1
LT1246/LT1247
1MHz Off-Line
Current Mode PWM
and DC/DC Converter
+
+
1
2
FEEDBACK
COMPENSATION
2.5V
2R
R
1V
1.5V
+
3
I
SENSE
BLANKING
1mA
5.6V
4
R
T
/C
T
OSCILLATOR
R
S
18V
7
V
CC
UV
LOCKOUT
5
GND
6
OUTPUT
8
V
REF
5V REF
MAIN BIAS
REFERENCE PULLDOWN
OUTPUT
PULLDOWN
REFERENCE ENABLE
LT1246 · BD01
D
U
ESCRIPTIO
S
FEATURE
U
S
A
O
PPLICATI
s
Current Mode Operation to 1MHz
s
30ns Current Sense Delay
s
< 250
µ
A Low Start-Up Current
s
Current Sense Leading Edge Blanking
s
Pin Compatible with UC1842
s
Undervoltage Lockout with Hysteresis
s
No Cross-Conduction Current
s
Trimmed Bandgap Reference
s
1A Totem Pole Output
s
Trimmed Oscillator Frequency and Sink Current
s
Active Pull-Down on Reference and Output During
Undervoltage Lockout
s
18V High Level Output Clamp
s
Off-Line Converters
s
DC/DC Converters
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 250
µ
A. 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.
W
I
D AGRA
BLOCK
Minimum
Start-Up
Operating
Maximum
Device
Threshold
Voltage
Duty Cycle
Replaces
LT1246
16V
10V
100%
UC1842
LT1247
8.4V
7.6V
100%
UC1843
LT1246/LT1247
2
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Reference Section
Output Voltage
I
O
= 1mA, T
J
= 25
°
C
4.925
5.000
5.075
V
Line Regulation
12V < V
CC
< 25V
q
3
20
mV
Load Regulation
1mA < I
REF
< 20mA
q
6
25
mV
Temperature Stability
0.1
mV/
°
C
Total Output Variation
Line, Load, Temperature
q
4.87
5.13
V
Output Noise Voltage
10Hz < F < 10kHz, T
J
= 25
°
C
50
µ
V
Long-Term Stability
T
A
= 125
°
C, 1000 Hrs.
5
25
mV
Output Short-Circuit Current
q
30
90
180
mA
Oscillator Section
Initial Accuracy
R
T
= 10k, C
T
= 3.3nF, T
J
= 25
°
C
47.5
50
52.5
kHz
R
T
= 6.2k, C
T
= 500pF, T
J
= 25
°
C
465
500
535
kHz
Voltage Stability
12V < V
CC
< 25V, T
J
= 25
°
C
1
%
Temperature Stability
T
MIN
< T
J
< T
MAX
0.05
%/
°
C
Amplitude
Pin 4
1.7
V
Clock Ramp Reset Current
V
OSC
(Pin 4) = 2V, T
J
= 25
°
C
7.9
8.2
8.5
mA
Error Amplifier Section
Feedback Pin Input Voltage
V
PIN
1
= 2.5V
q
2.42
2.50
2.58
V
Input Bias Current
V
FB
= 2.5V
q
2
µ
A
Open-Loop Voltage Gain
2 < V
O
< 4V
q
65
90
dB
Unity-Gain Bandwidth
T
J
= 25
°
C
1
2
MHz
Power Supply Rejection Ratio
12V < V
CC
< 25V
q
60
dB
Output Sink Current
V
PIN
2
= 2.7V, V
PIN
1
= 1.1V
q
2
6
mA
Output Source Current
V
PIN
2
= 2.3V, V
PIN
1
= 5V
q
0.5
0.75
mA
(Notes 1, 2)
ELECTRICAL C
C
HARA TERISTICS
W
U
U
PACKAGE/ORDER I FOR ATIO
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
Supply Voltage ....................................................... 25V
Output Current ......................................................
