LT1246/LT1247
1MHz Off-Line
Current Mode PWM
and DC/DC Converter
DESCRIPTIO
The LT
®
1246/LT1247 are 8-pin, fixed frequency, current
mode, pulse width modulators. These devices are de-
signed to be improved plug compatible versions of the
industry standard UC1842 PWM circuit. The LT1246/
LT1247 are optimized for off-line and DC/DC converter
applications. They contain a temperature compensated
reference, high gain error amplifier, current sensing com-
parator, and a high current totem pole output stage ideally
suited to driving power MOSFETs. Start-up current has
been reduced to less than 250µA. Cross-conduction cur-
rent spikes in the totem pole output stage have been
eliminated, making 1MHz operation practical. Several new
features have been incorporated. Leading edge blanking
has been added to the current sense comparator. This
minimizes or eliminates the filter that is normally required.
Eliminating this filter allows the current sense loop to
operate with minimum delays. Trims have been added to
the oscillator circuit for both frequency and sink current,
and both of these parameters are tightly specified. The
output stage is clamped to a maximum V
OUT
of 18V in the
on state. The output and the reference output are actively
pulled low during under-voltage lockout.
, LTC and LT are registered trademarks of Linear Technology Corporation.
FEATURES
s
s
s
s
s
s
s
s
s
s
s
s
Current Mode Operation to 1MHz
30ns Current Sense Delay
< 250µA Low Start-Up Current
Current Sense Leading Edge Blanking
Pin Compatible with UC1842
Undervoltage Lockout with Hysteresis
No Cross-Conduction Current
Trimmed Bandgap Reference
1A Totem Pole Output
Trimmed Oscillator Frequency and Sink Current
Active Pull-Down on Reference and Output During
Undervoltage Lockout
18V High Level Output Clamp
APPLICATI
s
s
S
Off-Line Converters
DC/DC Converters
Start-Up
Threshold
16V
8.4V
Minimum
Operating
Voltage
10V
7.6V
Maximum
Duty Cycle
100%
100%
Device
LT1246
LT1247
Replaces
UC1842
UC1843
BLOCK DIAGRA
REFERENCE ENABLE
5V REF
MAIN BIAS
REFERENCE PULLDOWN
OUTPUT
PULLDOWN
R
T
/C
T
COMPENSATION
4
1
1mA
5.6V
OSCILLATOR
S
R
1V
2R
R
5
GND
6
OUTPUT
FEEDBACK
2
2.5V
–
+
–
+
BLANKING
I
SENSE
3
U
W
UO
UV
LOCKOUT
8
7
V
REF
V
CC
18V
+
1.5V
–
LT1246 • BD01
1
LT1246/LT1247
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
COMP 1
FB 2
I
SENSE
3
R
T
/C
T
4
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 100°C,
θ
JA
= 130°C/W (N8)
T
JMAX
= 100°C,
θ
JA
= 150°C/W (S8)
Supply Voltage ....................................................... 25V
Output Current ......................................................
±1A*
Output Energy (Capacitive Load per Cycle) ............. 5µJ
Analog Inputs (Pins 2, 3) .............................. – 0.3 to 6V
Error Amplifier Output Sink Current ..................... 10mA
Power Dissipation at T
A
≤
25°C ............................... 1W
Operating Junction Temperature Range
LT1246C/LT1247C ............................. 0°C to 100°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
*The 1A rating for output current is based on transient switching
requirements.
ORDER PART
NUMBER
8
7
6
5
V
REF
V
CC
OUTPUT
GND
LT1246CN8
LT1246CS8
LT1247CN8
LT1247CS8
S8 PART MARKING
1246
1247
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
(Notes 1, 2)
PARAMETER
Reference Section
Output Voltage
Line Regulation
Load Regulation
Temperature Stability
Total Output Variation
Output Noise Voltage
Long-Term Stability
Output Short-Circuit Current
Oscillator Section
Initial Accuracy
Voltage Stability
Temperature Stability
Amplitude
Clock Ramp Reset Current
Error Amplifier Section
Feedback Pin Input Voltage
Input Bias Current
Open-Loop Voltage Gain
Unity-Gain Bandwidth
Power Supply Rejection Ratio
Output Sink Current
Output Source Current
V
PIN 1
= 2.5V
V
FB
= 2.5V
2 < V
O
< 4V
T
J
= 25°C
12V < V
CC
< 25V
V
PIN 2
= 2.7V, V
PIN 1
= 1.1V
V
PIN 2
= 2.3V, V
PIN 1
= 5V
q
q
q
q
q
q
CONDITIONS
I
O
= 1mA, T
J
= 25°C
12V < V
CC
< 25V
1mA < I
REF
< 20mA
Line, Load, Temperature
10Hz < F < 10kHz, T
J
= 25°C
T
A
= 125°C, 1000 Hrs.
