L6668
SMART PRIMARY CONTROLLER
PRELIMINARY DATA
1
General Features
Figure 1. Package
MULTIPOWER BCD TECHNOLOGY
LOAD-DEPENDENT CURRENT-MODE CONTROL: FIXED-FREQUENCY (HEAVY LOAD),
FREQUENCY FOLDBACK (LIGHT LOAD),
BURST-MODE (NO-LOAD)
■ ON-BOARD HIGH-VOLTAGE START-UP
■ IMPROVED STANDBY FUNCTION
■ LOW QUIESCENT CURRENT (< 2 mA)
■ SLOPE COMPENSATION
■ PULSE-BY-PULSE & HICCUP-MODE OCP
■ INTERFACE WITH PFC CONTROLLER
■ DISABLE FUNCTION (ON/OFF CONTROL)
■ LATCHED DISABLE FOR OVP/OTP FUNCTION
■ PROGRAMMABLE SOFT-START
■ 2% PRECISION REFERENCE VOLTAGE EXTERNALLY AVAILABLE
■ ±800 mA TOTEM POLE GATE DRIVER WITH
INTERNAL CLAMP AND UVLO PULL-DOWN
■ BLUE ANGEL, ENERGY STAR, EU CODE OF
CONDUCT COMPLIANT
Figure 2. Block Diagram
■
■
S-COMP
SO-16 (Narrow)
Table 1. Order Codes
Part Number
L6668
SO-16
L6668TR
SO-16 in Tape & Reel
SO16 PACKAGE ECOPACK®
■
1.1 APPLICATIONS
HI-END AC-DC ADAPTERS/CHARGERS FOR
NOTEBOOKS.
LCD/CRT MONITORS, LCD/CRT TV
DIGITAL CONSUMER
■
■
■
V CC
HV
15
Package
5
1
SLOPE
COMP.
VREG
25V
Vref
16
RCT
HV generator ON/OFF
and UVLO management
CLK
TIMING
8
VREF
Vcc_OK
DIS
R
+
7
S
Q
-
DIS
2.2V
N.C.
15V
4
OUT
6
S
BLANKING
Q
R
12
HICCUP
R
+
ISEN
+
PWM
-
S
Vcc_OK
DIS
OCP
Q
3
VREG
GND
-
-
HYST. CTRL
1.5V
+
0.4mA
PFC_STOP
13
+
14
DIS
OCP
STANDBY
2.2/2.7V
ST-BY
11R
0.8V
4R
SOFT-START
11
9
SKIPADJ
SS
10
COMP
May 2005
This is preliminary information on a new product now in development or undergoing evaluation. Details are subject to change without notice.
Rev. 1
1/23
L6668
2
Description
L6668 is a current-mode primary controller IC, designed to build single-ended converters.
The IC drives the system at fixed frequency at heavy load and an improved Standby function causes a
smooth frequency reduction as the load is progressively reduced. At very light load the device enters a
special operating mode (burst-mode with fixed, externally programmed peak current) that, in addition to
the on-board high-voltage start-up and the very low quiescent current, helps keep low the consumption
from the mains and be compliant with energy saving regulations. To allow meeting compliance with these
standards in power-factor-corrected systems too, an interface with the PFC controller is provided that enables to turn off the pre-regulator when the load level falls below a threshold.
The IC includes also a programmable soft-start, slope compensation for stable operation at duty cycles
greater then 50%, a disable function, a leading edge blanking on current sense to improve noise immunity,
latched disable for OVP or OTP shutdown and an effective two-level OCP able to protect the system even
in case the secondary diode fails short.
Table 2. Absolute Maximum Ratings
Symbol
Pin
Vcc
5
VHV
IHV
Value
Unit
IC Supply voltage (Icc = 20 mA)
Self-limited
V
1
High-voltage start-up generator voltage range
-0.3 to 700
V
1
High-voltage start-up generator current
Self-limited
A
Analog Inputs & Outputs, except pin 14
-0.3 to 8
V
---
Parameter
IPFC_STOP
14
Max. sink current (low state)
2
mA
VPFC_STOP
14
Max. voltage (open state)
16
V
0.75
W
Junction Temperature Operating range
-25 to 150
°C
Storage Temperature
-55 to 150
°C
Ptot
Tj
Tstg
Power Dissipation @Tamb = 50°C
Table 3. Thermal Data
Symbol
Rth j-amb
Parameter
Thermal Resistance Junction to AmbientMax
Figure 3. Pin Connection (Top view)
2/23
HV
RCT
HVS
S-COMP
GND
PFC_STOP
OUT
STBY
Vcc
ISEN
N.C.
SS
DIS
COMP
VREF
SKIPADJ
Value
Unit
120
°C/W
L6668
Table 4. Pin Description
Pin
Number
Pin Name
Function
1
HV
High-voltage start-up. The pin is to be connected directly to the rectified mains voltage. A
0.8 mA internal current source charges the capacitor connected between pin Vcc and GND
to start up the IC. When the voltage on the Vcc pin reaches the start-up threshold the generator is shut down. Normally it is re-enabled when the voltage on the Vcc pin falls below
5V, except under latched shutdown conditions, when it is re-enabled as the Vcc voltage falls
0.5V below the start-up threshold.
2
HVS
High-voltage spacer. The pin is not connected internally to isolate the high-voltage pin and
comply with safety regulations (creepage distance) on the PCB.
