AN016 EN

Application Note
Gary Zheng
AN016 – Jun 2014
Guidelines for the RT7321 Off-line Linear LED Driver
Abstract
The RT7321 linear LED driver is designed for dynamically time- and segment-based driving in 220V/230V systems. This
document describes the principles and methods for use of the RT7321 to provide guidance for initial use and references for LED
lighting application engineers. These methods and principles are also applicable to the RT7322, a version for 110V systems.
Contents
1. Overview ..................................................................................................................................................2
2. Introduction to the RT7321 ........................................................................................................................5
3. Internal Constant-current Source...............................................................................................................7
4. Operating Mode of the RT7321 .................................................................................................................8
5. How to Choose the Optimal Current and VF............................................................................................. 13
6. System Design Considerations ............................................................................................................... 16
7. PCB Design ............................................................................................................................................ 18
8. Conclusion .............................................................................................................................................. 18
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Guidelines for the RT7321 Off-line Linear LED Driver
1. Overview
What challenges the most in design of an off-line LED driver is that the forward voltage of an LED is relatively constant, while the
power voltage from the power grid is in sine wave. For this reason, most solutions are subject only to the switch-type conversion
architecture with the function of changing the voltage. The linear drive is essentially to connect variable resistors and LEDs in
series to share the input voltage. Simple use of the linear architecture may cause mismatching between the input and output
voltages. When the input voltage is lower than the forward voltage of an LED, no current passes through the LED. Otherwise, a
resistor or an equivalent device must be added to undertake the excessive voltage, from which the energy is completely wasted.
This may give rise to current deformity, posing a threat to normal operation of the power grid.
To take advantage of such a simple linear circuit, and maximize matching with the supply waveform in the power grid, the best
method is to divide an LED into segments as many as possible, and timely combine different numbers of LED segments by
different input voltage conditions for matching to minimize the voltage loss. In terms of current, different currents need to pass
through the LED at different voltages to achieve a high power factor. Theoretically, we can obtain the nearly 100% conversion
efficiency and the power factor approaching 1 in the case of infinite LED voltage and current segments. However, we can only
make a compromise between performance and cost to achieve feasibility. As a result, the RT7321 comes out.
Let’s see how to connect the circuits of the RT7321 in practice.
As shown in the preceding figure, the voltage from the power grid, after being rectified, is added to the RT7321 which is subject to
the PSOP-8 package. Therefore, no excessive pin is available. However, the LED is divided into several segments for separate
use. This may be different from the use of the previous LED drivers.
The previous linear LED drivers may be divided into two types:
One is the so-called constant-current diode, which is a two-pin element being capable of one-way breakover, but the passing
current is almost stable. When thinking that the element is expensive, you can easily replace it with a simple constant-current
circuit consisting of two resistors, one voltage regulator diode, and one common transistor. The constant-current diode can be
directly connected to the LED in series and then placed in the circuit for use. However, large current and high
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Guidelines for the RT7321 Off-line Linear LED Driver
voltage applied on such a diode may cause severe power dissipation and low efficiency. Therefore, the high-voltage and
low-current LED series should be used to lower the dissipation and improve the efficiency. However, the downside is that stable
current can pass through the LED only when the input voltage is higher than the LED forward voltage and the minimum voltage
drop of the constant-current diode. No current is output in other situations. Moreover, higher efficiency may result in longer time in
which no current passes through the LED as well as a lower power factor.
The other one is the improved and segment-based pass-through LED drive. With such a drive, the LED is divided into several
segments and connected in series. Then the connecting points between segments are connected to the constant-current source
via a switch. When the input voltage is applied, the power volume of the LED shall be adjusted in real time based on the voltage
for the maximal utilization ratio of the LED and full use of the electric energy. The circuit structure is shown as follows:
The constant-current source also involves detection of the voltage and control of switches K1 and K2. It determines the status of
K1 and K2 based on the relationship between the input voltage VIN and breakover voltages of LED G1, G2 and G3. When the
input voltage is higher than the breakover voltage of G1, and lower than the voltage of G1+G2, K1 is connected. The input current
returns to the power supply via the G1-K1-constant-current source, and only G1 flashes. When the input voltage is higher than
the breakover voltage of G1+G2, K1 is disconnected, and K2 is connected. The input current returns to the power supply via the
G1-G2-K2-constant-current source, and both G1 and G2 flash, but G3 does not flash. When the input voltage is higher than the
breakover voltage of G1+G2+G3, K1 and K2 are disconnected, and the input current returns to the power supply via the
G1-G2-G3-constant-current source.
