LT1241 Series
High Speed Current Mode
Pulse Width Modulators
FEATURES
s
s
s
s
s
s
s
s
s
s
DESCRIPTIO
s
s
Low Start-Up Current: < 250µA
50ns Current Sense Delay
Current Mode Operation: To 500kHz
Pin Compatible with UC1842 Series
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
High Level Output Clamp: 18V
Current Sense Leading Edge Blanking
APPLICATI
s
s
S
Off-Line Converters
DC/DC Converters
, LTC and LT are registered trademarks of Linear Technology Corporation.
The LT
®
1241 series devices are 8-pin, fixed frequency,
current mode, pulse width modulators. They are improved
plug compatible versions of the industry standard UC1842
series. These devices have both improved speed and
lower quiescent current. The LT1241 series is optimized
for off-line and DC/DC converter applications. They con-
tain a temperature-compensated reference, high gain er-
ror amplifier, current sensing comparator 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 current spikes in
the output stage have been eliminated, making 500kHz
operation practical. Several new features have been incor-
porated. Leading edge blanking has been added to the
current sense comparator. 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 undervoltage lockout.
BLOCK DIAGRA
REFERENCE ENABLE
5V REF
MAIN BIAS
REFERENCE PULL-DOWN
R
T
/C
T
4
OSCILLATOR
T
S
R
6
OUTPUT
COMP
FB
1
1mA
2
2.5V
5.6V
1V
–
+
2R
–
R
BLANKING
+
1.5V
I
SENSE
3
U
UV
LOCKOUT
8
7
V
REF
V
CC
OUTPUT
PULL-DOWN
5
18V
GND
W
UO
+
–
1241 BD01
1
LT1241 Series
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
COMP 1
FB 2
I
SENSE
3
R
T
/C
T
4
J8 PACKAGE
8-LEAD CERDIP
8
7
6
5
V
REF
V
CC
OUTPUT
GND
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
LT124XC ............................................. 0°C to 100°C
LT124XI......................................... – 40°C to 100°C
LT124XM........................................ – 55°C to 125°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
LT124XCJ8
LT124XCN8
LT124XCS8
LT124XIN8
LT124XIS8
LT124XMJ8
S8 PART MARKING
124X
124XI
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 100°C/W (J8)
T
JMAX
= 100°C,
θ
JA
= 130°C/W (N8)
T
JMAX
= 100°C,
θ
JA
= 150°C/W (S8)
ELECTRICAL CHARACTERISTICS
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
PIN1
= 2.5V
V
FB
= 2.5V
2 < V
O
< 4V
T
J
= 25°C
12V < V
CC
< 25V
Line, Load, Temp
I
O
= 1mA, T
J
= 25°C
12V < V
CC
< 25V
1mA < I
VREF
< 20mA
CONDITIONS
(Notes 1, 2)
MIN
4.925
q
q
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
10Hz < F < 10kHz, T
J
= 25°C
T
A
= 125°C, 1000 Hrs.
q
25
– 180
52.5
268
1
– 30
47.5
228
– 90
50
248
– 0.05
1.7
R
T
= 10k, C
T
= 3.3nF, T
J
= 25°C
R
T
= 13.0k, C
T
= 500pF, T
J
= 25°C
12V < V
CC
< 25V, T
J
= 25°C
T
MIN
< T
J
< T
MAX
T
J
= 25°C (Pin 4)
V
OSC
(Pin 4) = 2V, T
J
= 25°C
q
q
q
7.9
2.42
65
0.7
q
q
q
8.2
2.50
90
1.3
6
– 0.75
8.5
2.58
–2
2
60
2
– 0.5
V
PIN2
= 2.7V, V
PIN1
= 1.1V
V
PIN2
= 2.3V, V
PIN1
= 5V
2
U
mA
V
µA
dB
MHz
dB
mA
mA
W
U
U
W W
W
LT1241 Series
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
LT1241
LT1242/LT1244
LT1243/LT1245
Minimum Operating Voltage
LT1241/LT1243/LT1245
LT1242/LT1244
Hysteresis
LT1241
LT1242/LT1244
LT1243/LT1245
PWM
Maximum Duty Cycle
LT1241/LT1244/LT1245
LT1242/LT1243
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
q
q
q
q
(Notes 1, 2)
MIN
q
q
CONDITIONS
V
PIN2
= 2.3V, R
L
= 15k to GND
V
PIN2
= 2.7V, R
L
= 15k to Pin 8
TYP
5.6
0.2
MAX
UNITS
V
5
1.1
3.15
1.10
10
100
V
V/V
V
dB
µA
ns
ns
V
2.85
0.90
3.00
1.00
70
–1
50
100
1.5
V
PIN3
< 1.1V
q
q
q
q
I
OUT
= 20mA
I
OUT
= 200mA
I
OUT
= 20mA
I
OUT
= 200mA
C
L
= 1nF, T
J
= 25°C
C
L
= 1.0nF, T
J
= 25°C
I
O
= 1mA
q
q
q
q
0.25
0.75
12.0
11.75
50
30
0.4
2.2
V
V
V
V
80
60
19.5
ns
ns
V
q
18
9.0
15
7.8
7.0
9.0
1.6
5.5
0.4
9.6
16
8.4
7.6
10
2.0
6.0
0.8
10.2
17
9.0
8.2
11
V
V
V
V
V
V
V
V
T
J
= 25°C
T
J
= 25°C
q
46
94
48
96
0
170
7
250
10
%
%
%
µA
mA
Note 2:
Low duty cycle pulse techniques are used during test to maintain
junction temperature close to ambient.
