Download AN-9744 Smart LED Lamp Driver IC with PFC Function

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AN-9744
Smart LED Lamp Driver IC with PFC Function
Introduction
The FL7701 is a PWM peak current controller for a buck
converter topology operating in Continuous Conduction
Mode (CCM) with an intelligent PFC function using a
digital control algorithm. The FL7701 has an internal selfbiasing circuit that is a current source using a high-voltage
switching device. When the input voltage is applied to the
HV pin is over 25 V to 500 V, the FL7701 maintains a
15.5 VDC at the VCC pin. The FL7701 also has a UVLO
block for stable operation. When the VCC voltage reaches
higher than VCCST+, the UVLO block starts operation. When
the VCC drops below the VCCST-, IC operation stops. Please
note that this application note could also be used for
FLS0116, which is a Smart LED Lamp driver IC with the
same features and functions as FL7701 but includes built-in
1 A, 500 V MOSFET.
Internal Digital Reference by ZCD
Hysteresis is provided for stable operation of the IC when the
input voltage is in noisy circumstances or unstable conditions.
The FL7701 has a “smart” internal block for AC input
condition. If an AC source with 50 Hz or 60 Hz is applied, the
IC automatically changes the internal reference to adjust to
input conditions with an internal fixed transient time.
When a DC source connects to the IC, the internal reference
immediately changes to DC waveform. The internal
DAC_OUT reference signal is dependent on the V CC
voltage. Using the DAC_OUT signal and internal clock,
CLK_GEN; the FL7701 automatically makes a digital
reference signal, DAC_OUT. If too high of a capacitor
value is connected to VCC, the VCC voltage may not drop
enough during the AC input voltage valley region. The
ZCD_OUT signal does not trigger correctly and the IC will
have an abnormal internal reference signal, see Figure 2 for
a normal trigger sequence. This abnormal internal reference
signal causes LED light flicker.
Soft-Start Function
The FL7701 has an internal soft-start to reduce inrush
current at IC startup. When the IC starts operation, the
internal reference of the IC slowly increases up to a fixed
level for around seven cycles. After this transient period,
the internal reference is fixed at a certain DC level, see
Figure 3. In this transient time, the IC continually tries to
find input phase information from the VCC pin. If the IC
succeeds in detecting phase information from the VCC
input, the IC automatically follows a similar shape to the
AC input voltage, see Figure 4. If not, the IC has a DC
reference level.
Vsup
D1
+ VLED -
Iline
LED Load
L
IL
Drain Voltage,Vdrain
FL7701
VCC
VSUP_SEN
DAC
C
ZCD_OUT
HV
HV
Device
VSUP_ SEN
DAC: Digital to Analog
HV Device : High-Voltage Device
VCC
DAC_OUT
Driver
S
Reference
OUT
Isw
Q
ZCD_ OUT
R
CS
CLK_ GEN
GND
Figure 1.
Basic Block of FL7701
DAC_ OUT
Gate Voltage,VG
Figure 2.
© 2012 Fairchild Semiconductor Corporation
Rev. 1.2 • 12/30/14
FL7701 Operation
www.fairchildsemi.com
AN-9744
APPLICATION NOTE
To precisely and reliably calculate the input voltage phase
on the VCC pin, the FL7701 uses a digital technique
(sigma/delta modulation/demodulation). After finishing this
digital technique, the FL7701 has new reference that is the
same phase as input voltage, as shown in Figure 6.
VRECTIFIED
time
ILED
Sine wave generation
(when PFC function is enabed.)
time
Figure 3.
DC Input Condition
Constant reference generation
(when PFC function is disabled.)
ZCD_OUT
Soft change period
VRECTIFIED
Reference
Vp
time
ILED
Vp / 2
Figure 6.
This signal enters the final comparator and current
information from the sensing resistor. Pin 1 is compared. As
a result, the FL7701 has a high power factor and can operate
as a normal peak current controller as shown in Figure 6, in
the DC input condition. The relationship between AC Input
Mode and DC Input Mode is 2 .
time
Figure 4.
AC Input Condition
Internal Power Factor (PF) Function
The FL7701 application circuit does not use the input
electrolytic capacitor for voltage rectification after a bridge
diode because this system design results in a high pulse
shape input current. This pulse shape current contains many
harmonic components, so the total system cannot have high
PF. To get high PF performance, the FL7701 uses a
different approach.
Output Frequency Programming
The FL7701 can program output frequency using an RT
resistor or with the RT pin in open condition. The FL7701
can have a fixed output frequency around 45 kHz when the
RT pin is left open. For increasing system reliability, a
small-value capacitor is recommended below 100 nF in RTopen condition. The relationship between output frequency
and the RT resistor is:
The FL7701 has an intelligent internal PFC function that
does not require additional detection pins or other
components. The IC does not need a bulk capacitor on the
VCC pin for supply voltage stabilization.
fOSC 
The FL7701 detects the VCC changing point for generating
the Zero Crossing Detection (ZCD) signal, which is an
internal timing signal for generating DAC_OUT. Normally,
a capacitor connected to the VCC pin is used for voltage
stabilization and acts as low-pass filter or noise-canceling
filter. This increases the ability to get a stable timing signal
at the VCC pin, even is there may be noise on other pins.
Vbridge
2.02  109 [Hz]
RT
(1)
Output Open-Circuit Protection
The recommended connection method is shown in Figure 7.
The FL7701 has a high-voltage power supply circuit, which
self biases using high-voltage process device. If the LED
does not connect to the chip, the IC cannot start.
Bridge Diode
Output Voltage
Input Voltage
Peak
Internal Reference
BD
EMI filter
LED
L1
JFET Output
Voltage
HV
t
D2
ADIM
Vdd Charging Voltage
VCC
L
D1
C2
C1
OUT
RT
VCC
JFET Output Voltage
FL7701
R3
R1
CS
t
C3
ZCD
L2
C4
GND
R2
t
DAC_OUT
Figure 7.
LED Open Condition
t
Figure 5.
Internal PFC Function
© 2012 Fairchild Semiconductor Corporation
Rev. 1.2 • 12/30/14
www.fairchildsemi.com
2
AN-9744
APPLICATION NOTE
For example, if VIN(max) = 220 V, η=85% and ten LEDs are
in series connection, the minimum duty ratio is:
Inductor Short-Circuit Protection
The FL7701 has an Abnormal Over-Current Protection
(AOCP) function. If the voltage of the LED current-sensing
resistor is higher than 2.5 V, even within Leading EdgeBlanking (LEB) time of 350 ns; the IC stops operation.
VCC
Dmin 
Step 2: Maximum Duty Ratio
Similar to Step 1, calculate maximum duty ratio as:
HV
JFET
VCC
10  3.5
 0.132
0.85  2  220
ZCD
UVLO
time
Dmax 
ZCD
nVF
(3)
  Vin(min)
DAC
Soft start
TSD
Digital Block
RT
S
Oscillator
60
OUT
Q
[%]
R
Reference
+
CS
LEB
40
Leading Edge
Blanking
GND
Duty
50
+
30
AOCP
2.5V
Figure 8.
20
AOCP Function
10
Analog Dimming Function
0
The Analog Dimming (ADIM) function adjusts the output
LED current by changing the voltage level of the ADIM pin.
0
Figure 9.
Application Information
Vin (min) 
Table 1 shows one example of a design target using the
FL7701 device.
Target Design Specification
Item
Specification
[ms]
Duty Variation vs. Time
nVF
35

