Fairchild FL7701 service manual

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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 25V to 450V, the FL7701 maintains a

15.5V block for stable operation. When the V higher than V the V

at the VCC pin. The FL7701 also has a UVLO

voltage reaches

, the UVLO block starts operation.hen

CCST+

drops below the V

IC operation stops.

CCST-,

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 50Hz or 60Hz 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.

Device

sup

Driver

line

FL7701

DSG

DAC_OUT

Reference

ZCD_OUT

VSUP_SEN

DSG: Digital Sine-Wave Generator HV Device : High-Voltage Device

OUT

+ V

LED

GND

LED Load

Figure 1. Basic Block of FL7701

The internal DAC_OUT reference signal is dependent on the V

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.

Figure 2. FL7701 Operation

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 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.

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

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.

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.

bridge

Input Voltage

Peak

Bridge Diode

Output Voltage

/2Vp

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 between AC Input Mode and DC Input Mode is

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 45kHz when the RT pin is left open. For increasing system reliability, a small-value capacitor is recommended below 100nF in RTopen condition. The relationship between output frequency and the RT resistor is:

1002.2 



OSC

[Hz]

(1)

Output Open-Circuit Protection

VCC

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

ZCD

DAC_OUT

Figure 5. Internal PFC Function

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

does not connect to the chip, the IC cannot start.

EMI filter

ADIM

VCC

FL7701

GND

OUT

LED

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

Figure 7. LED Open Condition

pin, even is there may be noise on other pins.

AN-9744 APPLICATION NOTE



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 2.5V, even within Leading EdgeBlanking (LEB) time of 350ns; the IC stops operation.

VCC

RT OUT

GND

VCC

ZCD

Oscillator

time

ZCD

DAC

Soft start

Digital Block

Reference

UVLO

TSD

LEB

Leading Edge

Blanking

JFET

AOCP

2.5V

Figure 8. AOCP Function

Analog Dimming Function

The Analog Dimming (ADIM) function adjusts the output LED current by changing the voltage level of the ADIM pin.

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 level can be up to 305V

Table 1 shows one example of a design target using the FL7701 device.

or 400VDC.

For example, if V

= 220V and ten LEDs are in series

IN(max)

connection, the minimum duty ratio is:

5.310

D

min



220285.0



132.0

Step 2: Maximum Duty Ratio

Similar to Step 1, calculate maximum duty ratio as:

max







(min)

(3)

[%]

051015

Duty

[ms]

Figure 9. Duty Variation vs. Time

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, recalculate the minimum input voltage for CCM operation:



(min)







5.085.0



][35.82

(4)

Table 1. Target Design Specification

Item Specification Note

Frequency 45kHz

Output Voltage 35 VF=3.5V,

Output LED Current 0.3 rms

Output LED Current 0.5 Peak

Input Voltage (Max.) 220 V

AC(rms)

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.



min





(max)

(2)

Figure 10. Estimated Waveforms

where η is efficiency of system; V input voltage; V

is forward-drop voltage of LED; and n

is maximum

IN(max)

is LED number in series connection.

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