ST AN1320 Application note

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AN1320

APPLICATION NOTE

December 2000

1.0 INTRODUCTION

Roughly speaking a HF-TL ballast converts the 50-60Hz input to a high frequency output, usually in the range

of 25-125KHz.

A rectifier block and a DC to high frequency inverter usually make up a ballast. The half bridge of the inverter

can be driven in different ways with different ICs.

We will focus on a specific driver: the L6574. We will see first how the L6574 can drive and control a ballast,

then how it can communicate and be supervised by a

The aim of this paper is to examine if there are advantages in having a

C work with L6574, the feasibility of

this “cooperation”, and a practical example.

2.0 L6574

L6574 is a BCD off line 16 pin IC specifically designed for ballast applications [r ef.1](see fig.1). It has both driver

functions and controller functions on board.

The most useful characteristics to control the lamp are:

■

Preheat and frequency shifting timing

■

Cmos shut down input

■

Sense op-amp for closed loop control or protection failures

The parameters of the application are set by external components (resistors and capacitors) connected to the

IC. L6574 allows the user to set all the parameters according to the lamp characteristics, and the ballast wi ll be

a high performance one. There is a specific application note on this IC (ref. [2]) : here you find the description of

58W TL ballast with PFC section. Please refer to this application note and to the L6574 datasheet for the IC

details. In the following paragraphs we will focus our attention on a way to interface the L6574 with the micro-

controller rather than on “L6574 - stand alone” perfor mances. The aim of this “supervision” is to control the three

points mentioned above.

by Francesca Sandrini and Luca Rodeschini

L6574 & MICROCONTROLLER

IN BALLAST APPLICATIONS

There is an increasing demand for flexibility in ballast applications. This means a request for having

ballast that can be used for different tubes without changing the soldered components. The aim is to

save money by using less parts (resistors, capacitors and so on) and less ballast models to be stored

and managed.

A way that is going to be investigated is the use of micr ocontroller which can “supervise” the application

in such a way that the key parameters of the application can be modi fied accordi ng to the tube char ac-

teristics just by changing the micro-code.

In this application note we will exploit a way to interface a microcontroller with our integrated ballast

controller: the L6574.

AN1320 APPLICATION NOTE

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Figure 1. L6574 Block Diagram

3.0 L6574: HOW TO SET FREQUENCIES, TIMING, FAULT SIGNALS IN AN “ANALOG APPLICATION”

In this paragraph we will have a snap shot of the L6574 working, just as far as the characteristics important for

the micro interface are concerned. For further details please refer to [1] and [2].

The L6574 typical behavior is shown in fig. 2: there is a starting frequency (f

MAX

or f

PRE

) that is constant for a

set preheat time T

PRE

, than there is a frequency shift towards f

min

that last 0.1 of T

PRE

and is called T

IGN

or T

Figure 2. Frequency shift

Leads 1, 2, 3, 4 are used to set frequencies and timing.

The capacitor connected to pin 1[CPRE] sets the preheat time:

PRE

= K1 · C

PRE

The ignition time, or better, the time to let frequency shift from preheat value to the min. value is one tenth of T

PRE

The current that charges and discharges the capacitor connected to pin 3 (C

) sets the half bridge oscillating

frequency.

The current that charges C

is set by the current that flows out from pin 2 and 4 during preheat and from pin 4

GND

REF

Imin

IGN

VCO

EN1

THE

EN2

BOOT

OUT

BOOT

LOAD

H.V.

LVG

DETECTION

HVG

BOOTSTRAP

DRIVER

HVG

DRIVER

LVG DRIVER

Vthpre

Ifs

PRE

REF

Imax

PRE

OP AMP

OPOUT

OPIN-

OPIN+

DEAD

TIME

DRIVING

LOGIC

CONTROL

LOGIC

Ipre

LEVEL

SHIFTER

D97IN493B

fmax

fmin

freq

time

Tpre

Tsh

normal operation

preheating phase

ignition phase

AN1320 APPLICATION NOTE

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alone during the on phase. As pin 2 and 4 are at 2V, the currents that flow out of them is inversely proportional

to the resistance connected between gnd and pin 2 (R

PRE

) and between gnd and pin 4 (R

IGN

There are some useful formula:

Choosing properly the resistor and the c apacitor values the designer can set the desired fr equencies and timing.

When the designer has to do another application for another lamp type, he has to change the resistors and ca-

pacitors in order to have another range of frequencies.

For protection in case of lamp failure two logic input are provided: pin8 [EN1] and pin9 [EN2]. Both are active

high, but they have different functions: when EN2 is activated it forces the IC to start again the preheat se-

quence. When EN1 is activated it shut down the IC until V

is removed or until EN2 is pulled high.

EN2 is usually used as “ignition fault”: if the lamp is not ignited, the preheat sequence starts again.

EN1 can be used to sense lamp removal / replacement or disconnection.

Figure 3. EN1

max

1.41 R

pre

ign

+()⋅

pre

ign

Cƒ

⋅⋅

---------------------------------------------------=

min

1.41

ign

Cƒ

⋅

----------------------- -=

LVG

HVG

EN1

SUVP

D97IN490

AN1320 APPLICATION NOTE

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Figure 4. EN2

L6574 has also a sense op-amp th at can be used to have a closed loop contr ol of the lam p. We can give a volt-

age reference to the non-inverting input, a signal proportional to the load current to the inverting input, and we

can connect the op amp output pin to pin #4. In this way if the current in the load exceeds the reference the op

amp will sink from pin 4 an additional amount of current that has to be added to the current that flows through

IGN

. So the current charging C

will increase, that means a higher half bridge osci llation frequency, that m eans

a lower current in the load. C hanging the reference v oltage on the non- in verting input of the op amp we change

the frequency of the oscillator, that means we change the current in the load, and this allows lamp dimming.