 82.35[V ]
  Dmax 0.85  0.5
(4)
311V
Input voltage
Frequency
45 kHz
35
VF=3.5 V, n=10
Output LED Current RMS
0.3
ILED(rms)
Output LED Current Peak
0.5
ILED(peak)
Input Voltage (Max.)
220
VAC(rms)
DCM
Expected min. input voltage (CCM) :
Vin(min)=82.35V
DCM
time
Current Limit on the
DAC reference
Average
LED Current(ILED(ave)
∆i
Step 1: Minimum Duty Ratio
The FL7701 has a fixed internal duty ratio range between
2% and 50%. This range depends on the input voltage and
the number of LEDs in the string.
Dmin
15
Note
Output Voltage
nVF

  Vin (max)
10
The FL7701 has a 50% maximum duty cycle to prevent subharmonic instability. Assume the minimum input voltage
enters 50% duty ratio. Using Equation (2), re-calculate the
minimum input voltage for CCM operation:
The FL7701 is an innovative buck converter control IC
designed for LED applications. It can operate from DC and
AC input voltages without limitation and its input voltage
level can be up to 308 VAC.
Table 1.
5
CCM
Dmin
1-Dmin
time
ton toff
(2)
Figure 10.
Estimated Waveforms
where η is efficiency of system; VIN(max) is maximum
input voltage; VF is forward-drop voltage of LED; and n
is LED number in series connection.
© 2012 Fairchild Semiconductor Corporation
Rev. 1.2 • 12/30/14
www.fairchildsemi.com
3
AN-9744
APPLICATION NOTE
Step 3: Maximum On/Off Time
The FL7701 has internally fixed maximum duty ratio
around 0.5 to prevent sub-harmonic instability. Assume the
maximum on/off time. For example, the maximum on/off
time at 45 kHz operation condition is:
t on  t off 
Step 5: Inductance
Derive one more formula for the minimum inductance value
of the inductor using the Step 4 results:
L
(VF  n)(1  Dmin ) 3.5  10  (1  0.132)