3
GND
Chip ground. Current return for both the gate-drive current and the bias current of the IC. All
of the ground connections of the bias components should be tied to a track going to this pin
and kept separate from any pulsed current return.
4
OUT
Gate-drive output. The driver is capable of 0.8A min. source/sink peak current to drive
MOSFET’s. The voltage delivered to the gate is clamped at about 15V so as to prevent too
high values when the IC is supplied with a voltage close to or exceeding 20V.
5
Vcc
Supply Voltage of both the signal part of the IC and the gate driver. The internal high voltage generator charges an electrolytic capacitor connected between this pin and GND as
long as the voltage on the pin is below the start-up threshold of the IC, after that it is disabled. Sometimes a small bypass capacitor (0.1µF typ.) to GND might be useful to get a
clean bias voltage for the signal part of the IC.
6
N.C.
Connect the pin to GND.
7
DIS
Latched device shutdown. Internally the pin connects a comparator that, when the voltage
on the pin exceeds 2.2V, shuts the IC down and brings its consumption to a value barely
higher than before start-up. The information is latched and it is necessary to recycle the
input power to restart the IC: the latch is removed as the voltage on the Vcc pin goes below
the UVLO threshold. Connect the pin to GND if the function is not used.
8
VREF
Voltage reference. An internal generator furnishes an accurate voltage reference (5V±4%,
all inclusive) that can be used to supply up to 5 mA to an external circuit. A small film
capacitor (0.1µF typ.), connected between this pin and GND is recommended to ensure the
stability of the generator and to prevent noise from affecting the reference.
9
SKIPADJ
Burst-mode control threshold. A voltage is applied to this pin, derived from the reference
voltage VREF via a resistor divider. When the control voltage at pin COMP falls 50 mV
below the voltage on this pin the IC is shutdown and the consumption is reduced. The chip
is re-enabled as the voltage on pin COMP exceeds the voltage on the pin. The high-voltage
start-up generator is not invoked. The function is disabled during the soft-start ramp. The
pin must always be biased between 0.8 and 2.5V. A voltage between 0.8 and 1.4V disables
the function, if the pin is pulled below 0.8V the IC is shut down.
10
COMP
Control input for PWM regulation. The pin is to be driven by the phototransistor (emittergrounded) of an optocoupler to modulate the voltage by modulating the current sunk from
(sourced by) the pin (0.4 mA typ.). It is recommended to place a small filter capacitor
between the pin and GND, as close to the IC as possible to reduce switching noise pick up,
to set a pole in the output-to-control transfer function. A voltage 50 mV lower than that on
pin SKIPADJ shuts down the IC and reduces its current consumption.
11
SS
Soft start. An internal 20µA generator charges an external capacitor connected between
the pin and GND generating a voltage ramp across it. This ramp clamps the voltage at pin
COMP during start-up, thus the duty cycle of the power switch starts from zero. During the
ramp all functions monitoring the voltage at pin COMP are disabled. The SS capacitor is
quickly discharged as the chip goes into UVLO.
12
ISEN
Current sense (PWM comparator) input. The voltage on this pin is internally compared with
an internal reference derived from the voltage on pin COMP and when they are equal the
gate drive output (previously asserted high by the clock signal generated by the oscillator)
is driven low to turn off the power MOSFET. The pin is equipped with 200 ns. min. blanking
time for improved noise immunity. A second comparison level located at 1.5V shuts the
device down and brings its consumption almost to a “before start-up” level.
3/23
L6668
Table 4. Pin Description (continued)
Pin
Number
Pin Name
Function
13
STBY
Standby function. This pin is a high-impedance one as long as the voltage on pin COMP is
higher than 3V. When the voltage on pin COMP falls below 3V, the voltage on the pin tracks
that on pin COMP and is capable of sinking current. A resistor connected from the pin to the
oscillator allows programming frequency foldback at light load.
14
PFC_STOP
Open-drain ON/OFF control of PFC controller. This pin is intended for driving the base of a
PNP transistor in systems comprising a PFC pre-regulator, to stop the PFC controller at
light load by cutting its supply. The pin, normally low, opens if the voltage on COMP is lower
than 2.2V and goes back low when the voltage on pin COMP exceeds 2.7V. Whenever the
IC is shutdown, either latched (DIS>1.5V, ISEN >1.5V) or not (UVLO, SKIPADJ<0.8), the
pin is open as well.
15
S-COMP
Voltage ramp for slope compensation. When the gate-drive output is high the pin delivers a
voltage tracking the oscillator ramp (shifted down by one VBE); when the gate-drive output
is low the voltage delivered is zero. The pin is to be connected to pin ISEN via a resistor to
make slope compensation and allow stable operation at duty cycles close to and greater
than 50%.
16
RCT
Oscillator pin. A resistor to VREF and a capacitor to GND define the oscillator frequency (at
full load). A resistor connect to STBY modifies the oscillator frequency when the voltage on
pin COMP is lower than 3V.
Table 5. Electrical Characteristcs
(Tj = 0 to 105°C, Vcc=15V, Co=1nF; RT =13.3k , CT =1nF; unless otherwise specified)
Symbol
Parameter
Test Condition
Min.
Typ.
Max.