We can see that the current fed to the LED always stays at the same value if the input voltage is higher than the breakover
voltage of LED. As a result, the load in the power grid is a current square wave sequence (shown in the following figure). This is
quite different from the current sine wave which we desire for.
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Guidelines for the RT7321 Off-line Linear LED Driver
Is it possible to design the current waveform as follows?
According to the above figure, the current waveform is more similar to the sine wave upon comparison with the preceding square
wave. Therefore, the waveform can achieve the power factor more close to 1, and the RT7321 results from the attempt.
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Guidelines for the RT7321 Off-line Linear LED Driver
2. Introduction to the RT7321
In design of the RT7321, the LED is designed in four segments, which can be partially or completely connected in series or in
parallel based on the voltage relationship. In each connection mode, the current can also be user-defined to provide a user with
expected current waveform and fully use the LED. Proper design can also achieve the best result of correcting the power factor,
and fully fit with LEDs at different specifications. The following figure shows the voltage segmentation and LED connection, as
well as traditional segmentation mode for comparison.
AC input voltage waveform
Time span
(Voltage)
Lighting mode for traditional
4S segments
Lighting mode for RT7321
segment combination
t1 - t2
(VHV >1 VF)
t2 - t3
(VHV > 2VF)
t3 - t4
(VHV > 3VF)
t4 - t5
(VHV > 4V)
The RT7321 is packed in two types. One is in the 5mm x 5mm WQFN-20L package, which features good thermal property and
many pins, enabling a user to easily set the current value in each time span. The following block diagram shows the internal
structure of a device in this package.
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Guidelines for the RT7321 Off-line Linear LED Driver
Functions of the pins in the package are described as follows:
HV—the power supply pin of IC, with the operating voltage of 1–400V, and the maximum allowable voltage of 500V.
S1-S4—the input pin of the constant-current source, which is connected to the LED cathode. Each constant-current source works
in a state based on the operating mode. It can work in the constant-current or cut-off state. The operating current of each
constant-current source relates to two sets of data: the basic current of 10mA and the current increment which is determined by
the connection mode at the relevant current setting pin. For S3 and S4, the current setting pin in series connection is different
from that in parallel connection, and they are described in detail later. The settings of S1/2 are also different from those of S3/4.
The former ranges from 10 mA to 30 mA, and the latter ranges from 10 mA to 50 mA.
I11, I12, I13—the current setting pins of constant-current sources S1 and S2. When these pins are grounded, the current
increases by 5 mA, 10 mA and 20 mA respectively for these constant-current sources.
I21, I22, I23—the current setting pins of constant-current sources S3 and S4 in parallel connection. When these pins are
grounded, the current increases by 5 mA, 10 mA, and 20 mA respectively for these constant-current sources. The settings also
take effect in series connection, and can be used with the settings of I31-33 to form the series current.
I31, I32, I33—the current setting pins of constant-current sources S3 and S4 in series connection. When these pins are grounded,
the current increases by 5 mA, 10 mA and 20 mA respectively for these constant-current sources.
HV1— the output pin of the internal short-circuit switch. In parallel connection mode, this pin is short-circuited to the HV pin via an
internal MOSFET. In series connection mode, this pin is connected to S2 via an internal diode.
The RT7321 also involves the PSOP-8 package. Since this package is subject to a limited number of pins, the current setting pin
is preset by jumpers in the package instead of being pulled out. The specific setting relates to the selected model, The following
figure shows the block diagram of the circuit.
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Guidelines for the RT7321 Off-line Linear LED Driver
3. Internal Constant-current Source
The constant-current source serves as the core of the RT7321, and we can know the most fundamental knowledge by learning it.
