LT1508
Power Factor and PWM
Controller (Voltage Mode)
FEATURES
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DESCRIPTION
The LT
®
1508 is a complete solution for universal off-line
switching power supplies utilizing active power factor
correction. The PFC section is identical to the LT1248 PFC
controller except the EN/SYNC pin is removed because
PFC and PWM are synchronized internally.
The voltage mode PWM section (LT1509 is the current
mode counterpart) contains all the primary side functions
to convert the PFC preregulated high voltage output to an
isolated low voltage output. The PWM duty cycle is
internally limited to 47% (maximum 50%) to prevent
transformer saturation. PWM soft start begins when the
PFC output reaches the preset voltage. In the event of brief
line loss, PWM will be shut off when the PFC output
voltage drops below 73% of the preset value.
, LTC and LT are registered trademarks of Linear Technology Corporation.
PFC and PWM Single Chip Solution
Synchronized Operation up to 300kHz
99% Power Factor Over 20:1 Load Current Range
Voltage Mode PWM
Instantaneous Overvoltage Protection
Dedicated Overvoltage Protection (OVP Pin)
Minimal Line Current Dead Zone
Typical 250µA Start-Up Supply Current
Line Switching Noise Filter
Low Quiescent Current: 13mA
Fast 1.5A Peak Current Gate Drivers
Separate Soft Start Control
APPLICATIONS
s
Universal Power Factor Corrected Power Supplies
and Preregulators
BLOCK DIAGRAM
V
CC
16V TO 10V
VA
OUT
10
V
REF
7.5V
V
REF
RUN
12
M
OUT
I
SENSE
8
7
CA
OUT
6
+
–
2.2V
7µA
+
M1
V
SENSE
14
7.5V
–
–
EA
–
I
M
CA
I
A
25k
I
B
I
M
=
I
A2
I
B
200µA
2
–
+
RUN
+
I
AC
9
+
+
0.7V
R
2R
7.9V
OVP
11
SS1
16
14µA
+
–
–
OSC
16V
GND2
2
+
1V
–
CL
4 C
SET
R
SET
15
55%
DELAY
S
–
14µA
SS2
13
PWMOK
–
7V TO
4.7V
+
+
–
+
NOTE: PWM PULSE IS DELAYED BY 55% DUTY CYCLE
AFTER PFC PULSE
19 I
LIM
V
C
18
50µA
U
W
U
GND1
3
PK
LIM
V
CC
5
17
–
+
R
R
S
Q
GTDR1
1
R
R
Q
GTDR2
20
200ns
BLANKING
16V
1508 BD
1
LT1508
DESCRIPTION
By using fixed high frequency PWM current averaging
without the need for slope compensation, the LT1508
achieves far lower line current distortion with a smaller
magnetic element than systems that use either peak
current detection, or zero current switching approach, in
both continuous and discontinuous modes of operation.
The LT1508 also provides filtering capability to reject line
switching noise which can cause instability when fed into
the multiplier. Line current dead zone is minimized with
low bias voltage at the current input to the multiplier. The
LT1508 provides many protection features including peak
current limiting and overvoltage protection. Implemented
with a very high speed process, the LT1508 can be
operated at frequencies as high as 300kHz.
ABSOLUTE
MAXIMUM
RATINGS
Supply Voltage ........................................................ 27V
GTDR Current Continuous ...................................... 0.5A
GTDR Output Energy ................................................ 5µJ
I
AC
, R
SET
, PK
LIM
Input Current .............................. 20mA
V
SENSE
, OVP Input Voltage .................................... V
MAX
I
LIM
, V
C
Input Voltage ................................................ 8V
I
SENSE
, M
OUT
Input Current ...................................
±5mA
Operating Junction Temperature Range
LT1508C ................................................ 0°C to 100°C
LT1508I ............................................ – 40°C to 125°C
Thermal Resistance (Junction-to-Ambient)
N Package ................................................... 100°C/W
SW Package ................................................ 120°C/W
PACKAGE/ORDER INFORMATION
TOP VIEW
GTDR1 1
GND2 2
GND1 3
C
SET
4
PK
LIM
5
CA
OUT
6
I
SENSE
7
M
OUT
8
I
AC
9
VA
OUT
10
N PACKAGE
20-LEAD PDIP
20 GTDR2
19 I
LIM
18 V
C
17 V
CC
16 SS1
15 R
SET
14 V
SENSE
13 SS2
12 V
REF
11 OVP
SW PACKAGE
20-LEAD PLASTIC SO WIDE
ORDER PART
NUMBER
LT1508CN
LT1508CSW
LT1508IN
LT1508ISW
T
JMAX
= 125°C,
θ
JA
= 100°C/ W (N)
T
JMAX
= 125°C,
θ
JA
= 120°C/ W (SW)
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
Maximum operating voltage (V
MAX
) = 25V, V
CC
= 18V, R
SET
= 15k to GND, C
SET
= 1nF to GND, I
AC
= 100µA, I
SENSE
= 0V, CA
OUT
= 3.5V,
VA
OUT
= 5V, OVP = V
REF
. No load on any outputs unless otherwise noted.