3
LT1241 Series
TYPICAL PERFOR A CE CHARACTERISTICS
Undervoltage Lockout – LT1241
11
17
START-UP THRESHOLD
10
START-UP THRESHOLD
16
10
V
CC
(V)
V
CC
(V)
V
CC
(V)
9
8
MINIMUM OPERATING VOLTAGE
7
6
–50 –25
25
75
0
50
TEMPERATURE (°C)
Start-Up Current
200
START-UP
THRESHOLD
START-UP CURRENT (µA)
150
LT1243/5
100
LT1241
LT1242/4
START-UP CURRENT (µA)
100
80
60
40
I
CC
(mA)
50
T
J
= 25°C
0
0
2
4
6
8 10
V
CC
(V)
12
14 16
18
Supply Current vs
Oscillator Frequency
10
9
8
SUPPLY CURRENT (mA)
LT1242, LT1243
OSCILLATOR SINK CURRENT (mA)
FREQUENCY (kHz)
7
6
5
4
3
2
1
0
10k
V
CC
= 15V
R
T
= 10k
C
L
= 15pF
LT1241, LT1244, LT1245
100k
OSCILLATOR FREQUENCY (Hz)
4
U W
100
LT1241 • TPC01
LT1241 • TPC04
LT1241 • TPC18
Undervoltage Lockout –
LT1242, LT1244
11
Undervoltage Lockout –
LT1243, LT1245
15
9
START-UP THRESHOLD
8
MINIMUM OPERATING VOLTAGE
11
MINIMUM OPERATING VOLTAGE
10
7
125
9
–50 –25
0
50
25
75
TEMPERATURE (°C)
100
125
6
–50 –25
25
75
0
50
TEMPERATURE (°C)
100
125
LT1241 • TPC02
LT1241 • TPC03
Start-Up Current
200
180
160
140
120
8
9
10
Supply Current
V
CC
= 15V
R
T
= 10k
C
T
= 3300pF
7
6
20
0
–50 –25
25
75
0
50
TEMPERATURE (°C)
100
125
5
–50 –25
25
75
0
50
TEMPERATURE (°C)
100
125
LT1241 • TPC05
LT1241 • TPC06
Oscillator Frequency
60
58
56
54
52
50
48
46
44
42
1M
40
–50 –25
25
75
0
50
TEMPERATURE (°C)
100
125
V
CC
= 5V
R
T
= 10k
C
T
= 3300pF
8.7
8.6
8.5
8.4
8.3
8.2
8.1
8.0
7.9
7.8
Oscillator Sink Current
V
PIN4
= 2V
7.7
–50 –25
0
50
25
75
TEMPERATURE (°C)
100
125
LT1241 • TPC07
LT1241 • TPC08
LT1241 Series
TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage
5.04
I
O
= 1mA
REFERENCE SHORT-CIRCUIT CURRENT (mA)
5.05
120
100
80
60
40
20
–50 –25
FEEDBACK PIN INPUT VOLTAGE (V)
REFERENCE VOLTAGE (V)
5.03
5.02
5.01
5.00
4.99
4.98
4.97
4.96
4.95
–50 –25
0
50
25
75
TEMPERATURE (°C)
100
125
Error Amplifier Open-Loop Gain
and Phase
100
A
VOL
OPEN-LOOP VOLTAGE GAIN (dB)
80
60
40
20
0
–20
10
100
GAIN
180
135
CURRENT SENSE CLAMP VOLTAGE (V)
V
CC
= 15V
V
O
= 2.0V - 4.0V
R
L
= 100k
T
A
= 25°C
1.04
1.03
1.02
1.01
1.00
0.99
0.98
0.97
0.96
0.95
–50 –25
0
50
25
75
TEMPERATURE (°C)
100
125
CURRENT SENSE INPUT THRESHOLD (V)
PHASE
1k
100k
10k
FREQUENCY (Hz)
High Level Output
Saturation Voltage
4.0
OUTPUT SATURATION VOLTAGE (V)
OUTPUT SATURATION VOLTAGE (V)
3.0
2.5
2.0
1.5
1.0
0.5
0
0
OUTPUT SATURATION VOLTAGE (V)
3.5
T
J
= –55°C
T
J
= 25°C
T
J
= 125°C
100
OUTPUT SOURCE CURRENT (mA)
U W
LT1241 • TPC10
Reference Short-Circuit Current
140
2.55
2.54
2.53
2.52
2.51
2.50
2.49
2.48
2.47
2.46
0
50
25
75
TEMPERATURE (°C)
100
125
Feedback Pin Input Voltage
2.45
–50 –25
0
25
50
75
100
125
TEMPERATURE (°C)
LT1241 • TPC09
LT1241 • TPC11
Current Sense Clamp Voltage
225
1.05
1.2
1.0
0.8
Current Sense Input Threshold
PHASE (DEG)
T
J
= –55°C
0.6
T
J
=125°C
0.4
0.2
0
0
1
2
4
5
3
ERROR AMP OUTPUT VOLTAGE (V)
6
T
J
= 25°C
90
45
0
–45
10M
1M
LT1241 • TPC16
LT1241 • TPC12
LT1241 • TPC17
Low Level Output
Saturation Voltage
1.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
Low Level Output Saturation
Voltage During Undervoltage
Lockout
T
J
= 125°C
0.5
T
J
= –55°C
T
J
= 25°C
T
J
= –55°C
T
J
= 25°C
T
J
= 125°C
0
200
0
100
OUTPUT SINK CURRENT (mA)
200
0
5
OUTPUT SINK CURRENT (mA)
10
LT1241 • TPC13
LT1241 • TPC14
LT1241 • TPC15
5
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