The model is shown as follows:
As shown in the figure, Sx corresponds to pins S1-4, and pins Ix1 and Ix3 can be construed as pins I11-33. The current fed into
Sx passes through M1 and the resistor successively, and finally enters the GND pin to form the voltage VCS on the resistor. Then,
VCS is delivered to the error amplifier for comparison with the reference voltage V REF. The comparison results, after being
amplified, are used to adjust the grid voltage of M1 to ensure that the output current reaches the setting value. This is the same
as the preceding constant-current diode, except that the current can be adjusted in line with the condition at your discretion.
To ensure the constant current of the constant-current source, the voltage at the Sx pin must be higher than a value calculated in
the following formula:
VSx_MIN  3000  ISx_SET 2  4  V 
It is clear that ISx_SET is the current setting value of the constant-current pin Sx, and may change in different stages. For this
reason, the value needs to be separately calculated. This value actually presents the internal resistance of MOSFET, and shall be
concerned in the detailed design.
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Guidelines for the RT7321 Off-line Linear LED Driver
4. Operating Mode of the RT7321
Now let’s figure out how to use the segments of an LED, and provide the methods of setting and calculating the current in
different operating modes. This is crucial to understanding the operating mode of the RT7321. I hope that my interpretation can
eliminate your problems. First, let’s see the rectified voltage waveform.
The half-wave in the figure is divided into eight time spans in the lateral axis and five voltage segments in the vertical axis. This
arrangement is subject to the voltage instead of the time, and the voltage is the forward breakover voltage V F at each LED
segment. Normally, the forward voltage VF of the LED and the voltage drop of the constant-current source are different in case of
different currents. Therefore, it is impossible to accurately divide the half-wave via several integral multiplies of VF. We should
know that this is only an ideal division, because the accurate analysis is a hard task.
The RT7321 is designed as follows:
When VHV < VF, no current passes through the LED for the time span (t0-t1) regardless of the constant-current source.
When VF < VHV < 2VF, the parallel connection mode is activated for the time span (t1-t2). HV and HV1 are short-circuited, the
current passes through S1 and S3, and LED G1 and G3 flash. See the following figure.
When 2VF < VHV < 3VF, the parallel connection mode is activated for the time span (t2-t3). HV and HV1 are short-circuited, S1
and S3 are cut-off, the current passes through S2 and S4, and all LEDs flash. See the following figure.
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Guidelines for the RT7321 Off-line Linear LED Driver
In the previous two cases, the currents passing through the constant-current sources are as follows:
IP_S1/2 = I10 + I11 (if I11 = GND) + I12 (if I12 = GND) + I13 (if I13 = GND)
IP_S3/4 = I20 + I21 (if I21 = GND) + I22 (if I22 = GND) + I23 (if I23 = GND)
Note that the current of S1/2 relates to the basic current I 10 = 10 mA and I11-I13. When these pins are grounded, the current
increases by 5 mA, 10 mA and 20 mA respectively. The current of S3/4 depends on the basic current I 20 = 10mA and I21-I23.
When these pins are grounded, the current increases by 5mA, 10mA and 20mA respectively. According to the previous formula,
we can get the following table for calculating the current.
0
1
0
1
0
1
0
1
I21
0
0
1
1
0
0
1
1
I22
0
0
0
0
1
1
1
1
I23
10
15
20
25
30
35
40
45
IP-S3/4
0
0
0
10
20
25
30
35
40
45
50
55
1
0
0
15
25
30
35
40
45
50
55
60
0
1
0
20
30
35
40
45
50
55
60
65
1
1
0
25
35
40
45
50
55
60
65
70
0
0
1
30
40
45
50
55
60
65
70
75
1
0
1
35
45
50
55
60
65
70
75
80
0
1
1
40
50
55
60
65
70
75
80
85
1
1
1
45
55
60
65
70
75
80
85
90
I11
I12
I13
IP-S1/2
Total current in
parallel connection
mode
In the table, Ixx is represented by 0 and 1. The value 1 indicates that Ixx is grounded, and the value 0 indicates that Ixx is floating.
The combination of I11-13 in the same line provides IP-S1/2, and the combination of I21-23 in the same row provides IP-S3/4. The
data obtained at the intersection of the lines and rows where the two combinations are located is the total current in parallel
connection mode.