Figure 5. Cl osed Loop

PRE

ING

EN2

OUT

SUVP

D97IN491B

STP4NB50/

STP6NB50

R25

0.68

C9 8.2nF

R18 100K R19 100K

1N4148

R33 9.1K

RIGN

D98IN818A

5/20

AN1320 APPLICATION NOTE

Figure 6. AN993 Demo Application Circuit

Q2 STP4NB50/

STP6NB50

C12

100nF

C14

1µF

C13

470pF

R25

0.68

Bridge

Fuse

NTC

L1 2.1mH

22µF

450V

DZ1

14V

4.7µF

25V

100nF

D1 BYT11600

C17

100nF 250V

C19

8.2nF

1500V

LAMP

1N4148

R15

1.5K

R14

4.7K

R18

100K

R19

100K

R21

R22 22

Q3 STP4NB50/

STP6NB50

1612

L6574

R13

100K

R33 9.1K

R17 9.1K

8.2nF

L6561

1.5M

68K

120K

R6 22

0.68

R12

9.53K

R11

750K

R16 47

STP6NB50

220nF

400V

10nF

C4 680nF

C11

680pF 630V

R23

47K

R20

47K

120K

R27

6.8K

R30

3.9K

C16 1µF

R32 20K

C15

330nF

R29

750K

R28

750K

R26

390K

R24

6.8K

100nF

R10

750K

220nF

630V

C18

100nF 250V

1N4148

RSENSE

0.68

T1 1.24mH (E25*13*7)

10 Turns 1.3mm gapped

C20

100nF

10.1K

1N4148

D99IN1064

AN1320 APPLICATION NOTE

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4.0 L6574: HOW TO SET FREQUENCIES, TIMING, FAULT SIGNALS IN A “MICROCONTROLLED

APPLICATION”

We have seen that the frequencies in L6574 are set by fixing the current that flows out from pin 2 and 4 and

fixing the value of the C

capacitor.

A microcontroller output pin can give us a high logic level (5V) a low logic one (0V) or a PWM output at fixed

frequency and variable duty cycle.

We can not use the P WM to act directly on C

pin, because the rising edge on C

is the low side mosfet “on

time” and the falling edge is the high side mosfet one. The half bridge would oscillate in asymmetrical way at

fixed frequency instead of at 50% duty cycle and variable frequency. So we have to interface the

C with th e

pins that set the current that charges C

. A push pull output that gives us just 0V or 5V can not be used to in-

terface pin 2 and 4 because they have a maximum voltage level up to 2V. We have to use the integrated value

of a PWM signal to set a voltage level between 0V and 2V.

We can use a PWM output also to gi ve the op amp the vo ltage reference to c hange the load c urrent (and so the

lamp power to perform dimming)

Acting on L6574 pin 4 and (or) on pin 7 (opamp+) we can control the inverter working frequency.

If we want to control the preheat timing and frequency we have to act on pins 2 and 1. First we have to avoid

that the L6574 fixes the preheat time by itself. If we connect to pin 1 a ver y small cap (e.g. 1nF), the L6574 “an-

alog T

PRE

” will be so small to be “invisible” to the lamp (i.e. less than 2ms). During these 2 ms, the oscillating

frequency has to be high enough to avoid lamp filaments preheat (> 150KHz). The resistor connected to pin 2

has to be sized properly.

After these 2 ms L6574 is in “working mode”: it means that pin 4 is no more involved in fixing the frequency.

Only C

and R

IGN

(pin 3 and 4) set it.

Now the effective preheat time can be decided by the

C just acting on the PWM that gives the voltage reference

to pin 4. For example, it can have a certain duty cycle (appropriate for the preheat freq.) for a fixed time, than it

can change the duty cycle (i.e. the voltage reference) to set the ignition profile and the final working frequency.

Now we are able to change all the frequencie s and the tim ing invo lved in lamp tur ning on a nd dimming with two

connections between L6574 and the

The fault management can al so be d one by the

C: all the fault si gnals will be br ought to it, and then it will react

according to the code. A connection that can be useful is the one to pin 8 (shut down pin) that can be direct

because the ICs levels are compatible. In this way the

C can react to a signal either by stopping the inverter

or by changing the frequency (i.e. r epeating a preheat sequ ence if there is the no-i gnition al arm, or brin ging the

frequency to a very high value…).

Just with these 3 connection between the L6574 and the

C we can set nearly all the parameters of the appli-

cation by software.

The number of

C inputs we need for fault signals depends only on what we want to control.

We need another input pin to give the

C the information about the dimming level: this is the interface between

the ballast and the “final user”. We can use either switches or an AD input. The first solution is more expensive

in terms of number of pin, the AD input requires some attention for the co de part but allows a much larger num-

ber of levels.

5.0 HOW TO APPLY THIS INTERFACE TO A BALLAST

We started from AN993 demo boar d to build a

C application with the s ame performances and s ome additi onal

degrees of freedom.

We will now apply all the concepts already discussed and put them into a working board.

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