AN019 EN

Application Note
AN019 – Jun 2014
Low Cost 8W Off-line LED Driver using RT8487
Abstract
RT8487 is a boundary mode constant current controller with internal high side driver, which can be used in buck and buck-boost
configuration, to provide a constant output current to the (LED) load. It contains special circuitry for achieving high power factor
and low input current THD, while minimizing external component count. The small SOT23-6 package keeps application footprint
small, and makes RT8487 a cost effective solution for off-line LED drivers.
This application note provides details on how to design a cost effective 8W Buck LED driver with RT8487.
Contents
1. Introduction ...............................................................................................................................................2
2. Application Circuit .....................................................................................................................................2
3. Calculation of the Key Components...........................................................................................................3
4. Key Performance Measurements ..............................................................................................................8
5. Total Bill of Materials .................................................................................................................................9
6. PCB Layout ............................................................................................................................................ 10
7. Conclusion .............................................................................................................................................. 11
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Low Cost 8W Off-line LED Driver using RT8487
1. Introduction
Important requirements for low cost off-line LED drivers are high efficiency, good Power Factor with low THDi fulfilling IEC61000
class C, accurate LED current, fast start-up, and simple design using low cost standard components. The following 8W LED
driver design with RT8487 in floating buck configuration fulfills all of the above.
Key specifications:

Input range 230V +/-15% (with some alterations, the design can be extended to full range input)

LED string 27V , I-LED = 300mA +/- 5% , P-out=8W for general retrofit lighting applications

THDi < 20% fulfilling IEC61000 class C

Start-up time < 300msec

Full protections: output short-circuit, LED open circuit, over current and over temperature protection.

Key focus on low BOM cost by using transformer-free design with standard low cost components.
2. Application Circuit
The total application of the 8W LED driver is shown below.
Figure 1
RT8487 is used in floating controller buck configuration. The complete application circuit is shown in figure 1. The IC controls the
switch-on time of the high side MOSFET Q1, and it senses the average LED current via RS which is in series with the buck
inductor for true load current sense. Boundary conduction mode switching is obtained by sensing the zero inductor current (also
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Low Cost 8W Off-line LED Driver using RT8487
via RS). High efficient resonant switching at minimum drain-source voltage is achieved by programmable switch-on delay from
zero current detection point (via R3). A smart algorithm controls the ON time to obtain high input power factor and low THDi. IC
bias is provided by a simple bootstrap circuit D2 & C2, thereby eliminating the need for a separate auxiliary winding. This makes
it possible to use a simple standard drum coil instead of a more expensive custom wound transformer. Low IC start-up bias
current allows the use of high value start-up resistors R1 & R2 while still achieving fast start-up (typically 123msec). The total
circuit can be built on a small single-sided PCB measuring 18 x 36mm.
3. Calculation of the Key Components
The following sections explain the settings of the various application parameters.
Setting Average Output Current
The average output current that flows through the LED string is set by an external resistor, R S, which is connected between the
IC GND and SENSE terminals. Since RS is connected in series with the inductor, the average LED current can be accurately
sensed via this resistor. The relationship between output current, I OUT, and RS is shown below :
IOUT  250mV
RS
In this application, LED current was defined as 300mA, so RS 
250mV  0.833Ω
300mA
We select 1Ω//4.7Ω to obtain 0.824Ω
Start-up Resistor
The start-up resistors (R1+R2) should be chosen such that the current flow through these resistors at minimum line voltage
exceeds the IC start-up current. Otherwise, the RT8487 Vcc may never reach the start-up voltage. The typical IC start-up current
is 25μA.
Start-up resistor should be chosen not to exceed the operating current. Otherwise, the VCC voltage may rise higher than set by
the Vcc bootstrap circuit, and could trigger OVP. The typical operating current is 1mA.
The value of the start-up resistor together with the VCC capacitor C2 will determine the start-up time, which is defined by:
t start up  C 2
VUVLO
Istart up
where VUVLO is 17V and Istart-up can be approximated by:
Vin 2
 R1  R2 
 25μA
For most applications, C2 can be chosen 1μF.
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Low Cost 8W Off-line LED Driver using RT8487
R1 and R2 are chosen 1MΩ each, which gives a typical start-up current of 230 2  25μA  138μA
2MΩ
The start-up time will become: 1μF  17V  123m sec
138μA
Figure 2 shows the IC start-up waveforms at 230V ac
input.
When AC mains is applied, the current through R1 and
R2 will charge C2.
When the IC VCC voltage exceeds the UVLO level, the
power MOSFET starts switching, quickly charging the
output.
Total measured start-up time was 150msec.
Figure 2
Input Capacitor Selection
For High Power Factor application, the input capacitor C1 should be sufficiently small to achieve rectified line voltage sine-wave.
The voltage rating of the input filter capacitor, VCIN, should be large enough to handle the maximum input voltage. A 100nF /
500V film capacitor is a suitable choice. For reducing differential mode EMI, a pi filter can be used by means of two 47nF
capacitors and a suitable inductor.
Buck Inductor Value Selection
Due to Boundary Conduction Mode switching, the buck inductance value will influence the converter switching frequency. For
smaller size coil, a small inductance value could be selected, but the limitations are set by the IC minimum ON time (0.5μsec
typically) and minimum OFF time (0.5μsec typically).
The maximum inductance value is limited by the IC maximum ON time (15μsec typically) and maximum OFF time (33μsec
typically).
To calculate the inductance, we first need to calculate the maximum peak current (I peak) at the top of the rectified sine wave
(Vpeak) :
Ipeak 
2Pin
Vpeak  F  K  a  
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Low Cost 8W Off-line LED Driver using RT8487
where Pin is the converter input power, a is the ratio of LED voltage versus BUCK input voltage: a  VLED
Vpeak
and F(K(a)) is a complex function to achieve low THD for PFC buck:
K  a   1  a ,F  K  a    0.411a4  0.296a3  0.312a2  0.638a  0.0000846,a|0 ~ 0.7 (1)
a
Figure 3
In the 8W application, Pin can be calculated from
PLED 27V  0.3A