 4.5 mH 
f s  i
45000  0.1516
(7)
1
1

 11.11 [μs]
2 f s 90000
Step 4: Calculate the LED Current Ripple, ∆i
Figure 11 shows the typical LED current waveforms of a
FL7701 application. For more stable or linear LED current,
operate in CCM.
Current peak at LED current
maximum point (ILED(peak))
Current peak at LED average
current maximum point
(ILED(ave.peak))
DCM
Average
LED Current (ILED(ave))
0.5∆i
∆i
DCM
0.5∆i
Figure 12.
Current Ripple (∆I) vs. Inductance
CCM
Current min at LED current
maximum point (ILED(min))
Dmin
1-Dmin
ton toff
Figure 11.
Target Waveforms of LED Current
Using the typical LED current waveform in Figure 11,
derive the formula as:
i or
2
i

2
I LED ( peak)  I LED ( ave. peak) 
I LED (min)  I LED ( ave. peak)
(5)
Figure 13.
Step 6: Sensing Resistor
The current sensing resistor value can be determined by:
In Table 1, the desired LED current average is always
located between LED peak current value, ILED(peak)=500 mA,
which is limited by the IC itself, and the LED minimum
current. Using this characteristic, the inductor value for the
desired output current ripple range (∆i) is:
i  2( I LED ( peak)  I LED ( ave. peak) ) or
i  2( I LED ( ave. peak)  I LED (min) )
Expected Waveforms
R
VCS
0.5
[]

