising Time
tR
VDD = 15V, CL = 1nF
--
140
250
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
0.9
1.25
1.6
s
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
Minimum Comp Voltage
Maximum Sourcing Current
VDIM = 3V
)
Current Sense Section
Leading Edge Blanking Time
tLEB
Gate Driver Section
Timing Control Section
IZCD = 150A
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 under natural convection (still air) at TA = 25C 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 recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Note 5. Guarantee by design.
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2016
RT7306D
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
RT7306D
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
RT7306D
VDD
CS
3
7 RG
DIM
CVDD
CCOMP
GD
6
RCS
COMP
ZCD
RAUX
RPC
GND
2
DAUX
RZCD1
CZCD
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September
2016
RZCD2
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RT7306D
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
Junction Temperature (°C)
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10
125
-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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2016
RT7306D
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) (μA)
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
Junction Temperature (°C)
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DS7306D-01
September
2016
125
-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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RT7306D
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)
tON(MIN) vs. Junction Temperature
25
50
75
100
125
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)
-50
-25
0
25
50
75
100
125
Junction Temperature (°C)
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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is a registered trademark of Richtek Technology Corporation.
DS7306D-01
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2016
RT7306D
Application Information
Output Current Setting
Considering the conversion efficiency, the programmed
DC level of the average output current (IOUT(t)) can be
derived as :
IOUT_CC 1 NP KCC CTRTX1
2 NS RCS
CTRTX1
ISEC_PK NS
IPRI_PK NP
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 the RT7306D and the turn-off
transition of the main MOSFET. The sourcing current
from CS pin of the RT7306D (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 :
VCS tD RCS RZCD1 NP
ICS
Lm KPC
NA
When GD pin is turned off, CS pin sources a current
(100A, typ.) flowing through RPC and RCS. If the
voltage on CS pin is lower than its under voltage
threshold (50mV, typ.), CS under voltage protection
(UVP) will be triggered and the GD pin will be forced
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September
Thus, RPC must be higher than 750 to prevent false
triggering CS UVP. In addition, RPC is recommended to
be higher than 1.5k if the ambient temperature will be
lower to 40C
Feed-Forward Compensation Design
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 :
RPC
low to turn off the main switch.
2016
The COMP voltage, VCOMP, is a function of the resistor
RZCD1 as following :
RZCD1
tON Gm
ramp tON
t
N
S
A
= VIN_pk
KIV
NP
2 Cramp VCOMP VD
in which KIV, Gmramp, and Cramp are fixed parameters in
the RT7306D, and the typical value are : KIV = 2.5V/mA,
Gmramp = 8A/V, Cramp = 6.5pF.
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 RT7306D 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.
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RT7306D
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
For continuous operation, the maximum operating
junction temperature indicated under Recommended
Operating Conditions is 125C. The junction-to-ambient
thermal resistance,JA, is highly package dependent. For
a SOP-8 package, the thermal resistance, JA, is
206.9°C/W
on
a
standard
JEDEC
low
effective-thermal-conductivity two-layer test board. The
maximum power dissipation at TA = 25C can be
calculated as below :
PD(MAX) = (125C 25C) / (206.9°C/W) = 0.48W for a
SOP-8 package.
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 RT7306D provides
adaptive blanking time (tBK). It varies with the peak
voltage of the CS pin (VCS_PK), as shown by the
following formula :
tBK = 2s + VCS_PK x 2s/V (typ.)
The maximum power dissipation depends on the
operating ambient temperature for the fixed TJ(MAX) and
the thermal resistance, JA. The derating curves in
Figure 6 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)
Thermal Considerations
Figure 6. Derating Curve of Maximum Power
The junction temperature should never exceed the
absolute maximum junction temperature TJ(MAX), listed
Dissipation
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) / JA
where TJ(MAX) is the maximum junction temperature, TA is
the
ambient
temperature,
and
JA is
the
junction-to-ambient thermal resistance.
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14
is a registered trademark of Richtek Technology Corporation.
DS7306D-01
September
2016
RT7306D
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
RT7306D
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
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS7306D-01
September
2016
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
15
RT7306D
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
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16
is a registered trademark of Richtek Technology Corporation.
DS7306D-01
September
2016
RT7306D
Footprint Information
Package
Footprint Dimension (mm)
Number of Pin
SOP-8
8
Tolerance
P
A
B
C
D
M
1.27
6.80
4.20
1.30
0.70
4.51
±0.10
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 © 2016 Richtek Technology Corporation. All rights reserved.
DS7306D-01
September
2016
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
17
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