 9.4W (efficiency was estimated at 86%)
η
0.86
The factor a can be calculated at the peak of the rectified sine wave: a 


From Figure 3 or formula (1) we can derive, F K  a   0.051, so Ipeak 
The range of inductance can now be calculated from: L 
27V  0.082
230 2
2  9.4
 1.12A
230 2  0.051
Vpeak  VLED 
VLED
Ton
Toff and L 
Ipeak
Ipeak
L1 was chosen 330μH with current rating 1.2A to achieve best compromise between size, cost and efficiency.
The frequency at the top of the sine wave can be calculated: FSW 
1
Ton  Toff  Tdelay
(Tdelay is determined by the resistor connected to AND pin, see next section)
Setting the switch-on delay time
After the inductor current has reached zero, a resonance will occur between the inductor and the total capacitance at the switch
node, which is mainly determined by the MOSFET drain-source capacitance.
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Low Cost 8W Off-line LED Driver using RT8487
In order to minimize the MOSFET switching losses, RT8487 provides the flexibility to adjust the delay time of next switch-on cycle
in order to switch-on at the maximum point of the resonance, which corresponds to the minimum drain-source voltage value.
Figure 4
The delay time from zero current point to the maximum of the switch resonance (T delay in figure 4) which can be
calculated from:  = √1 ∙  where CSW is the capacitance at the switch node, mostly determined by
the MOSFET drain-source capacitance, which in this application equals 38pF. The resonance delay becomes:
 = √330µ ∙ 38 = 352
The total required delay time for optimal resonant switching needs to be chosen a bit larger to include zero current detection delay
(around 290nsec in this case). So total delay time becomes 270nsec + 352nsec = 622nsec.
The delay time (Tdelay ) from zero current detection point to next MOSFET switch-on cycle can be adjusted by the resistor value
R3 connected between AND pin and IC GND