1
I LED ( peak) 0.5
(8)
The power rating is under 0.25 W even when considering
power consumption at peak-current condition.
Step 7: Frequency Set Resistor
1
Rt 
 2.0213  109  44.919 [k]
f sw
(6)
I LED ( ave. peak)
where I
LED ( rms ) 
(9)
If the frequency setting resistor Rt is not connected, the
system operates at the IC’s default switching frequency
which is is 45 kHz.
2
From the Table 1, the target LED current rms is defined as
0.3A and the LED current peak is set to 0.5 A.
i  2( I LED ( peak)  2  I LED ( rms) )
 2(0.5  2  0.3)  0.1516 [ A]
© 2012 Fairchild Semiconductor Corporation
Rev. 1.2 • 12/30/14
www.fairchildsemi.com
4
AN-9744
APPLICATION NOTE
Figure 17 and Figure 18 show performance of FL7701
following the input source changes from high-line frequency,
to lower frequency, then to higher frequency.
System Verification
Figure 14 shows the recommended circuit of a FL7701
system with just a few components.
EMI filter
BD
VDRAIN[100V/div]
L1
LED
D1
L
HV
D2
ADIM
C2
C1
OUT
RT
VCC
FL7701
R3
C3
C4
L2
Figure 14.
R1
ILED[0.2A/div]
CS
GND
R2
Test Circuit
Figure 15 and Figure 16 show the startup waveforms from a
FL7701 application in DC and AC input conditions at 220 V
with ten LEDs.
Figure 17.
VCC[5V/div]
VDRAIN[100V/div]
ILED[0.2A/div]
Input Source Changing: 45 Hz to 100 Hz
VDRAIN[100V/div]
ILED[0.2A/div]
Figure 18.
Figure 15.
Input Source Changing: 100 Hz to 45 Hz
Figure 19 shows the analog dimming performance with
changing VADIM. The output LED current changes according
to the control voltage.
Soft-Start Performance in DC
Input Condition
VCC[5V/div]
VDRAIN[100V/div]
ILED[0.2A/div]
Figure 19.
Figure 16.
VADMIN vs. LED Current
Soft-Start Performance in AC
Input Condition
© 2012 Fairchild Semiconductor Corporation
Rev. 1.2 • 12/30/14
www.fairchildsemi.com
5
AN-9744
APPLICATION NOTE
Figure 20 shows the typical function of AOCP performance.
The FL7701 limits output LED current pulse-by-pulse with
Leading-Edge Blanking (LEB), ignoring current noise. Even
though the IC limits the output LED current pulse-by-pulse,
it cannot prevent inrush current during an inductor short. To
prevent this kind of abnormal situation, the IC has an AOCP
function to protect the system.
Design Tips
LED Current Changing
Figure 22 shows the recommended circuit for achieving
high PF. In this condition, the LED current goes to 0 every
half cycle period.
VCC[10V/div]
VDC[40V/div]
VCS[1V/div]
VGATE[7V/div]
VDD[3V/div]
ILED[0.2A/div]VD
RVDRAIN[100V/div
]
Figure 20.
AOCP Function
Figure 22.
Figure 21 shows the typical waveforms of FL7701 system.
The LED current has the same phase as the input voltage
source and rectified sinusoidal waveform.
Typical Waveform
To design around this, add an electrolytic capacitor in
parallel to the LED load, as shown in Figure 23. This added
capacitor provides a truer DC LED current.
VDD[3V/div]
ILED[0.1A/div]
VDRAIN[100V/div]
Electrolytic Capacitor
EMI filter
BD
LED
FRD
AC INPUT
L
HV
ADIM
OUT
RT
VCC
FL7701
CS
GND
Figure 23.
Figure 21.
Circuit with Electrolytic Capacitor
VDD[3V/div]
Vgate[7V/div]
ILED[0.2A/div]
VDRAIN[100V/div]
Typical Operating Waveforms
Figure 24.
© 2012 Fairchild Semiconductor Corporation
Rev. 1.2 • 12/30/14
Typical with Bulk Capacitor
www.fairchildsemi.com
6
AN-9744
APPLICATION NOTE
Preventing LED Flicker Due to ZCD Error
During Analog Dimming
Minimum Dimming Range
Operating range of analog dimming pin is 0.5 V ~ 3.5 V. If
a dc signal is not injected into the ADIM pin, this pin is
internally pulled high to 3.5 V and the system operates at
full brightness.
HV pin of FL7701 is usually connected behind the EMI filter
components. Waveform of HV pin has distortion due to L-C
resonance of the EMI filter and discharging of EMI capacitor.
This could lead to VCC waveform distortion as well as ZCD
detection error as shown in Figure 25. The abnormal
waveform could cause LED light flicker to happen.
When voltage of ADIM pin is below 0.5 V, FL7701operates
with minimum turn on time which decides the minimum
dimming current as shown in Figure 27.
Figure 27.
Figure 25.
Increasing System Reliability
LED Flicker at Analog Dimming
The reasons for the LED flickering are as follow;
1.
Example of Analog Dimming Curve
To increase system reliability in noisy conditions, add a small
capacitor with below 100 pF to the RT and ADIM pins. In
normal conditions, these components are unnecessary.
EMI filter is optimized for the rated output power.
When analog diming is used, the output power is
decreased. As the output power decreases, the EMI
filter capacitor is not fully discharged.
2.
When the EMI filter capacitor is fully discharged,
VCC voltage does not have a drop point and this
causes ZCD error
3.
When ZCD error happens, FL7701 changes to DC
mode operation for the next 6 rectified line cycles
LED string has a flicker due to current deviation.
PCB Layout Guidelines
The PCB layout is important because a common application
would be to retrofit a lamp application, which requires a
small product size. The IC could be affected by noise, so
carefully follow the PCB layout guide lines:
 Locate the IC on the external powering path.
 Separate power GND and signal GND.
 VCC capacitor should be located close to the VCC pin.
In order to prevent ZCD error when FL7701 is used for
analog dimming, HV connection point must change from
the drain side to the input side via series connected diodes as
shown in Figure 26.
Powering Path
LED
Capacitor for VCC pin
FLS0116
Fuse
500mA/250V
CS
BD
MB6S
VCC
IC
One point GND
DRAIN
HV
PGND
GND
RT

ADIM
Analog
Dimming Signal
with MCU
Figure 26.
SGND
Figure 28.
Example LED Layout
FLS0126 Application with Analog Dimming
© 2012 Fairchild Semiconductor Corporation
Rev. 1.2 • 12/30/14
www.fairchildsemi.com
7
AN-9744
APPLICATION NOTE
Related Datasheets
FL7701 - Smart LED Lamp Driver IC with PFC Function
FLS0116 - MOSFET Integrated Smart LED Lamp Driver IC with PFC Function
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS
HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE
APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS
PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION.
As used herein:
1.
Life support devices or systems are devices or systems which,
(a) are intended for surgical implant into the body, or (b)
support or sustain life, or (c) whose failure to perform when
properly used in accordance with instructions for use provided
in the labeling, can be reasonably expected to result in
significant injury to the user.
© 2012 Fairchild Semiconductor Corporation
Rev. 1.2 • 12/30/14
2.
A critical component is any component of a life support device
or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
www.fairchildsemi.com
8
ON Semiconductor and
are trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries.
ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent
coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein.
ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
Buyer is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards,
regardless of any support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor data sheets and/or
specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer
application by customer’s technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are not
designed, intended, or authorized for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification
in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized
application, Buyer shall indemnify and hold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and
expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such
claim alleges that ON Semiconductor was negligent regarding the design or manufacture of the part. ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This
literature is subject to all applicable copyright laws and is not for resale in any manner.
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