q
q
q
MIN
4.925
TYP
5.000
3
–6
0.1
MAX
5.075
20
– 25
5.13
UNITS
V
mV
mV
mV/°C
V
µV
mV
mA
kHz
kHz
%
%/°C
V
q
4.87
50
5
– 30
47.5
465
– 90
50
500
– 0.05
1.7
7.9
2.42
65
1
60
2
– 0.5
6
– 0.75
8.2
2.50
90
2
25
– 180
52.5
535
1
R
T
= 10k, C
T
= 3.3nF, T
J
= 25°C
R
T
= 6.2k, C
T
= 500pF, T
J
= 25°C
12V < V
CC
< 25V, T
J
= 25°C
T
MIN
< T
J
< T
MAX
Pin 4
V
OSC
(Pin 4) = 2V, T
J
= 25°C
8.5
2.58
–2
2
U
mA
V
µA
dB
MHz
dB
mA
mA
W
U
U
W W
W
LT1246/LT1247
ELECTRICAL CHARACTERISTICS
PARAMETER
Error Amplifier Section
Output Voltage High Level
Output Voltage Low Level
Current Sense Section
Gain
Maximum Current Sense Input Threshold
Power Supply Rejection Ratio
Input Bias Current
Delay to Output
Blanking Time
Blanking Override Voltage
Output Section
Output Low Level
Output High Level
Rise Time
Fall Time
Output Clamp Voltage
Undervoltage Lockout
Start-Up Threshold
Minimum Operating Voltage
Hysteresis
PWM
Maximum Duty Cycle
Minimum Duty Cycle
Total Device
Start-Up Current
Operating Current
The
q
denotes those specifications which apply over the full operating
temperature range.
Note 1:
Unless otherwise specified, V
CC
= 15V, R
T
= 10k, C
T
= 3.3nF.
q
q
q
q
(Notes 1, 2)
MIN
q
q
CONDITIONS
V
PIN 2
= 2.3V, R
L
= 15k to GND
V
PIN 2
= 2.7V, R
L
= 15k to Pin 8
TYP
5.6
0.2
MAX
UNITS
V
5
1.1
3.15
1.10
–10
V
V/V
V
dB
µA
ns
ns
V
2.85
0.90
3.00
1.00
70
–1
30
60
1.5
V
PIN 3
< 1.1V
q
I
OUT
= 20mA
I
OUT
= 200mA
I
OUT
= 20mA
I
OUT
= 200mA
C
L
= 1nF, T
J
= 25°C
C
L
= 1nF, T
J
= 25°C
I
O
= 1mA
LT1246
LT1247
LT1246
LT1247
LT1246
LT1247
T
J
= 25°C
T
J
= 25°C
q
q
q
q
0.25
0.75
12.0
11.75
30
20
0.4
2.2
V
V
V
V
70
60
19
17
9.0
11
8.2
ns
ns
V
V
V
V
V
V
V
q
18
15
7.8
9.0
7.0
5.5
0.4
94
0
170
13
16
8.4
10
7.6
6.0
0.8
q
q
q
q
q
q
100
%
%
250
20
µA
mA
Note 2:
Low duty cycle pulse techniques are used during test to maintain
junction temperature close to ambient.
