FL7734MX datasheet(11/14 Pages) ONSEMI

Part #	FL7734MX

Description	Single-Stage Primary-Side-Regulation PWM Controller

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Manufacturer	ONSEMI [ON Semiconductor]

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FL7734

• Rev. 1.0

10

As a function of dimming reference modulation shown in

Figure 18, output current is constantly regulated with

constant VREF when VDIM is higher than 3 V and VREF is

set lower than VEAI (TRUECURRENT

®

calculation

result) when VDIM is lower than 2.25 V. Once VDIM is less

than 2.25 V, the error amplifier always pulls down

current in the output and FB voltage is clamped by MOD

voltage so that open loop control starts for stable LED

current control at low phase angle range.

IDIM

VVIN-DIM

(3 V)

Input

Voltage

VIN

RVIN1

RVIN2

VDIM-CLAMP

(3.25 V)

CDIM

RDIM1

DIM

RDIM2

MOD

FB

CFB

Dimming

Reference

Modulation

VEAI

Error

Amp.

VREF

Figure 18.

Dimming Control

High dimmer compatibility is implemented by bleeding

current generated through BLD to RBLD and having the

switching mode management by TCIC.

PFC and THD

In

a

conventional

boost

converter,

Boundary

Conduction Mode (BCM) is generally used to keep

input current in phase with input voltage for Power

Factor (PF) and Total Harmonic Distortion (THD).

However, BCM switching distorts input current in the

single stage flyback / buck boost converter because

power inductor current is not the same as input

current. Moreover, it becomes more difficult to meet

PF and THD once passive bleeder (resistor and

capacitor in the driver input) is added for successful

firing in the single stage TRIAC dimming system.

IIN

IIN_AVG

GATE

Constant Frequency

Figure 19.

PFC Control in DCM

In order to optimize PF and THD in the single stage

flyback topology, constant turn-on time and constant

frequency in Discontinuous Conduction Mode (DCM)

are the best method to make the input current

proportional to the input voltage as shown in Figure 19.

FL7734 basically adopts the DCM operation with

constant turn-on and frequency in a half line cycle.

Once dimmer absence is detected during startup

sequence, FL7734 selects voltage mode in which

constant turn-on time is maintained by an internal error

amplifier and a large external capacitor (typically

>1 µF) at the FB pin. Constant frequency and DCM

operation are managed by linear frequency control.

Linear Frequency Control

DCM should be guaranteed for high power factor in

flyback topology. To maintain DCM in the wide range of

output voltage, frequency is linearly adjusted by output

voltage in linear frequency control. Output voltage is

detected by auxiliary winding and resistive divider

connected to the VS pin, as shown in Figure 20. When

output voltage decreases, secondary diode conduction

time is increased and the linear frequency control

lengthens

switching

period,

which

retains

DCM

operation in the wide output voltage range. The

frequency control lowers primary rms current for better

power efficiency in full-load condition.

OSC

Linear Frequency

Controller

VVS

freq.

VOUT

VS

tDIS

Detector

DCM

BCM

Figure 20.

Linear Frequency Control

BCM Control

The end of secondary diode conduction time (tDIS) can

be over a switching period set by linear frequency

control. In this case, FL7734 doesn

’t allow CCM and

operation mode changes from DCM to BCM. Therefore,

magnetizing inductance can be largely designed to add

BCM for better efficiency if PF and THD meet

specification with enough margin.

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