PARAMETER
Overall
Supply Current (V
CC
in Undervoltage Lockout)
Supply Current On
V
CC
Turn-On Threshold (Undervoltage Lockout)
V
CC
Turn-Off Threshold
Voltage Amplifier (PFC Section)
Voltage Amp Offset
Input Bias Current
Voltage Gain
Voltage Amp Unity-Gain Bandwidth
Voltage Amp Output High (Internally Clamped)
Voltage Amp Output Low
Voltage Amp Short-Circuit Current
CONDITIONS
V
CC
= Lockout Voltage – 0.2V
11.5V
≤
V
CC
≤
V
MAX
q
q
q
q
MIN
TYP
0.25
13
16.5
10.5
MAX
0.45
19
17.5
11.5
10
– 250
UNITS
mA
mA
V
V
mV
nA
dB
MHz
V
V
mA
15.5
9.5
– 10
70
VA
OUT
= 3.5V
V
SENSE
= 0V to 7V
q
q
q
q
11.3
3
VA
OUT
= 0V
q
– 25
100
3
13.3
1.1
8
2
17
2
U
W
U
U
W W
U
W
LT1508
ELECTRICAL CHARACTERISTICS
Maximum operating voltage (V
MAX
) = 25V, V
CC
= 18V, R
SET
= 15k to GND, C
SET
= 1nF to GND, I
AC
= 100µA, I
SENSE
= 0V, CA
OUT
= 3.5V,
VA
OUT
= 5V, OVP = V
REF
. No load on any outputs unless otherwise noted.
PARAMETER
Current Amplifier (PFC Section)
Current Amp Offset Voltage
I
SENSE
Bias Current
Current Amp Voltage Gain
Current Amp Unity-Gain Bandwidth
Current Amp Output High
Current Amp Output Low
Current Amp Short-Circuit Current
Input Range, I
SENSE
, M
OUT
(Linear Operation)
Reference
Reference Output Voltage
V
REF
Load Regulation
V
REF
Line Regulation
V
REF
Short-Circuit Current
V
REF
Worst Case
Current Limit
PK
LIM
Offset Voltage
PK
LIM
Input Current
PK
LIM
to GTDR Propagation Delay
Multiplier
Multiplier Output Current
Multiplier Output Current Offset
Multiplier Maximum Output Current
Multiplier Gain Constant (Note 1)
I
AC
Input Resistance
Oscillator
Oscillator Frequency
C
SET
Ramp Peak-to-Peak Amplitude
C
SET
Ramp Valley Voltage
Overvoltage Comparator (PFC Section)
Comparator Trip Voltage Ratio (V
TRIP
/ V
REF
)
Hysteresis
OVP Bias Current
OVP Propagation Delay
Gate Drivers (GTDR1 and GTDR2)
Max Output Voltage
Output High
Output Low (Device Unpowered)
Output Low (Device Active)
Peak Output Current
Rise and Fall Time
Max Duty Cycle (PFC)
Max Duty Cycle (PWM) (Note 2)
CONDITIONS
q
q
MIN
TYP
±1
– 25
110
3
8.5
1.1
8
MAX
±4
– 250
UNITS
mV
nA
dB
MHz
V
V
mA
V
V
mV
mV
mA
V
mV
µA
ns
µA
µA
µA
V
–2
kΩ
kHz
kHz
V
V
80
q
q
7.2
3
– 0.3
7.39
CA
OUT
= 0V
q
q
2
17
1
7.60
20
50
7.68
25
– 100
I
REF
= 0mA, T
A
= 25°C
– 5mA < I
REF
< 0mA
11.5V < V
CC
< V
MAX
V
REF
= 0V
Load, Line, Temperature
q
q
q
q
– 20
12
7.32
– 25
7.50
5
5
28
7.5
PK
LIM
= – 0.1V
PK
LIM
Falling from 50mV to – 50mV
I
AC
= 100µA, R
SET
= 15k
R
AC
= 1M from I
AC
to GND
I
AC
= 450µA, R
SET
= 15k, VA
OUT
= 7V, M
OUT
= 0V
I
AC
from 50µA to 1mA
R
SET
= 15k, C
SET
= 1000pF
R
SET
= 15k, C
SET
= 1500pF
q
– 50
400
35
– 0.05
– 260
0.035
25
100
68
4.7
1.3
1.05
0.35
0.2
100
15
0.9
0.5
0.2
2
25
96
q
q
– 286
15
– 0.5
– 235
35
115
78
5.0
1.55
1.06
1
q
q
85
58
4.35
1.15
1.04
q
OVP = 7.5V
q
V
µA
ns
V
V
V
V
V
A
ns
%
%
0mA Load, 18V < V
CC
– 200mA Load, 11.5V
≤
V
CC
≤
15V
V
CC
= 0V, 50mA Load (Sinking)
200mA Load (Sinking)
10mA Load
10nF from GTDR to GND
1nF from GTDR to GND
q
q
q
q
q
12
V
CC
– 3.0
17.5
1.5
1
0.4
90
44
50
3
LT1508
ELECTRICAL CHARACTERISTICS
V
CC
= 18V, R
SET
= 15k to GND, C
SET
= 1nF to GND, I
AC
= 100µA, I
SENSE
= 0V, CA
OUT
= 3.5V, VA
OUT
= 5V, OVP = V
REF
. No load on any
outputs, unless otherwise noted.