Unit
22
V
12.5
13.5
14.5
V
8.0
8.7
9.4
V
24
28
V
SUPPLY VOLTAGE
Vcc
Operating range
After turn-on
VCCOn
Turn-on threshold
(1)
VCCOff
Turn-off threshold
(1)
Hys
Hysteresis
VZ
Zener Voltage
After turn-on
9.4
4.0
Icc = 20 mA
22
V
SUPPLY CURRENT
Istart-up
Iq
Start-up Current
Before turn-on,
Vcc=VccON-0.5
Quiescent Current
After turn-on
ICC
Operating Supply Current
Iqdis
Shutdown quiescent
current
150
2
VDIS > 2.2, or VISEN > 1.5
µA
2.5
mA
4
mA
180
VSKIPADJ<0.8
1
0.8 <VCOMP < VSKIPADJ
µA
1.8
1.3
mA
mA
HIGH-VOLTAGE START-UP GENERATOR
VHV
VHVstart
Breakdown voltage
IHV < 100 µA
Start voltage
IVcc < 100 µA
Icharge
Vcc charge current
VHV > VHvstart, Vcc > 3V
IHV, ON
ON-state current
IHV, OFF
Leakage current (OFF state)
VCCrestart
HV generator restart voltage
V
50
80
110
V
0.55
0.85
1
mA
VHV > VHvstart, Vcc > 3V
1.6
mA
VHV > VHvstart, Vcc = 0
0.8
VHV = 400 V
(1) After
4/23
700
DIS tripping
40
µA
4.4
5
5.6
V
12
13
14
V
L6668
Table 5. Electrical Characteristcs (continued)
(Tj = 0 to 105°C, Vcc=15V, Co=1nF; RT =13.3k , CT =1nF; unless otherwise specified)
Symbol
Parameter
Test Condition
Min.
Typ.
Max.
Unit
4.925
5
5.075
V
REFERENCE VOLTAGE
VREF
Output voltage
(2)
VREF
Total variation
Vcc= 9.4 to 22 V,
IREF = 1 to 5 mA
4.8
5.13
V
IREF
Short circuit current
VREF = 0
10
30
mA
Sink capability in UVLO
Vcc = 6V; Isink = 0.5 mA
0.2
0.5
V
400
480
µA
Tj = 25 °C; IREF = 1 mA
PWM CONTROL
VCOMPH
Maximum level
ICOMP =0
5.5
ICOMP
Max. source current
VCOMP = 1 V
320
RCOMP
Dynamic resistance
VCOMP = 2 to 4 V
Dmax
Maximum duty cycle
VCOMP = 5 V
Dmin
Minimum duty cycle
V
22
70
kΩ
75
%
VCOMP = 1 V
0
%
-1
µA
ns
CURRENT SENSE COMPARATOR
IISEN
Input Bias Current
VISEN = 0
tLEB
Leading Edge Blanking
After gate drive low-to-high
transition
td(H-L)
VISENdis
225
290
100
ns
3.56
3.75
3.94
V/V
0.725
0.8
0.875
V
1.35
1.5
1.65
V
Delay to Output
Gain
VISENx
160
Maximum signal
VCOMP = 5 V
Hiccup-mode OCP level
(2)
STANDBY FUNCTION
Vdrop
Vth
VCOMP - VSTBY
ISTBY = 0.8 mA, VCOMP<3V
Threshold on VCOMP
(2)
Voltage falling
Hysteresis
35
mV
3
V
50
mV
LATCHED DISABLE FUNCTION
IDIS
Input Bias Current
VDIS = 0 to Vth
Vth
Disable threshold
(2)
voltage rising
-1
µA
2.1
2.2
2.3
V
95
100
105
kHz
OSCILLATOR
fsw
Oscillation Frequency
Tj = 25°C, VCOMP = 5 V
93
100
107
kHz
Vpk
Oscillator peak voltage
(2)
2.85
3
3.15
V
Vvy
Oscillator valley voltage
(2)
0.75
0.9
1.05
V
Vcc = 9.4 to 22V, VCOMP = 5 V
SLOPE COMPENSATION
S-COMPpk Ramp peak
RS-COMP = 3 kΩ to GND,
OUT pin high, VCOMP = 5 V
1.6
1.75
1.9
V
S-COMPvy Ramp starting value
RS-COMP = 3 kΩ to GND,
OUT pin high
0.15
0.35
0.55
V
Ramp voltage
OUT pin low
Source capability
VS-COMP = VS-COMPpk
0.8
0
Tj = 25 °C
14
mA
SOFT-START
ISSC
Charge current
20
26
µA
5/23
L6668
Table 5. Electrical Characteristcs (continued)
(Tj = 0 to 105°C, Vcc=15V, Co=1nF; RT =13.3k , CT =1nF; unless otherwise specified)
Symbol
Parameter
VSSsat
Low saturation voltage
VSSclamp
High saturation voltage
Test Condition
Min.
Typ.
Duty cycle = 0
Max.