When 3VF < VHV < 4VF, the series connection mode is activated for the time span (t3-t4). HV and HV1 are open-circuited, S1 and
S2 are cut-off, S2 is connected to HV1 via an internal diode, the current passes through S3, and the LEDs G1-G2-G3 flash. See
the following figure.
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Guidelines for the RT7321 Off-line Linear LED Driver
When VHV > 4VF, the series connection mode is activated for the time span (t4-t5). HV and HV1 are open-circuited, S1, S2 and
S3 are cut-off, S2 is connected to HV1 via an internal diode, the current passes through S4, and the LEDs G1-G2-G3-G4 flash.
See the following figure.
In these two cases, the current passing through the LED is calculated as follows:
IS_S3/4 = IP_S3/4 + I31 (if I31 = GND) + I32 (if I32 = GND) + I33 (if I33 = GND)
Note that the basic current in series connection mode is the current I P-S3/4 of S3/4 obtained in parallel connection mode. When
pins I31-33 are grounded, the total current increases by 5mA, 10mA and 20mA respectively. The table for calculating the current
based on the formula is as follows:
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0
1
0
1
0
1
0
1
I21
0
0
1
1
0
0
1
1
I22
0
0
0
0
1
1
1
1
I23
10
15
20
25
30
35
40
45
IP-S3/4
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
5
5
5
5
5
5
5
5
5
0
1
0
10
10
10
10
10
10
10
10
10
1
1
0
15
15
15
15
15
15
15
15
15
0
0
1
20
20
20
20
20
20
20
20
20
1
0
1
25
25
25
25
25
25
25
25
25
0
1
1
30
30
30
30
30
30
30
30
30
1
1
1
35
35
35
35
35
35
35
35
35
I31
I32
I33
IS-S3/4+
© 2014 Richtek Technology Corporation
Total current
(IS-S3/4) in series
connection mode
IS-S3/4
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Guidelines for the RT7321 Off-line Linear LED Driver
Being the same as the previous table, the bold data in the table refers to the total current in series connection mode. Only IS-S3/4+
should be concerned. The data in the row presents only the current increment arising from the combination of I31-33. This
increment always appears with IP-S3/4 in the total current, and cannot be separately measured.
Due to the symmetry of the sine wave, the later time spans are similar to the previous ones, except that the voltage changes in
different sequences. As a result, the connection and current value also change in a reverse sequence.
Currents passing
through pins
Total current
LED connection
Cut-off
Short circuit
Operating mode
Endpoint
Starting
point
VHV
Time span
The following table lists the preceding data for easy understanding and use.
S1
S2
S3
S4
0
0
0
0
0
G1||G3
IP-S1
0
IP-S3
0
IP-S1 + IP-S3
0
VF
t0-t1
VF
2VF
t1-t2
P
HV-HV1
2VF
3VF
t2-t3
P
HV-HV1
S1,
S3
(G1+G2)||(G3+G4)
0
IP-S2
0
IP-S4
IP-S2 + IP-S4
3VF
4VF
t3-t4
S
S2-D-HV1
S1,
S2
G1+G2+G3
0
0
IS-S3
0
IS-S3
4VF
4VF
t4-t5
S
S2-D-HV1
G1+G2+G3+G4
0
0
0
IS-S4
IS-S4
4VF
3VF
t5-t6
S
S2-D-HV1
S1,
S2
G1+G2+G3
0
0
IS-S3
0
IS-S3
3VF
2VF
t6-t7
P
HV-HV1
S1,
S3
(G1+G2)||(G3+G4)
0
IP-S2
0
IP-S4
IP-S2 + IP-S4
2VF
VF
t7-t8
P
HV-HV1
G1||G3
IP-S1
0
IP-S3
0
IP-S1 + IP-S3
VF
0
t8-t9
0
0
0
0
0
S1,
S2,
S3
As shown in previous formula for calculating the current, I P-S1 = IP-S2, IP-S3 = IP-S4,and IS-S3 = IS-S4. Therefore, we know the total
current output involves only three levels, with one level being 0. In practice, only two levels are available for choice in the time
span when the current passes by. The two tables for calculating the current are integrated as follows for easy use. It is also
possible to integrate these two tables, because I P-S3/4 can work in both parallel and series connection modes.
