AN-9744 [FAIRCHILD]
Smart LED Lamp Driver IC with PFC Function; 智能LED灯驱动器IC，具有PFC功能
| 型号： | AN-9744 |
| 厂家： | 
FAIRCHILD SEMICONDUCTOR |
| 描述： | Smart LED Lamp Driver IC with PFC Function |
| 文件： | 总7页 (文件大小：988K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
www.fairchildsemi.com
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 self-
biasing 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.hen
the VCC drops below the VCCST-, IC operation stops.
The internal DAC_OUT reference signal is dependent on
the VCC voltage. Using the DAC_OUT signal and internal
clock, CLK_GEN; the FL7701 automatically makes a
digital reference signal, DAC_OUT. If the FL7701 cannot
detect the ZCD_OUT signal, the IC has an abnormal
internal reference signal. In this situation, this phenomenon
causes a lighting flicker.
Hysteresis is provided for stable operation of the IC when
input the 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.
Vsup
Iline
LED Load
D1
Figure 2.
FL7701 Operation
IL
L
FL7701
Soft-Start Function
VSUP_SEN
VCC
HV
HV
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 settling down this
transient period, the internal reference is fixed at a certain
DC level. In this time, the IC continually tries to find input
phase information from the VCC pin. If the IC succeeds in
getting phase information from the VCC, the IC
automatically follows a similar shape reference, which it
made during the transient times, seven periods. If not, the
IC has a DC reference level.
Device
DAC: Digital to Analog
HV Device : High-Voltage Device
DAC
ZCD_OUT
C
DAC_OUT
Driver
OUT
CS
S
Q
R
Isw
Reference
GND
Figure 1.
Basic Block of FL7701
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 11/9/12
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.
Figure 3.
DC Input Condition
Vp
Vp /
2
Figure 6.
Internal Reference
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
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
between AC Input Mode and DC Input Mode is
.
2
diode because this system design results in a high pulse Output Frequency Programming
shape input current. This pulse shape current contains many
The FL7701 can program output frequency using an RT
harmonic components, so the total system cannot have high
resistor or with the RT pin in open condition. The FL7701
PF. To get high PF performance, the FL7701 uses a
can have a fixed output frequency around 45 kHz when the
different approach.
RT pin is left open. For increasing system reliability, a
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.
small-value capacitor is recommended below 100 nF in RT-
open condition. The relationship between output frequency
and the RT resistor is:
2.02×109
[Hz]
(1)
fOSC
=
Vbridge
Bridge Diode
RT
Output Voltage
Input Voltage
Peak
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.
VCC
ZCD
EMI filter
BD
DAC_OUT
LED
L1
D1
L
HV
D2
Figure 5.
Internal PFC Function
ADIM
OUT
C2
C1
RT
FL7701
The FL7701 detects the VCC changing point for making the
Zero Crossing Detection (ZCD) signal, which is an internal
timing signal for making 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.
VCC
R1
R3
CS
C3
C4
R2
GND
L2
Figure 7.
LED Open Condition
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 11/9/12
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 Edge-
Blanking (LEB) time of 350 ns; the IC stops operation.
10×3.5
0.85× 2 × 220
Dmin
=
= 0.132
Step 2: Maximum Duty Ratio
Similar to Step 1, calculate maximum duty ratio as:
VCC
HV
VCC
JFET
ZCD
UVLO
time
nV
F
D
=
(3)
max
ZCD
DAC
η ×V
in(min)
TSD
Soft start
Digital Block
S
R
Oscillator
Q
RT
OUT
CS
[%]
-
Reference
+
LEB
Leading Edge
Blanking
GND
-
+
AOCP
2.5V
Figure 8.
AOCP Function
Analog Dimming Function
[ms]
The Analog Dimming (ADIM) function adjusts the output
LED current by changing the voltage level of the ADIM pin.
Figure 9.
Duty Variation vs. Time
Application Information
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
The FL7701 has a 50% maximum duty cycle to prevent
sub-harmonic instability. Assume the minimum input
voltage enters 50% duty ratio. Using Equation (2), re-
calculate the minimum input voltage for CCM operation:
level can be up to 308 VAC
.
nVF
η × Dmax 0.85× 0.5
35
Vin(min)
=
=
= 82.35[V ]
(4)
Table 1 shows one example of a design target using the
FL7701 device.
311V
Input voltage
Table 1. Target Design Specification
Item
Specification
Note
Expected min. input voltage (CCM) :
Frequency
45 kHz
35
Vin(min)=82.35V
DCM
DCM
time
Output Voltage
VF=3.5 V, n=10
ILED(rms)
Current Limit on the
DAC reference
Output LED Current RMS
Output LED Current Peak
Input Voltage (Max.)