Unit
0.6
V
7
V
SKIPADJ FUNCTION
Ibias
VSKIP
Input Bias Current
VSKIP = 0 to 4.5 V
-1
Operating range
Hys
Hysteresis
Below VSKIP
VOFF
Shutdown threshold
Voltage falling
µA
1.4
2.5
V
25
85
mV
0.8
V
PFC_STOP FUNCTION
Ileak
High level leakage current
VPFC_STOP < 16V, VCOMP = 2V
1
µA
VL
Low saturation level
IPFC_STOP = 1mA, VCOMP = 4V
0.1
V
Vth
Threshold for high level
VCOMP falling
Vth
Threshold for high level
VCOMP rising (2)
(2)
2.1
2.2
2.3
V
2.55
2.7
2.85
V
1.0
V
GATE DRIVER
VOL
Output Low Voltage
Isink = 200 mA
VOH
Output High voltage
Isource = 5 mA, Vcc = 12V
Isourcepk
Peak source current
-0.8
A
Isinkpk
Peak sink current
0.8
A
tf
Current Fall Time
30
ns
tr
Current Rise Time
55
ns
VOclamp
(1), (2)
Output clamp voltage
Isource = 5mA; Vcc = 20V
UVLO saturation
Vcc= 0 to Vccon, Isink= 2mA
9.8
10
10.3
12
Parameters in tracking each other
Figure 4. Typical System Block Diagram
PFC PRE-REGULATOR
DC-DC CONVERTER
Vinac
Voutdc
PWM is turned off in case of PFC's
anomalous operation, for safety
L6561/2
or
L6563
L6668
PFC can be turned off at light
load to ease compliance with
energy saving regulations.
6/23
V
15
V
1.1
V
L6668
3
Typical Electrical Performance
Figure 5. High-voltage generator ON-state sink
current vs. Tj
Figure 8. High-voltage generator start voltage
vs. Tj
HV
IHV (pin 1)
120%
1.2
[mA]
Vcc ≥ 3V
1
110%
0.8
VHV = 100 V
100%
0.6
90%
0.4
Vcc = 0
Values normalized to VHV @ 25°C
0.2
-50
0
50
100
80%
-50
150
0
50
100
150
Tj (°C)
Tj (°C)
Figure 6. High-voltage generator output (Vcc
charge current) vs. Tj
Figure 9. High-voltage generator Vcc restart
voltage vs. Tj
Vcc (pin 5)
Icc (pin 5)
14
120%
[V]
VHV = 100 V
12
while
latched off
10
VHV = 100 V
110%
100%
8
90%
normal
operation
6
Values normalized to Icc @ 25°C
80%
-50
0
50
100
150
4
-50
0
50
100
150
Tj (°C)
Tj (°C)
Figure 7. High-voltage pin leakage vs. Tj
Figure 10. IC consumption vs. Tj
Icc (pin 5)
IHV (pin 1)
5
40
[mA]
[µA]
Operating
3
VHV = 400 V
Vcc = 15V
Quiescent
2
30
1
20
0.5
Disabled or
during burst-mode
Vcc = 15 V
Co = 1 nF
f = 100 kHz
0.3
10
0.2
Latched off
Before start-up (Vcc=12V)
0
-50
0
50
Tj (°C)
100
150
0.1
-50
0
50
100
150
Tj (°C)
7/23
L6668
Figure 11. Start-up & UVLO vs. Tj
Figure 14. COMP source current vs. Tj
VCC (pin 5)
(V)
14
ICOMP(pin 10)
140%
ON
Vcc = 15 V
VCOMP= 1 V
13
12
OFF while
latched off
120%
100%
11
10
80%
9
OFF
Values normalized to COMP
I
@ 25°C
8
-50
0
50
100
60%
-50
150
0
50
100
150
Tj (°C)
Tj (°C)
Figure 12. Vcc Zener voltage vs. Tj
Figure 15. COMP dynamic resistance vs. Tj
RCOMP
VccZ (pin 5)
(V) 26
(kW)
(pin 10)
32
30
25
28
24
26
23
24
22
22
Vcc = 15 V
21
20
-50
20
0
50
100
150
18
-50
0
Tj (°C)
50
100
150
Tj (°C)
Figure 13. COMP voltage upper clamp level vs. Tj
Figure 16. Max. duty-cycle vs. Tj
(%)
VCOMP(pin 10)
7
(V)
75
Vcc = 15 V
Vcc = 15 V
74
6.5
73
6
72
5.5
5
-50
71
0
50
Tj (°C)
8/23
100
150
70
-50
0
50
Tj (°C)
100
150
L6668
Figure 17. Oscillator frequency vs. Tj
Figure 20. Disable level on current sense vs. Tj
fsw
Vpin12
(V) 2.0
102%
Vcc = 22V
RT = 13.3 kΩ
CT = 1 nF
101.5%
Vcc = 15 V
1.8
101%
Vcc = 15V
100.5%
1.6
Vcc = 9.4V
1.4
100%
1.2
99.5%
Values normalized to sf w @ Tj=25°C, Vcc=15V
99%
-50
0
50
100
150
1.0
-50
0
50
Tj (°C)
Figure 18. Oscillator ramp vs. Tj
150
Figure 21. Reference voltage vs. Tj
Vpin14
(V)
100
Tj (°C)
VREF (pin 8)
(V) 5.1
3.5
Peak
Vcc = 15 V
3.0
5.05
Vcc = 15 V
2.5
5
2.0
1.5
4.95
Valley
1.0
0.5
-50
0
50
100
150
4.9
-50
0
Tj (°C)
Figure 19. Current sense clamp vs. Tj
V ISENx
(V) 1
50
100
150
Tj (°C)
Figure 22. PFC_STOP open/low thresholds on
VCOMP vs. Tj
(pin 12)
VCOMP (pin 10)
(V) 2.8
PFC_STOP low (voltage rising)
2.7
Vcc = 15 V
VCOMP= Upper clamp
0.9
2.6
Vcc = 15 V
2.5
0.8
2.4
2.3
0.7
PFC_STOP open (voltage falling)
2.2
0.6
-50
0
50
Tj (°C)
100
150
2.1
-50
0
50
100
150
Tj (°C)
9/23
L6668
Figure 23. Standby thresholds vs. Tj
Figure 26. SKIPADJ hysteresis vs. Tj
VCOMP (pin 10)
3.6
(V)