±
1A*
Output Energy (Capacitive Load per Cycle) ............. 5
µ
J
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.
S8 PART MARKING
LT1246CN8
LT1246CS8
LT1247CN8
LT1247CS8
ORDER PART
NUMBER
1246
1247
Consult factory for Industrial and Military grade parts.
1
2
3
4
8
7
6
5
TOP VIEW
COMP
FB
I
SENSE
R
T
/C
T
V
REF
V
CC
OUTPUT
GND
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 100
°
C,
JA
= 130
°
C/W (N8)
T
JMAX
= 100
°
C,
JA
= 150
°
C/W (S8)
3
LT1246/LT1247
ELECTRICAL C
C
HARA TERISTICS
(Notes 1, 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Error Amplifier Section
Output Voltage High Level
V
PIN
2
= 2.3V, R
L
= 15k to GND
q
5
5.6
V
Output Voltage Low Level
V
PIN
2
= 2.7V, R
L
= 15k to Pin 8
q
0.2
1.1
V
Current Sense Section
Gain
q
2.85
3.00
3.15
V/V
Maximum Current Sense Input Threshold
V
PIN 3
< 1.1V
q
0.90
1.00
1.10
V
Power Supply Rejection Ratio
70
dB
Input Bias Current
q
1
10
µ
A
Delay to Output
30
ns
Blanking Time
60
ns
Blanking Override Voltage
1.5
V
Output Section
Output Low Level
I
OUT
= 20mA
q
0.25
0.4
V
I
OUT
= 200mA
q
0.75
2.2
V
Output High Level
I
OUT
= 20mA
q
12.0
V
I
OUT
= 200mA
q
11.75
V
Rise Time
C
L
= 1nF, T
J
= 25
°
C
30
70
ns
Fall Time
C
L
= 1nF, T
J
= 25
°
C
20
60
ns
Output Clamp Voltage
I
O
= 1mA
q
18
19
V
Undervoltage Lockout
Start-Up Threshold
LT1246
q
15
16
17
V
LT1247
q
7.8
8.4
9.0
V
Minimum Operating Voltage
LT1246
q
9.0
10
11
V
LT1247
q
7.0
7.6
8.2
V
Hysteresis
LT1246
q
5.5
6.0
V
LT1247
q
0.4
0.8
V
PWM
Maximum Duty Cycle
T
J
= 25
°
C
94
100
%
Minimum Duty Cycle
T
J
= 25
°
C
0
%
Total Device
Start-Up Current
q
170
250
µ
A
Operating Current
q
13
20
mA
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.
Note 2: Low duty cycle pulse techniques are used during test to maintain
junction temperature close to ambient.
LT1246/LT1247
4
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
LT1246 Undervoltage Lockout
LT1247 Undervoltage Lockout
Start-Up Current
Oscillator Sink Current
Reference Short-Circuit Current
Reference Voltage
TEMPERATURE (
°
C)
50
9
V
CC
(V)
10
11
16
17
25
25
75
125
LT1246 · TPC01
0
50
100
MINIMUM OPERATING VOLTAGE
START-UP THRESHOLD
15
TEMPERATURE (
°
C)
50
6
V
CC
(V)
7
8
9
10
11
25
25
75
125
1247 TPC02
0
50
100
MINIMUM OPERATING VOLTAGE
START-UP THRESHOLD
TEMPERATURE (°C)
50
0
START-UP CURRENT (
µ
A)
40
80
120
160
200
25
25
75
125
LT1246 · TPC03
0
50
100
180
140
100
60
20
TEMPERATURE (