0.3
Average
LED Current(ILED(ave
)
0.5
ILED(peak)
i
220
VAC(rms)
CCM
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
1-Dmin
time
toff
ton
Figure 10. Estimated Waveforms
nVF
Dmin
=
(2)
η ×Vin(max)
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
www.fairchildsemi.com
Rev. 1.0.1 • 11/9/12
3
AN-9744
APPLICATION NOTE
Step 3: Maximum On/Off Time
Step 5: Inductance
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:
Derive one more formula for the minimum inductance value
of the inductor using the Step 4 results:
(VF ×n)(1− Dmin
fs ×Δi
)
3.5×10×(1−0.132)
45000×0.1516
L =
=
= 4.5
[
mH
]
(7)
1
1
[μs]
ton = toff
=
=
=11.11
2 fs 90000
Step 4: Calculate the LED Current Ripple, ∆i
The Figure 11 shows the typical LED current waveforms of
a FL7701 application. For more stable or linear LED current,
operate in CCM.
Figure 12. Current Ripple (∆I) vs. Inductance
Figure 11. Target Waveforms of LED Current
Using the typical LED current waveform in Figure 11,
derive the formula as:
Δi
2
or
ILED( peak ) = ILED(ave. peak)
+
(5)
Δi
ILED(min) = ILED(ave.peak)
−
Figure 13. Expected Waveforms
2
Step 6: Sensing Resistor
The FL7701 was calculated the sensing resistor value as:
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:
VCS
ILED( peak) 0.5
0.5
[Ω]
=1
R =
=
(8)
The power rating is under 0.25 W even when considering
power consumption at peak-current condition.
or
)
Step 7: Frequency Set Resistor
Δi = 2(ILED( peak) − ILED(ave. peak)
Δi = 2(ILED(ave. peak ) − ILED(min)
(6)
1
⋅2.0213⋅109 = 44.919[kΩ]
)
Rt =
(9)
fsw
ILED(ave. peak)
If there is not connected Rt resistance to the operation
frequency is 45 kHz.
Where
ILED(rms)
=
2
From the Table 1, the target LED current rms is defined as
0.3 A and the LED current peak is set to 0.5 A.
Δi = 2(ILED( peak )
− 2 ⋅ ILED(rms) )
= 2(0.5 − 2 ⋅0.3) = 0.1516 [A]
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 11/9/12
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.
VDRAIN[100V/div]
ILED[0.2A/div]
Figure 14. Test Circuit
Figure 15 and Figure 16 show the startup waveforms from a
on FL7701 application in DC and AC input conditions at
220 V with ten LEDs.
Figure 17. Input Source Changing: 45 Hz to 100 Hz
VCC[5V/div]
VDRAIN[100V/div]
VDRAIN[100V/div]
ILED[0.2A/div]
ILED[0.2A/div]
Figure 18. Input Source Changing: 100 Hz to 45 Hz
The Figure 19 shows the analog dimming performance with
changing VADIM. The output LED current changes according
to the control voltage.
Figure 15. Soft-Start Performance in DC
Input Condition
VCC[5V/div]
V
DRAIN[100V/div]
ILED[0.2A/div]
Figure 19. VADMIN vs. LED Current
Figure 16. Soft-Start Performance in AC
Input Condition
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 11/9/12
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]
VDD[3V/div]
ILED[0.2A/div]VD
RVDRAIN[100V/div
]
V
DC[40V/div]
VCS[1V/div]
GATE[7V/div]
V
Figure 20. AOCP Function
Figure 22. Typical Waveform
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.
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]
Figure 23.
Circuit with Electrolytic Capacitor
VDD[3V/div]
Vgate[7V/div]
ILED[0.2A/div]
VDRAIN[100V/div]
Figure 21. Typical Operating Waveforms
Figure 24. Typical with Bulk Capacitor
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 11/9/12
www.fairchildsemi.com
6
AN-9744
APPLICATION NOTE
Increasing System Reliability
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.
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:
Figure 25. Example LED Layout
.
.
.
Locate the IC on the external powering path.
Separate power GND and signal GND.
VCC capacitor should be located close to the VCC pin.
Related Datasheets
FL7701 — Smart LED Lamp Driver IC with PFC Function
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support or sustain life, or (c) whose failure to perform when
properly used in accordance with instructions for use provided
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2. A critical component is any component of a life support device
or system whose failure to perform can be reasonably expected
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© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 11/9/12
www.fairchildsemi.com
7
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