Vpin9
100.0
(mV)
Vcc = 15 V
3.4
3.2
80.0
Vcc = 15 V
Disable (voltage rising)
60.0
Vskipadj = 2.5 V
3
Enable (voltage falling)
40.0
Vskipadj = 1.4 V
2.8
20.0
2.6
2.4
-50
0
50
100
0.0
-50
150
0
50
100
150
Tj (°C)
Tj (°C)
Figure 24. Standby pin dropout vs. Tj
Figure 27. SKIPADJ disable threshold vs. Tj
Vpin13 - Vpin10
Vpin9
1.0
0
(mV)
(V)
Vcc = 15 V
VCOMP= 2V
ISTBY = 0.8 mA
-5
Vcc = 15 V
0.9
-10
0.8
-15
0.7
-20
0.6
-25
-30
-50
0
50
100
0.5
-50
150
0
50
100
150
Tj (°C)
Tj (°C)
Figure 25. DIS threshold vs. Tj
Figure 28. Soft-start charge current vs. Tj
V pin 7
2.5
(V)
Iss ( pin11)
40.0
(µA)
Vcc = 15 V
Vcc = 15 V
2.4
30.0
2.3
20.0
2.2
10.0
2.1
2.0
-50
0
50
Tj (°C)
10/23
100
150
0.0
-50
0
50
Tj (°C)
100
150
L6668
Figure 29. S-COMP ramp vs. Tj
Figure 32. Gate-drive output low saturation
V pin4 [V]
Vpin15
2.0
(V)
5
Tj = 25 °C
Vcc = 12 V
SINK
Peak
4
Vcc = 15 V
RS-SCOMP = 3 kΩ
1.5
3
1.0
2
0.5
1
Valley
0
0.0
-50
0
50
100
0
200
400
150
600
800
1,000 1,200
IGD [mA]
Tj (°C)
Figure 30. UVLO saturation vs. Tj
Figure 33. Gate-drive output high saturation
Vpin4 [V]
Vpin4
(V)
1
Vcc = 0 V
Tj = 25 °C
Vcc = 12 V
SOURCE
Vcc - 2.0
0.9
Vcc - 3.0
0.8
Vcc - 4.0
0.7
Vcc - 5.0
0.6
Vcc - 6.0
0.5
-50
0
0
50
100
150
200
400
600
800
1,000
IGD [mA]
Tj (°C)
Figure 31. Gate-drive clamp vs. Tj
Vpin4 clamp
(V)
13
Vcc = 20 V
12.8
12.6
12.4
12.2
12
-50
0
50
100
150
Tj (°C)
11/23
L6668
4
Application Information
The L6668 is a versatile current-mode PWM controller specific for offline fixed-frequency, peak-currentmode-controlled flyback converters.
The device is able to operate in different modes (fig. 34), depending on the converter's load conditions:
1) Fixed frequency at heavy load. In this region the IC operates exactly like a standard current mode control
chip: a relaxation oscillator, externally programmable with a capacitor and a resistor, generates a sawtooth
and releases a clock pulse during the falling edge of the sawtooth; the power switch is turned on by the clock
pulses and is turned off by the control loop.
2) Frequency-foldback mode at medium and light load. As the load is reduced the oscillator frequency is reduced as well by slowing down the charge of the timing capacitor proportionally to the load itself.
3) Burst-mode control with no or very light load. When the load is extremely light or disconnected, the converter
will enter a controlled on/off operation with constant peak current. A load decrease will be then translated
into a frequency reduction, which can go down even to few hundred hertz, thus minimizing all frequencyrelated losses and making it easier to comply with energy saving regulations. Being the peak current very
low, no issue of audible noise arises.
Burst mode
Figure 34. Multi-mode operation of the L6668
Fixed-frequency mode
Frequency foldback mode
f sw
0
0
Pinmax
Pin
4.1 High-voltage start-up generator
Figure 35 shows the internal schematic of the high-voltage start-up generator (HV generator). It is made
up of a high-voltage N-channel FET, whose gate is biased by a 15 MΩ resistor, with a temperature-compensated current generator connected to its source.
Figure 35. High-voltage start-up generator: internal schematic
HV
L6668
1
15 MΩ
Vcc_OK
HV_EN
IHV
5
Vcc
CONTROL
I charge
3
GND
The HV generator is physically located on a separate chip, made with BCD off-line technology able to withstand 700V, controlled by a low-voltage chip, where all of the control functions reside.
12/23
L6668
With reference to the timing diagram of figure 36, when power is first applied to the converter the voltage
on the bulk capacitor (Vin) builds up and, as it reaches about 80V, the HV generator is enabled to operate (HV_EN is pulled high) so that it draws about 1 mA. This current, diminished by the IC consumption,
charges the bypass capacitor connected between pin Vcc (5) and ground and makes its voltage rise almost linearly.
