ST AN880 Application note

AN880

®

THE L6569: A NEW HIGH VOLTAGE IC DRIVER FOR

INTRODUCTION

Electronic lamp ballasts are now popular in both
consumer and industrial lighting. They offer power
saving, flicker free operation and reduced sizes.
Improvements to the light control and cost reduc­tion of the ballast will broaden the ir market accep­tance.

Today designers focus on reducing the cost of the
ballast, but also work to add features to the bal­last like saving energy by dimming the light, or in­creasing the life time with better preheat and pro­tections. Such requirements have contributed to
the development of dedicated high voltage con­trollers like the L6569, which ar e able to driv e the
floating transistor of a symmetric half bridge in­verter. This device is a simple, monolithic oscilla­tor-half bridge driver that allows quick design of
the ballast.

HIGH VOLTAGE I C DRI VERS I N B ALL AST AP­PLICATIONS

The voltage fed half bridge

Voltage fed series resonant half bridge inverters
are currently used for Compact Fl uorescent Lamp
ballasts (CFL), for Halogen Lamp transformers,
and for many European Tube Lamp (TL) ballasts.
This simple converter is preferred for new de­signs, because it minimizes the off state voltage
of the power transistors to the peak line voltage,
and requires only one resonant choke. In addition
this choke protects the half bridge against short
circuits across lamp terminals. However overheat­ing and overcurrent occur during open load op­eration. The inverter robustness must be im­proved, or some protections are required.

The half bridge inverter operates in Zero Voltage
Switching (ZVS) resonant mode [1], to reduce the
transistor switching losses and the electromag­netic interference generated by the output wiring
and the lamp.

Fully integrated ballast controllers

By varying the switching frequency, the half
bridge inverter is able to modulate the lamp
power. However most current designs use a sin-

APPLICATION NOTE

ELECTRONIC LAMP BALLAST

by G. Calabrese and T. Castagnet

Figure 1: CFL series resonant half bridge inverter.

Figure 2: Current and voltage of the STD3NA50

MOSFETs when driven in ZVS with
the L6569.

I

V

DS

GND

GND

GND

gle frequency with a saturable pulse transformer
(see fig. 1) to drive the transistors. This type of
design has a higher component count, a higher
tolerance on the switching frequency, and it can­not adjust the lamp power.

The only way to design a cost effective, compact
and smart control of the lamp is to use a dedi­cated I.C. that is able to drive the upper transistor
of an symmetric half bridge inverter. Such control­lers require a high voltage capability for the float­ing transistor driver [2]. MOSFETs are preferred
over Bipolar transistors as power switches be­cause their gate driver requires a lower supply
current and a smaller silicon size [3].

D

LVG

RF

2 µs/dv ; 50 V/dv ; 0.1 A/dv

February 2003

1/14

AN880 APPLICATION NOTE

THE L6569 AND ITS APPLICATIONS
The L6569

The L6569 is able to directly control a symmetric
half bridge inverter of a fluorescent lamp ballast,
or a low voltage halogen lamp transformer.Two
270mA buffers drive the inverter MOSFETs in
complementary fashion with a 1.25µs built-in
dead time to prevent cross conduction. The buffer
for the upper Mosfet is driven through a 600V
level shifter realized in BCD off line technology.
The oscil lator, simil ar to a CMOS 555 timer, ope r­ates fr om 25 to 150 kHz wi th a +/-5% maximu m
tolerance. The in ternal 15V shunt regulator has a
9V Unde r Volt age Lock Ou t with an 1V hysteres is,

Figure 3: Block diagram of the L6569.

VS

UVLO

CHARGE

PUMP

RF
CF

LOGIC CONTROL

with DEAD TIME

and the circui t req uires o nly 150 µA at power up.
The L6569 integrates a high voltage Lateral

DMOS transistor in place of the usual external di­ode [2] to charge the bootstrap capacitor for the
upper buffer. Figure 5 shows DMOS operating as
a synchronous rectifier.

The applications

The primary application for the L6569 is the Com­pact Fluorescent Lamp. With the oscillator, the
supply and the Mosfet drivers it is the core of the
application, and designers can customize the cir­cuit to their requirements.