PARAMETER
Soft Start Current
SS1 Current (PFC)
SS2 Current (PWM)
Comparators in PWM Section
I
LIM
Input Current
Current Limit Comparator (CL) Threshold
GTDR2 Switching Off Threshold at V
C
or at SS2
V
C
Input Current
PWMOK Comparator Low Threshold (in Terms of V
REF
)
V
C
Pin High Voltage
GTDR2 Turn-On Blanking Time
CONDITIONS
SS1 = 2.5V
SS2 = 1V
I
LIM
= 0V, V
C
= 1.6V
V
C
> 2.6V
I
LIM
= 0V
V
C
= 0V
1mA into V
C
Pin
q
q
q
q
q
q
q
q
MIN
5
5
TYP
12
12
– 0.3
1.1
MAX
30
30
–2
1.20
– 80
0.70
7.5
UNITS
µA
µA
µA
V
V
µA
V
ns
0.95
1
– 20
0.57
6.2
0.63
6.9
180
The
q
denotes specifications which apply over the full operating
temperature range.
I
M
Note 1:
Multiplier Gain Constant: K =
I
AC
(VA
OUT
– 2)
2
Note 2:
GTDR2 (PWM) pulse is delayed by 53% duty cycle after GTDR1
(PFC) is set. See PFC/PWM Synchronization graph in the Typical
Performance Characteristics section.
TYPICAL PERFORMANCE CHARACTERISTICS
PFC Voltage Amplifier Open-Loop
Gain and Phase
100
80
GAIN
60
GAIN (dB)
GAIN (dB)
40
20
0
–20
PHASE
10
100
1k
10k 100k
FREQUENCY (Hz)
4
U W
1M
1508 G01
PFC Current Amplifier Open-Loop
Gain and Phase
0
–20
–40
–60
–80
–100
100
80
GAIN
60
40
20
0
–20
–40
–60
–80
PHASE (DEG)
PHASE (DEG)
PFC/PWM Synchronization
0
–20
PFC (GTDR1)
53%
PHASE
PWM (GTDR2)
–100
–120
10M
1508 G02
–120
10M
10
100
1k
10k 100k
FREQUENCY (Hz)
1M
TIME
1508 G03
LT1508
TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage
vs Temperature
7.536
7.524
REFERENCE VOLTAGE (V)
7.512
7.500
7.488
7.476
7.464
7.452
7.440
7.428
–75 –50 –25 0 25 50 75 100 125 150
JUNCTION TEMPERATURE (°C)
1508 G04
I
M
(µA)
Supply Current vs Supply Voltage
16
15
14
T
J
= –55°C
SUPPLY CURRENT (mA)
13
TIME (ns)
SUPPLY CURRENT (µA)
12
11
10
9
8
7
6
5
10
T
J
= 125°C
T
J
= 25°C
21
SUPPLY VOLTAGE (V)
Frequency vs R
SET
and C
SET
500
450
400
R
SET
= 10k
R
SET
= 15k
R
SET
= 20k
R
SET
= 30k
1.00
0.99
0.98
MAXIMUM DUTY CYCLE
FREQUENCY (kHz)
350
300
250
200
150
100
50
0
200
600
0.97
0.96
0.95
0.94
0.93
0.92
0.91
R
SET
= 10k
R
SET
= 15k
R
SET
= 20k
R
SET
= 30k
600
1800
1400
C
SET
CAPACITANCE (pF)
1000
2200
1508 G10
M
OUT
CURRENT (mA)
1800
1400
C
SET
CAPACITANCE (pF)
1000
U W
1508 G06
1508 G09
Multiplier Current
300
VA
OUT
= 7V
VA
OUT
= 6.5V
VA
OUT
= 6V
150
VA
OUT
= 5V
VA
OUT
= 4.5V
VA
OUT
= 4V
VA
OUT
= 3.5V
0
VA
OUT
= 3V
VA
OUT
= 2.5V
500
1508 G05
VA
OUT
= 5.5V
0
250
I
AC
(µA)
GTDR Rise and Fall Time
400
550
500
450
Start-Up Supply Current
vs Supply Voltage
300
400
350
300
250
200
150
100
50
0
–55°C
25°C
125°C
FALL TIME
200
RISE TIME
100
NOTE: GTDR SLEWS
BETWEEN 1V AND 16V
32
0
0
10
20
30
40
LOAD CAPACITANCE (nF)
50
1508 G07
0
2
4
6 8 10 12 14 16 18 20
SUPPLY VOLTAGE (V)
1508 G08
GTDR1 Maximum Duty Cycle
vs R
SET
and C
SET
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
M
OUT
Pin Characteristics
T
J
= 125°C
T
J
= 25°C
T
J
= –55°C
2200
0.90
200
–2.4
–1.2
1.2
0
M
OUT
VOLTAGE (V)
2.4
1508 G11
5
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