Figure 36. Timing diagram: normal power-up and power-down sequences
Vin
V HVstart
regulation is lost here
Vcc
t
Vcc ON
Vcc OFF
Vcc restart
t
OUT
t
HV_EN
t
Vcc_OK
IHV
t
1 mA
Normal
operation
Power-up
Power-down
t
As the Vcc voltage reaches the start-up threshold (13.5V typ.) the low-voltage chip starts operating and
the HV generator is cut off by the Vcc_OK signal asserted high. The IC is powered by the energy stored
in the Vcc capacitor until the self-supply circuit (typically an auxiliary winding of the transformer and a
steering diode) develops a voltage high enough to sustain the operation.
The residual consumption of this circuit is just the one on the 15MΩ resistor (≈10 mW at 400 Vdc), typically
50-70 times lower, under the same conditions, as compared to a standard start-up circuit made with an
external dropping resistor.
Figure 37. Timing diagram showing short-circuit behavior
Vcc
Short circuit occurs here
Vcc ON
Vcc OFF
Vcc rest
OUT
Vcc_OK
IHV
t
t
t
1 mA
t
13/23
L6668
At converter power-down the system will lose regulation as soon as the input voltage is so low that either
peak current or maximum duty cycle limitation is tripped. Vcc will then drop and stop IC activity as it falls
below the UVLO threshold (8.7V typ.).
The Vcc_OK signal is de-asserted as the Vcc voltage goes below a threshold Vccrestart located at about
5V. The HV generator can now restart but, if Vin < Vinstart, as shown in figure 36, HV_EN is de-asserted
too and the HV generator is disabled.
This prevents converter's re-start attempts and ensures monotonic output voltage decay at power-down.
The low restart threshold Vccrestart ensures that, during short circuits, the restart attempts of the L6668 will
have a very low repetition rate, as shown in the timing diagram of figure 37, and that the converter will
work safely with extremely low power throughput.
4.2 Frequency Foldback Block and operation at medium/light load
At heavy load, namely as the voltage on pin COMP (VCOMP) is higher than 3V, the device works at a fixed
frequency like a standard current mode PWM controller.
As the load is reduced, and the VCOMP voltage falls below 3V (approximately corresponding to 50% of the
maximum load in a fully DCM system), the oscillator frequency can be made dependent on converter's
load conditions - the lower the load, the lower the frequency and vice versa.
Figure 38. Frequency foldback function: oscillator frequency is a function of COMP voltage
fosc
L6668
RSTBY
13 STBY
8
COMP
Vref
+
RT
RSTBY
16
3.0V
OSCILLATOR
RCT
CT
VCOMP
1.4
3.0
4.4
This is done by adding an external resistor RSTBY between pins RCT (#16) and STBY (#13), which activates the circuit shown in figure 38.
When VCOMP is below 3 V (oscillator's peak voltage) the voltage on the STBY pin, which is internally connected to a current sink, tracks VCOMP and then some of the current that charges CT is diverted to ground
through the STBY pin.
In this way the rate of rise of the voltage across CT is slowed down and the oscillator frequency decreased,
the lower VCOMP the lower the frequency. Instead, when VCOMP is greater than 3 V the STBY pin features
high impedance and the oscillator frequency fosc will be determined by RT and CT. These components can
be then calculated as it is usually done with this type of oscillator:
1.4
R T C T = --------f osc
14/23
L6668
Figure 39. Standby function: frequency shift vs. timing resistors (normalized quantities)
1
0.9
0.8
0.7
f min
f osc
0.6
0.5
V(SKIPADJ) = 1.6V
0.4
0.3
0.2
0.
2.5V
V(SKIPADJ) = 1.5V
2.2V
2V
V(SKIPADJ) = 1.4V
1.8V
1
10
RSTBY
RT
The determination of RSTBY can be done assuming that the minimum switching frequency before burstmode operation takes place (fmin) is specified. This value will be above the audible range to ensure a
noise-free operation. With the aid of the diagrams in figure 39, which show the relationship between the
frequency shift obtained and the ratio of RSTBY to RT for different values of the burst-mode threshold, it is
possible to determine RSTBY. Draw an horizontal line corresponding to the desired fmin/fosc ratio as long as
it intercepts the characteristic corresponding to the voltage set at the pin SKIPADJ (#9). From there, draw
a vertical line: on the horizontal axis it is possible to read the required RSTBY/RT ratio.
Note that the characteristic for V(SKIPADJ)=1.4V corresponds to the burst-mode operation not used (see
next section). Note also that, for a given V(SKIPADJ), there is both a lower limit to the RSTBY/RT ratio and
a maximum frequency shift allowed. Not observing these limits will result in erratic behavior.
In applications where the switching frequency needs not be tightly fixed for some specific reason there is
no major drawback to this technique. In case this function is not desired, the STBY pin shall be left open.
4.3 Operation at no load or very light load
When the PWM control voltage at pin COMP falls about 50 mV below a threshold externally programmable via pin 9 (SKIPADJ), the IC is disabled with the MOSFET kept in OFF state and its consumption reduced at a very low value to minimize Vcc capacitor discharge. The soft-start capacitor is not discharged.
The control voltage now will increase as a result of the feedback reaction to the energy delivery stop, the
threshold will be exceeded and the IC will restart switching again. In this way the converter will work in
burst-mode with a constant peak current defined by the disable level applied at pin 9. A load decrease will
then cause a frequency reduction, which can go down even to few hundred hertz, thus minimizing all frequency-related losses and maximizing efficiency. This kind of operation is noise-free provided the peak
current, which is user-defined by the bias voltage at pin 9, is very low.