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Guidelines for the RT7321 Off-line Linear LED Driver
Table for RT7321
current calculating
0
1
0
1
0
1
0
1
I21
0
0
1
1
0
0
1
1
I22
0
0
0
0
1
1
1
1
I23
10
15
20
25
30
35
40
45
IP-S3/4
0
0
0
10
20
25
30
35
40
45
50
55
1
0
0
15
25
30
35
40
45
50
55
60
0
1
0
20
30
35
40
45
50
55
60
65
1
1
0
25
35
40
45
50
55
60
65
70
0
0
1
30
40
45
50
55
60
65
70
75
1
0
1
35
45
50
55
60
65
70
75
80
0
1
1
40
50
55
60
65
70
75
80
85
1
1
1
45
55
60
65
70
75
80
85
90
I11 I12 I13
IP-S1/2
I31 I32 I33
IS-S3/4+
0
0
0
0
10
15
20
25
30
35
40
45
1
0
0
5
15
20
25
30
35
40
45
50
0
1
0
10
20
25
30
35
40
45
50
55
1
1
0
15
25
30
35
40
45
50
55
60
0
0
1
20
30
35
40
45
50
55
60
65
1
0
1
25
35
40
45
50
55
60
65
70
0
1
1
30
40
45
50
55
60
65
70
75
1
1
1
35
45
50
55
60
65
70
75
80
Total current in
parallel
connection mode
Total current
(IS-S3/4) in series
connection mode
Current increment
in series
connection mode
Note:
1.
For the data of I11-I13, I21-I23, I31-I33 in the table, “1” indicates that the pin is grounded, and “0”
indicates that the pin is floating.
Fixed data: I10 = I20 = 10mA, I11 = I21 = I31 = 1 = 5mA, I12 = I22 = I32 = 1 = 10mA, I13 = I23 =
I33 = 1 = 20mA.
2. The data obtained at the intersection of the horizontal and vertical lines constituted by I11-13
combination and I21-23 combination refers to the total output current in parallel connection
mode.
3. The data obtained at the intersection of the horizontal and vertical lines constituted by I31-33
combination and I21-23 combination refers to the total output current in series connection
mode.
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Guidelines for the RT7321 Off-line Linear LED Driver
4. The current unit is mA.
5. Calculation formula:
IP_S1/2 = I10 + I11 (if I11 = GND) + I12 (if I12 = GND) + I13 (if I13 = GND)
IP_S3/4 = I20 + I21 (if I21 = GND) + I22 (if I22 = GND) + I23 (if I23 = GND)
IS_S3/4 = IP_S3/4 + I31 (if I31 = GND) + I32 (if I32 = GND) + I33 (if I33 = GND)
5. How to Choose the Optimal Current and VF
It is difficult to answer how to choose the optimal current and VF. The linear driving system involves many variables. For this
reason, we have not yet developed a complete and separate tool for assessing the potential problems in all circumstances.
The calculation tool based on the time partitioning algorithm gives us a prompt that when the grid voltage is 220V, appropriate VF
of single LED series is about 70V, and in such case, the conversion efficiency may be 86% or so. With the voltage deviates from
220V, the corresponding conversion efficiency may be continuously lowered. In any case, increasing V F can improve the
efficiency, yet shorten the LED flashing time, which is similar with the case where the grid voltage is lowered at fixed VF, but the
possibility of flashing is higher.
Similar with reduction of VF, higher grid voltage experiences longer LED flashing time and lower possibility of flashing, but the
response efficiency may be lowered. How to set the current? The LED specification is a primary consideration, thus we can firstly
determine the maximum current of the LED, and use it as the current in series connection mode, shown as I S-S3/4 in the
computational formula. Secondly, determine the total current in parallel connection mode, In my opinion, preferably such total
current is slightly higher than 1/2 of I S-S3/4. This is because it is required to ensure that the final current waveform is as close as
possible to the sine wave, and parallel connection mode occurs when VF < VHV < 3VF and series connection mode occurs when
VHV > 3VF. If you don’t care about the power factor performance, or you face abnormal sine wave, you are certain to have other
options, and it is all up to you.