BOOT

LEVEL

SHIFTER

HIGH
SIDE

DRIVER

HVG

OUT

LVG

LOW
SIDE

DRIVER

GND

Figure 4: Basic application diagram using the L6569 and two STD4NK50Z MOSFETs.

100nF

22Ω

STD4NK50Z

22Ω

D02IN1385

2/14

AC LINE

180KΩ

10µF

10µF

10KΩ

1nF

L6569

LAMP

Figure 5: Bootstrap capacitor charge.

AN880 APPLICATION NOTE

15.6 V

600V 120

CHARGE PUM P CIRCUIT

LOGIC

ON

Ω

L6569

Figure 6: Basic diagram for 2x105 W lamp ballast in full bridge configuration.

HV

100nF

BOOT

HVG

OUT

LVG

47

47

EXTERNAL

OSCILLATOR

V

V

S

RF

CF

GND

S

L6569

100nF

47

47

ON

BOOT

HVG

OUT

LVG

L6569

V

S

RF

CF

GND

D02IN1386

Typical industrial TL ballasts requires complex
control with dimming or automation interface.
Here the L6569 is a driver between the power
and control blocks. To use it with an external os­cillator, pin CF is used as an 0-12V logic input,
and the L6569 becomes a high voltage buffer.
Applications with power above 150W require a full
bridge inverter. Figure 6 shows how two L6569
drive such a MOSFET bridge. If no external con­trol is required, t he fir st L6569 m aster can control
the switching with its oscillator, and synchronizes
the other driver as (slave).

STB9NK50Z

The L6569 start up

Two versions of the L6569 are available with dif­ferent start up characteristics. The L6569 drives
the lower MOSFET ON at power-up until the sup­ply voltage reaches the Under Voltage Lock Out.
The bootstrap capacitor is precharged to 4.6V
and both the lower and the upper MOSFETs will
switch immediately with the oscillator. This is in­tended for inverters which use only one DC block­ing capacitor connected to the power ground, as
shown on figure 4 for CFL ballast.

3/14

AN880 APPLICATION NOTE

The L6569A holds both MOSFETs OFF until the
Under Voltage Lock Out is reached. This is in­tended for inverters using 2 decoupling capacitors
in half bridge as shown on figure 12. The inverter
is totally off, so that the voltage at the capacitors
center node is not unbalanced by the leakage
path during power on.

CONSIDERATIONS ON THE L6569 ENVIRON­MENT

To illustrate the benefits of the L6569 in t he CFL
applications, a demonstration board was devel­oped to supply Sylvania 18W DULUX lamp (ref:
CF18DT/E). The following chapters summarize
the application considerations applied in t his de­sign. The schematic, lay out and components list
are shown in appendix A.

Symmetric half bridge operation

To supply a fluorescent lamp, the ballast has to
achieve 3 functions: pre heat, ignition, and normal
lamp operation. The serial resonance occurs be­tween the choke and the capacitor in parallel with
the lamp. The choice of these components deter­mines the lamp ignition voltage a nd the nominal
lamp current.

Since the inverter using the L6569 and MOSFETs
can operate at a higher frequency than conven­tional solutions, the size of the passive compo­nents will be reduced. Such inverter can operate
up to 150 kHz in ZVS mode, and the switching
losses of the power transistors only limits t he fre­quency. In new design this frequency should be
set between 50 and 100 kHz. For instance with
an 18W lamp, a frequency increase from 33 to 50
kHz will lead to a 40% reduction of the choke
size.

To operate in Zero V oltage Switching (ZVS), the
switching frequency is higher than the resonant
frequency. All operation phases of the ballast ar e
secure in this mode. When the bootstrap transis­tor is conducting, no pulse c urrent will flow from
pin BOOT to pin V

, as it might happen in Zero

S

Current Switching. The bootstrap transistor re­mains in its Safe Operating Area, and its dissipa­tion is negligible.

The MOSF ET dri ve

The ZVS drive technique requires only a fast turn
off capability as shown on f igure 2, and the tran­sistor buffers are designed with a stronger sink
current. The two MOSFET buffers of the L6569
can sink a 400 mA peak current on capacitive
load. Typically these buffers can drive any MOS­FETs in TO220 package.