The timing diagram of figure 40 illustrates this kind of operation along with the other ones, showing the
most significant signals.
15/23
L6668
Figure 40. Load-dependent operating modes: timing diagram
COMP
3.0V
50 mV
hyster.
V(SKIPADJ)
t
fosc
t
OUT
Fix. Freq.
Mode
Burst-mode
Fix. Freq.
Mode
t
Frequency Foldback Mode
The operating range of the voltage V(SKIPADJ) is practically limited upwards by the onset of audible
noise: typically, with a voltage above 2-2.1V some noise can be heard under some line/load conditions. If,
instead, V(SKIPADJ) is set below the low saturation value of the PWM control voltage (1.4V typ.) burstmode operation will never take place. Always bias the pin at some voltage, a floating pin will result in
anomalous behavior. The SKIPADJ pin doubles its function: if the voltage is pulled below 0.8V the IC is
disabled completely, except for the externally available reference voltage VREF, and its quiescent consumption reduced. The soft-start capacitor is discharged so that, when the voltage on the SKIPADJ pin is
pulled above 0.8V, the chip is soft-started just like exiting from UVLO. This function is useful for some kind
of remote ON/OFF control. The comparator referenced to 0.8V does not have hysteresis; hence make
sure that the voltage on the pin does not linger on the threshold to prevent uncertain behavior.
4.4 PWM control Block
The device is specific for secondary feedback. Typically, there is a TL431 at the secondary side and an
optocoupler that transfers output voltage information to the PWM control at the primary side, crossing the
isolation barrier. The PWM control input (pin #10, COMP) is driven directly by the phototransistor's collector (the emitter is grounded to GND) to modulate the duty cycle.
4.5 Current Comparator, PWM Latch and Hiccup-mode OCP
The current comparator senses the voltage across the current sense resistor (Rs) on pin 12 (ISEN) and,
by comparing it with the programming signal derived form the control voltage on pin 10 (COMP), determines the exact time when the external MOSFET is to be switched off. The PWM latch avoids spurious
switching of the MOSFET, which might result from the noise generated ("double-pulse suppression").
16/23
L6668
Figure 41. Hiccup-mode OCP: timing diagram
Secondary diode is shorted here
Vcc
Vcc ON
Vcc OFF
re
V CS
OUT
1.5 V
t
t
OCP latch
t
Vcc_OK
t
t
A second comparator senses the voltage on the current sense input and shuts the IC down if the voltage
at the pin exceeds 1.5 V. Such an anomalous condition is typically generated by either a short circuit of
the secondary rectifier or a shorted secondary winding or a saturated flyback transformer.
This condition is latched as long as the IC is supplied. When the IC is disabled, however, no energy is
coming from the self-supply circuit, then the voltage on the Vcc capacitor will decay and cross the UVLO
threshold after some time, which clears the latch. The internal start-up generator is still off, then the Vcc
voltage still needs to go below its restart voltage before the Vcc capacitor is charged again and the IC restarted.
Ultimately, either of the above mentioned failures will result in a low-frequency intermittent operation (Hiccup-mode operation), with very low stress on the power circuit. The timing diagram of figure 41 illustrates
this operation.
4.6 Power Management
The L6668 is specifically designed to minimize converter's losses under light or no-load conditions, and a
special function has been provided to help the designer meet energy saving requirements even in powerfactor-corrected systems where a PFC pre-regulator precedes the DC-DC converter.
Actually EMC regulations require compliance with low-frequency harmonic emission limits at nominal
load, no limit is envisaged when the converter operates with a light load. Then the PFC pre-regulator can
be turned off, thus saving the no-load consumption of this stage (0.5 to 1W).
To do so, the device provides the PFC_STOP (#14) pin: it is an open collector output, normally low, that
becomes open when the voltage VCOMP falls below 2.2V.
This signal will be externally used for switching off the PFC controller and the pre-regulator as shown in
figure 42. To prevent intermittent operation of the PFC stage, 0.5V hysteresis is provided: the PFC_STOP
pin is re-asserted low (which will re-enable the PFC pre-regulator) when VCOMP exceeds 2.7 V.
A capacitor (and a limiting resistor in the hundred ohms), shown in dotted lines, may be used if one wants
to delay PFC turn-off
When the L6668 is in UVLO (Vcc<8.7V) the pin is kept high so as to ensure that the PFC pre-regulator
will start up only after the DC-DC converter governed by the L6668 is activated.
17/23
L6668
Figure 43 shows a timing diagram where the PFC_STOP function operation is illustrated under different
operating conditions.
Figure 42. How the L6668 can switch off a PFC controller at light load
BC557
Vcc
Vcc
16
10 kΩ
L6668
14
8.2 V
PFC_STOP
PFC
controller
2.2 kΩ
Figure 43. Operation of PFC_STOP function
Vcc
Vcc ON
Vcc OFF
PFC_STOP
t
COMP
t
2.7V
2.2V
t
OUT
t
PFC
Gate
Drive
Start-up
Full-load
Light-load
Full-load
t
Figure 44. Operation after DIS pin activation: timing diagram
DIS
2.2V
t
Vcc
VccON
VccON -0.5
VccOFF
HV generator is turned on
Disable latch is reset here
Vccrest
OUT
HV generator is disabled here
t
Input source is removed here
Vin
t
VHVstart
t
18/23
L6668
4.7 Disable function
Latched OTP or OVP functions can be easily realized with the L6668: the IC is equipped with a comparator whose non-inverting input is externally available on pin #7 (DIS), and whose inverting input is internally referenced to 2.2V.