After the total currents in series connection mode and parallel connection mode are determined, we can find out such two data
from the same row of the above computation table, and then determine the setting mode of I21-23 from such row as well as the
setting modes of I11-13 and I31-33 from the line occupied by the two data. Please give preference to the data equivalent to I P-S1/2
and IP-S3/4, which can ensure that the two branch currents passing by in parallel connection mode are identical, and LED-related
resources are fully used. Here that's all for the design of system configuration.
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Guidelines for the RT7321 Off-line Linear LED Driver
V
264
V
10.78
W
16.50
W
I21
0
0
0
8.69
W
13.35
W
Efficiency
Operating
voltage
190
0
I13
I22
I23
I31
I32
I33
1
1
1
1
1
2.09
W
Efficiency
mA
I12
W
80.60%
3.15
W
Efficiency
50
I11
IC Power
dissipation
IP
Setting
configuration
1.9
IC Power
dissipation
mA
W
IC Power
dissipation
75
Output
power
IS-S3/4
11.29
Output
power
V
W
Output
power
70
Input power
VF
13.20
Input power
V
Input power
Operating
voltage
220
Operating
voltage
The following table shows one set of calculating data obtained by the above calculation tool.
80.90%
85.60%
The upper part of the table lists the data under the standard voltage of 220V, and the lower part describes the performance under
extreme voltages of 190V and 264V.
In such setting, the relationship between current and voltage in the next time span under the standard voltage of 220V is shown
400.0
80
350.0
70
300.0
60
250.0
50
200.0
40
150.0
Vin
30
100.0
I-input
20
50.0
0.0
0.0E+00
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I-in [mA]
Vin [V]
as follows:
10
5.0E-03
1.0E-02
1.5E-02
© 2014 Richtek Technology Corporation
0
2.0E-02
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Guidelines for the RT7321 Off-line Linear LED Driver
How about this? Does it live up to your expectation?
Maybe you wonder about the current passing through each LED segment, and the following figures may grant your wish. If you
find certain performance is different from what you think, you should review relevant contents described above, and then have a
look at here. You don’t have to be embarrassed, because I've been through it.
400.0
80
Vin
350.0
70
300.0
60
I-LED1
200.0
40
150.0
30
100.0
20
50.0
10
5.0E-03
1.0E-02
1.5E-02
Vin [V]
400.0
80
350.0
Vin
70
300.0
I-LED2
60
250.0
50
200.0
40
150.0
30
100.0
20
50.0
10
0.0
0.0E+00
5.0E-03
1.0E-02
1.5E-02
Vin [V]
400.0
0
2.0E-02
80
350.0
Vin
70
300.0
I-LED3
60
250.0
50
200.0
40
150.0
30
100.0
20
50.0
10
0.0
0.0E+00
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0
2.0E-02
I-LED [mA]
0.0
0.0E+00
I-LED [mA]
50
5.0E-03
1.0E-02
1.5E-02
© 2014 Richtek Technology Corporation
I-LED [mA]
Vin [V]
250.0
0
2.0E-02
15
Guidelines for the RT7321 Off-line Linear LED Driver
400.0
80
Vin
300.0
Vin [V]
70
I-LED4
60
250.0
50
200.0
40
150.0
30
100.0
20
50.0
10
0.0
0.0E+00
5.0E-03
1.0E-02
1.5E-02
I-LED [mA]
350.0
0
2.0E-02
6. System Design Considerations
After the above step, we need to determine the specific model of RT7321. Two questions shall be taken into account: Which
package can bear the power dissipation during operation? Which package can meet the selected current configuration?
We can obtain the following data from the specification of the RT7321:
This indicates that at the ambient temperature of 25°C, the PSOP-8 package can bear 3.44W power dissipation, while the
WQFN-20L can bear 3.54W. Based upon our calculation example described above, the worst case is that IC power dissipation
are 2.09W and 3.15W respectively at voltages of 190V and 264V, which means the two packages can meet these operating
conditions. However, it does not means everything is all right. As we all know, the internal temperature of the LED is generally
high. We assume the temperature is 60℃, can such packages still meet these requirements?
