ST AN1320 Application note

AN1320

APPLICATION NOTE

L6574 & MICROCONTROLLER

IN BALLAST APPLICATIONS

by Francesca Sandrini and Luca Rodeschini

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 acteristics 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.

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

C work with L6574, the feasibility of

December 2000

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AN1320 APPLICATION NOTE

Figure 1. L6574 Block Diagram

OP AMP

OPOUT

OPIN-

OPIN+

IGN

PRE

Imin

Imax

VCO

REF

CONTROL

LOGIC

DETECTION

DEAD

TIME

Ifs

Vthpre

Ipre

BOOTSTRAP

PRE

DRIVER

DRIVING

LOGIC

HVG

DRIVER

LEVEL

SHIFTER

LVG DRIVER

THE

D97IN493B

BOOT

HVG

OUT

LVG

GND

EN1

EN2

H.V.

BOOT

LOAD

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 set preheat time T

, than there is a frequency shift towards f

PRE

that last 0.1 of T

min

MAX

or f

PRE

) that is constant for a

PRE

and is called T

IGN

or TSH.

Figure 2. Frequency shift

freq

preheating phase

fmax

Tpre

ignition phase

normal operation

fmin

Tsh

time

Leads 1, 2, 3, 4 are used to set frequencies and timing. The capacitor connected to pin 1[CPRE] sets the preheat time: T

= 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 (CF) sets the half bridge oscillating frequency.

The current that charges C

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

AN1320 APPLICATION NOTE

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

There are some useful formula:

1.41 R

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

min

max

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

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.

) and between gnd and pin 4 (R

PRE

+()⋅

preRign

⋅⋅

preRign

1.41

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

ign

Cƒ

⋅

Cƒ

is removed or until EN2 is pulled high.

IGN

Figure 3. EN1

LVG

HVG

EN1

SUVP

D97IN490

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AN1320 APPLICATION NOTE

Figure 4. EN2

SUVP

OUT

PRE

ING

EN2

D97IN491B

PRE

PREtSH

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 voltage 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 R

. So the current charging CF will increase, that means a higher half bridge osci llation frequency, that m eans

IGN

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

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R25

0.68

Q3 STP4NB50/ STP6NB50

R33 9.1K

C9 8.2nF

1N4148

R18 100K R19 100K

RIGN 4

D98IN818A

Figure 6. AN993 Demo Application Circuit

47K

R16 47

R20

C12

100nF

C11

680pF 630V

C20

R21

100nF

1612

C18

C17

L1 2.1mH

STP6NB50

Q2 STP4NB50/

R22 22

R17 9.1K

100nF 250V

R28

750K

100nF 250V

R26

390K

STP6NB50

47K

R23

L6574

8.2nF

C19

8.2nF

1500V

R29

750K

Q3 STP4NB50/

1N4148

AN1320 APPLICATION NOTE

R27

6.8K

LAMP

R30

3.9K

C16 1µF

R32 20K

1N4148

R25

0.68

C15

330nF

R24

R18

100K

C14

C13

R19

6.8K

1µF

R33 9.1K

470pF

100K

D1 BYT11600

T1 1.24mH (E25*13*7)

10 Turns 1.3mm gapped

Bridge

Fuse

220nF

DZ1

68K

1.5M

630V

14V

1N4148

4.7K

RSENSE

C4 680nF

R15

R12

100nF

1.5K

9.53K

0.68

10nF

10.1K

D99IN1064

R13

100K

25V

4.7µF

120K

NTC

100nF

R6 22

STP6NB50

47K

L6561

220nF

R10

400V

22µF

750K

450V

R11

R14

750K

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AN1320 APPLICATION NOTE

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

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.

capacitor.

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

pin, because the rising edge on CF 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 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-

C with th e

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 “analog T

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

PRE

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

Now the effective preheat time can be decided by the

and R

(pin 3 and 4) set it.

IGN

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 We need another input pin to give the

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

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 number of levels.

5.0 HOW TO APPLY THIS INTERFACE TO A BALLAST

We started from AN993 demo boar d to build a µC 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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