As the voltage on the pin exceeds the threshold the IC is immediately shut down and its consumption reduced at a low value. The information is latched and it is necessary to let the voltage on the Vcc pin go
below the UVLO threshold to reset the latch and restart the IC.
To keep the latch supplied as long as the converter is connected to the input source, the HV generator is
activated periodically so that Vcc oscillates between the start-up threshold VccON and VccON - 0.5V. It is
then necessary to disconnect the converter from the input source to restart the IC. This operation is shown
in the timing diagram of figure 44. Activating the HV generator in this way cuts its power dissipation approximately by three and keeps peak silicon temperature close to the average value.
4.8 Slope compensation
A pin of the device (#15, S-COMP) provides a voltage ramp during MOSFET's ON-time which is a repetition of the oscillator sawtooth, buffered (0.8 mA min. capability) and level shifted down by one Vbe.
This ramp is intended for implementing additive slope compensation on current sense. This is needed to
avoid the sub-harmonic oscillation that arises in all peak-current-mode-controlled converters working in
continuous conduction mode with a duty cycle close to or exceeding 50%.
Figure 45. Slope compensation waveforms
RCT
OUT
t
S-COMP
t
t
The compensation will be realized by connecting a programming resistor between this pin and the current
sense input (pin 12, ISEN). The pin has to be connected to the sense resistor with another resistor to make
a summing node on the pin.
Since no ramp is delivered during MOSFET OFF-time (see figure 45), no external component other than
the programming resistor is needed to ensure a clean operation at light loads. If slope compensation is
not required the pin shall be left floating.
19/23
L6668
Figure 46. Typical Application: 80W, WRM flyback; Electrical Schematic
F1 T2A250V
NTC1
BD1
DF04M
J1
88 to264
Vac
T1
C1
100 µF
400 V
R1
3 MΩ
D1
1.5KE200
R2
3 MΩ
D2
N1
R5 47kΩ
R3 10Ω
18V/4.5A
C9
100 nF
C8A,B,C
680 µF
25 V
N2
J2
STTA106
C3 100 nF
R4
330 kΩ
D4
STPS20150CT
D3 1N4148
C2
25 V
47 µF
GND
C12
2.2 nF
Y1
N3
R6
9.1 kΩ
VREF S-COMP
STBY
8
VCC
DIS
15
7
HV
5
1
OUT
Q1
R710kΩ
STP9NK65
4
R14
4.3 kΩ
13
R8
4.03 kΩ
ISEN
L6668
12
R13A,B
0.56
1/2
16
R9
6.2 kΩ
RCT
6
N.C.
11
9
SS
SKIPADJ
10
3
COMP
R17
8.06 kΩ
R12 1kΩ
OC1
PC817A
R15
1.2 kΩ
GND
C10
100 nF
TL431
R10
82.5 kΩ
C4
100 nF
C5
3.3 nF
OC1
PC817A
C6
56 nF
R11
47 kΩ
R16
47 kΩ
1
3
C7
2.2 nF
2
R18
1.3 kΩ
Table 6. Light load measurements on the circuit of figure 46
Output power
Test condition
Input power
Pout = 0.5 W
Vin= 110 Vac
0.71 W
Vin= 230 Vac
0.86 W
Vin= 110 Vac
0.09 W
Vin= 230 Vac
0.17 W
Pout = 0 W
5
Package information
In order to meet environmental requirements, STMicroelectronics offers this device in ECOPACK® package. This package has a Lead-free second level interconnect. The category of second level Interconnect
is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97.
The maximum ratings related to soldering conditions are also marked on the inner box label.
ECOPACK is an STMicroelectronics trademark. ECOPACK specifications are available at: www.st.com.
20/23
L6668
Figure 47. SO16 (Narrow) Mechanical Data & Package Dimensions
mm
inch
DIM.
MIN.
TYP.
A
a1
MAX.
MIN.
TYP.
1.75
0.1
0.25
a2
MAX.
0.069
0.004
0.009
1.6
0.063
b
0.35
0.46
0.014
0.018
b1
0.19
0.25
0.007
0.010
45°
(typ.)
10
0.386
6.2
0.228
C
0.5
c1
D(1)
9.8
E
5.8
0.020
0.394
0.244
e
1.27
0.050
e3
8.89
0.350
F(1)
3.8
4.0
0.150
0.157
G
4.60
5.30
0.181
0.208
L
0.4
1.27
0.150
0.050
M
S
OUTLINE AND
MECHANICAL DATA
0.62
0.024
8 ° (max.)
SO16 (Narrow)
(1) "D" and "F" do not include mold flash or protrusions - Mold
flash or protrusions shall not exceed 0.15mm (.006inc.)
0016020 D
21/23
L6668
6
Revision History
Table 7. Revision History
22/23
Date
Revision
May 2005
1
Description of Changes
First Issue.
L6668
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics.
All other names are the property of their respective owners
© 2005 STMicroelectronics - All rights reserved
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23/23