Figure 7 shows an example with the STP8NA50
that has an 0.85 Ω resistance R

DS-ON

.

Figure 7: Cur re nt and voltage of the STP8NA50

MOSFET at turn off with the L6569.
T

= 245 ns ,Tc = 95 ns, E = 93 µJ

GD

@ Tj = 50°C, RG = 22 Ω.

GD

T

D

I

GS

V

D

V

50 ns/dv ; 1 A/dv ; 5 V/dv ; 50V/dv

Tc

GND
GND
GND

The built-in dead time circuit acts whe n a MOS­FET turns off, delaying the turn on of the opposite
transistor for 1.25 µs. The voltage V

between

OUT

the 2 MOSFETs must switch within the minimum
dead time (0.85 µs), as shown on figure 8, to
avoid bridge cross conductions and transistors
overheat.

Figure 8: STD3NA50 MOSFET turn off when

driven by the L6569. T

D

T

I

D

C

T

GD

T

200 ns/dv ; 50 V/dv ; 0.1 A/dv

C

+ T

LVG

RF

GD

V

< T

DS

D

GND

GND

GND

The MOSFET voltage selection

Since the ballast is connected to the ac mains, it
must handle any spurious voltage spikes. When
the front end RFI filter and t he clamping device,
such as a varistor, absorbes totally the spike en­ergy, MOSFETs can have the same 600V mini­mum breakdown voltage BV

as the L6569.

DSS

Otherwise when the upper MOSFET is on, the re­sidual default may be applied to the L6569. Al­though the pin OUT breakdown voltage is higher
than 600V, it has a poor avalanche robustness.
Therefore the lower MOSFET protects the driver
by having a lower BV
mum BV

up to 500V will achieve safely this

DSS

. A MOSFET with a mini-

DSS

task.

4/14

Figure 9. L6569 driver protection against voltage spikes.

AN880 APPLICATION NOTE

OUT

BV

> 600V

15V

L6569

The auxiliary supply of the converter

The circuit consumption is defined by the MOS-

H.V.+

ON

V

OUT

OFF

mA, a secondary winding on the resonant choke
is an easy supply alternative.

FETs gate charge, the I. C. consumption, the os­cillator, and the shunt regulator. Several circuits
are possible.

In many applications a snubber i s used to reduce
the dissipation in the MOSFETs. When this snub­ber is used in conjunction with a start up resistor

in Figure 10), a non dissipative supply is

(R

S

achieved almost for free.
At start up t he I. C. i s co ns uming 150 µA, and the r e-

fore only a small supply resistor is required. During
operat ion the capacitor provides the supply current.
To avoid cross conduction, the capacitance is lim­ited by the dri ver de ad tim e T

. Henc e the capaci-

D

tive supply current IC is also limited.For a CFL bal­last this circuit easily supplies the required
operat ing c ur rent . Usi ng a CF18 DT lamp ( I

> 230

L

The ballast shutdown

The L6569 allows several ways (see figg. 11, 12
and 13) to shutdown the ballast [4]: by acting on
the C

input oscillator pin to turn off the upper

F

MOSFET or by acting on the VS supply pin with
the Under Voltage Lock Out.

Acting on C

(Fig. 11) a limiting resistor RL has to

F

be used, and it has to be: RL ⋅ CF > 1µs.
When the shutdown is realized acting on Vs pin,

(see fig. 12) a limiting resistor Rs must be used to
slow down the discharge of the supply filter Cs.
The constant time of the discharge must be
greater than 10 periods of the switching fre­quency:

mA) the required capacitance is 470 pF on 230 Vac
line. At 50 kHz the average capacitive current is 6
mA, as described in appendix B.

When the required driver current is higher than 10

Connecting the CF pin to ground GND stops the
oscillator, and the lower MOSFET will remain ON.
Therefore the bootstrap capacitor remains

Figure 10: Non dissipative auxiliary supply using the transistor snubber.

I

10

≥

R

S

⋅ f

C

s

sw

1mA WHEN STARTING

220k

Rs

Cs

Ω

6 mA WHEN 50 kHz SWITCHING

C

470 pF

310 V

bootstrap

circuit

L6569

5/14