Tdelay  μs   0.6* R32  3600* R3  405200 *106

elay= approximate total delay time in μsec

R3 resister value in kΩ
(2)
The final value for R3 was set at 68kΩ.
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Low Cost 8W Off-line LED Driver using RT8487
Figure 5 below shows the switching waveform with optimal resonant switch-on point.
Figure 5
MOSFET Selection
The MOSFET voltage rating should be sufficient to handle the max line input voltage peak value + margin for line transients. A
MOSFET with minimum 500V drain-source rating is recommended. The MOSFET current rating depends on thermal aspects. A
2A MOSFET was selected for low dissipation and better efficiency.
Forward Diode Selection
When the power MOSFET turns off, the path for the current is through the diode connected between the switch output and
ground. This forward biased diode must have low forward voltage drop and fast recovery times. The reverse voltage rating of the
diode is should be greater than the maximum input peak voltage + margin and the current rating should be greater than the
inductor peak current. A fast 600V / 2A diode was selected for low dissipation and better efficiency.
Output Capacitor Selection
To achieve high power factor and low THDi, the inductor current contains considerable low frequency ripple. The output capacitor
will filter the switching and low frequency ripple current to deliver a low ripple voltage to the LED string. The amount of output
ripple voltage together with the differential resistance of the LED string will determine the ripple current through the LEDs. In this
low cost design, a 220μF capacitor was chosen, which gives around 330mApp ripple current through the LED string. To reduce
this ripple, a larger value output capacitor is required.
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Low Cost 8W Off-line LED Driver using RT8487
4. Key Performance Measurements
Figure 6 shows the input and output voltage and current waveforms.
Input AC waveform shows good PFC and low THD. The average output LED current is accurately set at 299mA.
Figure 6
Figure 7 below shows the switching waveforms. To achieve low THDi, the current peak value is around 4x higher than the
average current. The single switching cycle shows fully BCM switching with minimum drain-source voltage switch-on.
Figure 7
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Low Cost 8W Off-line LED Driver using RT8487
Below table shows the key performance parameters. Typical efficiency is 86% with excellent LED current stability over
mains voltage and low THD fulfilling IEC61000 class C.
Vin ac
Pin ac
PF
THD
V-LED
I-LED
Pout
Efficiency
Ploss
195.5
9.59
0.96
11.6
27.69
0.299
8.279
86.3%
1.31
231.8
9.64
0.94
13.3
27.74
0.299
8.294
86.0%
1.35
264.2
9.68
0.90
17.7
27.68
0.300
8.304
85.8%
1.38
5. Total Bill of Materials
The total BOM of the 8W LED driver is shown below:
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Item
Quantity
Reference
Part / Value
Footprint
Remark
1
1
F1
1A/250V
---
Fuse 1A
2
1
LX1
4.7mH (DR0612)
DR0612
EMI drum coil
3
1
L1
330μH (CKPK1012)
DR1012
Buck drum coil
4
1
CX1
0.1μF/275Vac
DIP
X-capacitor
5
1
C1
0.1μF/450V
DIP
MPP Cap
6
1
C2
1μF/50V
1206
7
1
C3
0.1μF/50V
0603
8
1
C4
1nF/50V
0603
9
1
EC1
220μF/35V
DIP
10
1
RX1
10K
1206
11
2
R1, R2
1M
1206
12
1
R3
6.8K
0603
13
1
R4
10R
0805
14
1
R5
100R
0603
15
1
R6
47k
0805
16
1
RS
1R // 4R7
1206
17
1
D1
SF28
DIP
Fast 2A/600V
18
1
D2
FFM107
SOD-123L
Fast 1A/1000V
19
1
Extra-D
1N4148
SOD-123
250mA/75V
20
1
DB
TB6S
---
600V/1A diode bridge
21
1
Q1
STD2HNK60
TO-251
2A/600V MOSFET
22
1
IC1
RT8487GJ6
TSOT23-6
LED driver controller
© 2014 Richtek Technology Corporation
Output E-cap
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Low Cost 8W Off-line LED Driver using RT8487
6. PCB Layout
The 8W LED driver application is build on a small single sided PCB. Due to the floating controller topology, the components
around the IC should be compact and close to the IC, and the layout should provide sufficient creepage and clearance margin for
the high voltage swing.
It should be noted that this layout is a preliminary version, and needs some further fine tuning to optimize performance: The buck
inductor orientation with respect to EMI coil needs some modification: currently the stray field of L1 couples into LX1 and causes
higher EMI readings.
Figure 8
Figure 9
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Low Cost 8W Off-line LED Driver using RT8487
7. Conclusion
RT8487 makes it possible to design a very cost effective 8W LED driver which has good performance and meets the
requirements of today’s LED driver market.
Please visit Richtek’s website for more information on Richtek’s innovative LED driver solutions at http://www.richtek.com/LED/
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14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: 886-3-5526789
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