°
C)
50
10
FREQUENCY CHANGE (%)
6
2
2
6
10
25
25
75
125
LT1246 · TPC06
0
50
100
8
4
0
4
8
V
CC
= 15V
Start-Up Current
Supply Current
Oscillator Frequency
TEMPERATURE (
°
C)
50
10
I
CC
(mA)
11
12
13
14
15
25
25
75
125
LT1246 · TPC05
0
50
100
V
CC
= 15V
R
T
= 10k
C
T
= 3300pF
V
CC
(V)
0
0
START-UP CURRENT (
µ
A)
50
100
150
200
2
8
12
18
1246/7 TPC04
4
10
14
6
16
LT1246
LT1247
T
J
= 25
°
C
START-UP
THRESHOLD
TEMPERATURE (
°
C)
50
7.7
OSCILLATOR SINK CURRENT (mA)
8.7
25
25
75
125
LT1246 · TPC07
0
50
100
7.8
8.0
8.2
8.4
8.6
8.5
7.9
8.1
8.3
V
PIN 4
= 2V
TEMPERATURE (
°
C)
50
20
REFERENCE SHORT-CIRCUIT CURRENT (mA)
140
25
25
75
125
LT1246 · TPC08
0
50
100
40
80
100
120
60
TEMPERATURE (
°
C)
50
4.95
REFERENCE VOLTAGE (V)
5.05
25
25
75
125
LT1246 · TPC09
0
50
100
4.96
5.00
5.01
4.97
4.98
4.99
5.02
5.03
5.04
I
O
= 1mA
5
LT1246/LT1247
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
High Level Output
Feedback Pin Input Voltage
Current Sense Clamp Voltage
Saturation Voltage
Low Level Output
Low Level Output Saturation Voltage
Supply Current vs
Saturation Voltage
During Undervoltage Lockout
Oscillator Frequency
OUTPUT SOURCE CURRENT (mA)
0
0
OUTPUT SATURATION VOLTAGE (V)
4.0
200
LT1246 · TPC12
0.5
2.0
2.5
1.0
1.5
3.0
3.5
T
J
= 55
°
C
T
J
= 25
°
C
T
J
= 125
°
C
100
Error Amplifier Open-Loop Gain
and Phase
Current Sense Input Threshold
TEMPERATURE (
°
C)
50
2.45
FEEDBACK PIN INPUT VOLTAGE (V)
2.55
25
25
75
125
LT1246 · TPC10
0
50
100
2.46
2.50
2.51
2.47
2.48
2.49
2.52
2.53
2.54
TEMPERATURE (
°
C)
50
0.95
CURRENT SENSE CLAMP VOLTAGE (V)
1.05
25
25
75
125
LT1246 · TPC11
0
50
100
0.96
1.00
1.01
0.97
0.98
0.99
1.02
1.03
1.04
OSCILLATOR FREQUENCY (Hz)
10k
9
SUPPLY CURRENT (mA)
10
13
14
100k
1M
LT1246 · TPC15
12
11
OUTPUT SINK CURRENT (mA)
0
0
OUTPUT SATURATION VOLTAGE (V)
4.0
5
10
LT1246 · TPC14
2.0
0.5
1.0
1.5
2.5
3.0
3.5
T
J
= 125
°
C
T
J
= 55
°
C
T
J
= 25
°
C
OUTPUT SINK CURRENT (mA)
0
0
OUTPUT SATURATION VOLTAGE (V)
1.0
100
200
LT1246 · TPC13
T
J
= 55
°
C
T
J
= 25
°
C
T
J
= 125
°
C
0.5
FREQUENCY (Hz)
10
20
A
VOL
, OPEN LOOP VOLTAGE GAIN (dB)
100
10k
10M
LT1246 · TPC16
40
0
20
60
80
100
1k
100k
1M
PHASE
GAIN
PHASE (DEGREES)
180
45
225
90
0
45
135
ERROR AMP OUTPUT VOLTAGE (V)
0
0
CURRENT SENSE INPUT THRESHOLD (V)
1.2
3
6
LT1246 · TPC17
0.6
0.2
0.4
0.8
1.0
1
2
4
5
T
J
= 55
°
C
T
J
= 25
°
C
T
J
=125
°
C
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