3
LT1246/LT1247
TYPICAL PERFOR A CE CHARACTERISTICS
LT1246 Undervoltage Lockout
17
START-UP THRESHOLD
11
START-UP CURRENT (µA)
16
V
CC
(V)
V
CC
(V)
15
11
MINIMUM OPERATING VOLTAGE
10
9
–50 –25
0
25
50
75
TEMPERATURE (°C)
LT1246 • TPC01
Start-Up Current
200
START-UP
THRESHOLD
START-UP CURRENT (µA)
150
FREQUENCY CHANGE (%)
I
CC
(mA)
LT1247
100
LT1246
50
T
J
= 25°C
0
0
2
4
6
8
10
12
14 16
18
V
CC
(V)
1246/7 TPC04
Oscillator Sink Current
OSCILLATOR SINK CURRENT (mA)
8.6
8.5
8.4
8.3
8.2
8.1
8.0
7.9
7.8
V
PIN 4
= 2V
REFERENCE SHORT-CIRCUIT CURRENT (mA)
8.7
REFERENCE VOLTAGE (V)
7.7
–50 –25
0
25
50
75
TEMPERATURE (°C)
LT1246 • TPC07
4
U W
100
100
LT1247 Undervoltage Lockout
200
180
10
Start-Up Current
160
140
120
100
80
60
40
20
9
START-UP THRESHOLD
8
MINIMUM OPERATING VOLTAGE
7
125
6
–50 –25
0
25
50
75
100
125
0
–50 –25
0
25
50
75
100
125
TEMPERATURE (°C)
1247 TPC02
TEMPERATURE (°C)
LT1246 • TPC03
Supply Current
15
Oscillator Frequency
10
8
V
CC
= 15V
14
6
4
2
0
–2
–4
–6
–8
13
12
V
CC
= 15V
R
T
= 10k
C
T
= 3300pF
0
25
50
75
100
125
11
10
–50 –25
–10
–50 –25
0
25
50
75
100
125
TEMPERATURE (°C)
LT1246 • TPC05
TEMPERATURE (°C)
LT1246 • TPC06
Reference Short-Circuit Current
140
120
100
80
60
40
20
–50 –25
5.05
5.04
5.03
5.02
5.01
5.00
4.99
4.98
4.97
4.96
0
25
50
75
100
125
Reference Voltage
I
O
= 1mA
125
4.95
–50 –25
0
25
50
75
100
125
TEMPERATURE (°C)
LT1246 • TPC08
TEMPERATURE (°C)
LT1246 • TPC09
LT1246/LT1247
TYPICAL PERFOR A CE CHARACTERISTICS
Feedback Pin Input Voltage
2.55
CURRENT SENSE CLAMP VOLTAGE (V)
FEEDBACK PIN INPUT VOLTAGE (V)
2.53
2.52
2.51
2.50
2.49
2.48
2.47
2.46
2.45
–50 –25
0
25
50
75
100
125
1.03
1.02
1.01
1.00
0.99
0.98
0.97
0.96
0.95
–50 –25
0
25
50
75
100
125
OUTPUT SATURATION VOLTAGE (V)
2.54
TEMPERATURE (°C)
LT1246 • TPC10
Low Level Output
Saturation Voltage
1.0
OUTPUT SATURATION VOLTAGE (V)
OUTPUT SATURATION VOLTAGE (V)
T
J
= 125°C
0.5
2.5
2.0
1.5
1.0
0.5
0
0
T
J
= 125°C
SUPPLY CURRENT (mA)
T
J
= 25°C
T
J
= – 55°C
0
0
100
OUTPUT SINK CURRENT (mA)
LT1246 • TPC13
Error Amplifier Open-Loop Gain
and Phase
100
225
GAIN
180
1.2
80
60
40
20
0
–20
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
LT1246 • TPC16
CURRENT SENSE INPUT THRESHOLD (V)
A
VOL
, OPEN LOOP VOLTAGE GAIN (dB)
U W
200
Current Sense Clamp Voltage
1.05
1.04
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
High Level Output
Saturation Voltage
T
J
= –55°C
T
J
= 25°C
T
J
= 125°C
0
100
OUTPUT SOURCE CURRENT (mA)
200
LT1246 • TPC12
TEMPERATURE (°C)
LT1246 • TPC11
Low Level Output Saturation Voltage
During Undervoltage Lockout
4.0
3.5
3.0
T
J
= – 55°C
13
14
Supply Current vs
Oscillator Frequency
12
T
J
= 25°C
11
10
5
OUTPUT SINK CURRENT (mA)
10
LT1246 • TPC14
9
10k
100k
OSCILLATOR FREQUENCY (Hz)
1M
LT1246 • TPC15
Current Sense Input Threshold
1.0
0.8
T
J
= – 55°C
0.6
T
J
=125°C
0.4
0.2
0
0
1
2
3
4
5
6
ERROR AMP OUTPUT VOLTAGE (V)
LT1246 • TPC17
PHASE (DEGREES)
135
PHASE
90
45
0
–45
10M
T
J
= 25°C
5
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