We quote another set of data from the specification:
These include the thermal resistance of each package and the maximum junction temperature which the RT7321 can withstand.
The relationship amongst the two data above, the power dissipation and the ambient temperature is given by:


PDmax  TJmax   TA / θJA
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Guidelines for the RT7321 Off-line Linear LED Driver
Based on this formula, the powers which the two packages can bear at 60°C are 3.1W (PSOP-8) and 3.19W (WQFN-20L). Oh!
Thank God! The power 3.19W is just larger than 3.15W above, indicating that only WQFN-20L package can be selected. This
calculation process has no prearrangement, and I'm not going to make any change to start over, please accept the predestined
arrangement and go on.
According to the above calculation results, the complete model of the RT7321 we should select is RT7321GQW. If you can
choose PSOP-8 package as per your calculation results, the RT7321XYGSP may be optional, in which X refers to the current of
two branches in parallel, Y refers to the current in series, and their relationship is defined in the following specification:
Obviously, the RT7321 in PSOP-8 package selects the option that two branch currents are identical in parallel connection mode.
This is much more reasonable than the previous calculation example. Please give preference to the package to avoid troubles in
use of WQFN package.
After package selection, the schematic design is a piece of cake, and we skip over it.
I don't think any more details are necessary on the bridge rectifier. The current passing through the rectifier diode which serves
as the linear driving circuit is identical with that passing through the LED. The high voltage is √2 times the maximum effective grid
voltage, and inductance of the circuit is generally small, thus less additional voltage allowance is required.
The waveform of the current passing through the LED is not sine wave, so the filter circuit may be considered in order to avoid
excessive conducted interference. According to the test results provided by AE (application engineer), the following circuits are
used. Where 220V/60Hz / IP = IS = 40mA, RRES = 50Ω, X-cap = 0.1mF, complying with requirements stated in EN55015.
AC
X-cap
MOV
7N391U
Source
S4
GND
RT7321
S3
RRES
HV
S1
S2
NA
How about your detailed design? In case of any discrepancy, please use the same parameters first, and make adjustments
during specific testing. I think it is very easy to pass the certification. With the X-cap added into the bridge rectifier, the design can
be completed by using MLCC, and I think it's certainly worth a try on your part.
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Guidelines for the RT7321 Off-line Linear LED Driver
7. PCB Design
Due to linear circuit, the PCB design is a no-brainer, leaving safety spacing and thermal dissipation the only concern. The thermal
pad at the chip bottom is bound to have a good contact with the PCB, and its area shall be maximized to lower the thermal
resistance. Meanwhile, avoid reduction in spacing due to the pin soldering tin. The copper trace of conductor shall be as short as
possible to reduce unnecessary loss.
8. Conclusion
The introduction to the RT7321 and description of its usage will come to an end, and I think the challenge lies in operating mode
and current design. Please try to figure out it patiently, and you don’t find it any hard once you make it through. Another concern is
how to configure voltage and current of the LED. I suggest that you could set parameters first based on the preliminary principles
herein, and then give trial runs and tune them accordingly to establish a relatively perfect system.
This paper describes the RT7321 on the basis of 220V/230V systems, and the same principles are also applicable to the 110V
system, where RT7322 is recommended for design. Actually the 110V system is not my cup of tea, because it uses the current
which is twice that of the 220V system to ensure the same power output, and all conductors are greatly enlarged to achieve the
same resistance and loss. How deep-pocketed it is! Also, the RT7322 has to adapt such case. Its current setting reference is set
to 20mA, and one more pin for setting series current is required to improve the current output capability, but all of these are costly.
I’ll stop here, and if you have any other questions, you can send us messages or call us. Your valuable opinions will help us
provide you with better services. Please visit http://www.richtek.com/LED/ for more about LED applications and other applications.
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Guidelines for the RT7321 Off-line Linear LED Driver
Related Parts
RT7321
Linear LED Driver for High-Voltage LED Lamps
Datasheet
RT7322
Linear LED Driver for High-Voltage LED Lamps
Datasheet
Next Steps
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Richtek Technology Corporation
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Hsinchu, Taiwan, R.O.C.
Tel: 886-3-5526789
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
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