Linear Technology LT1247, LT1246 Datasheet

LT1246/LT1247

1MHz Off-Line

Current Mode PWM

and DC/DC Converter

EATU

F

■

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

RE

S

Undervoltage Lockout

■

18V High Level Output Clamp

U

O

PPLICATI

A

■

Off-Line Converters

■

DC/DC Converters

Start-Up Operating Maximum

Device Threshold Voltage Duty Cycle Replaces

LT1246 16V 10V 100% UC1842
LT1247 8.4V 7.6V 100% UC1843

S

Minimum

DUESCRIPTIO

The LT®1246/LT1247 are 8-pin, fixed frequency, current
mode, pulse width modulators. These devices are de­signed 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 com­parator, and a high current totem pole output stage ideally
suited to driving power MOSFETs. Start-up current has
been reduced to less than 250µ A. Cross-conduction cur­rent 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
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.

of 18V in the

OUT

BLOCK

R

COMPENSATION

FEEDBACK

I

SENSE

IDAGRA

4

T/CT

1

2

3

2.5V

W

REFERENCE ENABLE

5V REF

MAIN BIAS

OSCILLATOR

5.6V

1mA

–

+

2R

R

1V

–

+

REFERENCE PULLDOWN

S
R

BLANKING

1.5V

LOCKOUT

OUTPUT

PULLDOWN

+

–

UV

18V

8

V

REF

7V

CC

6 OUTPUT

5 GND

LT1246 • BD01

1

LT1246/LT1247

WU

U

PACKAGE

/

O

RDER I FOR ATIO

A

W

O

LUTEXI T

S

A

WUW

ARB

U
G

I

S

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 TA ≤ 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.

LECTRICAL C CHARA TERIST

E

ICS

(Notes 1, 2)

ORDER PART

TOP VIEW

V

COMP

1

FB

2



I

3

SENSE

R

4

T/CT

N8 PACKAGE
8-LEAD PDIP

S8 PACKAGE

8-LEAD PLASTIC SO

T

= 100°C, θJA = 130°C/W (N8)

JMAX

T

= 100°C, θJA = 150°C/W (S8)

JMAX

Consult factory for Industrial and Military grade parts.

8
7
6
5





REF

V



CC

OUTPUT
GND

NUMBER

LT1246CN8
LT1246CS8
LT1247CN8
LT1247CS8

S8 PART MARKING

1246
1247

PARAMETER CONDITIONS MIN TYP MAX UNITS
Reference Section

Output Voltage IO = 1mA, TJ = 25°C 4.925 5.000 5.075 V
Line Regulation 12V < VCC < 25V ● 320 mV
Load Regulation 1mA < I
Temperature Stability 0.1 mV/°C
Total Output Variation Line, Load, Temperature ● 4.87 5.13 V
Output Noise Voltage 10Hz < F < 10kHz, TJ = 25°C50µV
Long-Term Stability TA = 125°C, 1000 Hrs. 5 25 mV
Output Short-Circuit Current ● –30 –90 –180 mA

Oscillator Section

Initial Accuracy RT = 10k, CT = 3.3nF, TJ = 25°C 47.5 50 52.5 kHz

RT = 6.2k, CT = 500pF, TJ = 25°C 465 500 535 kHz
Voltage Stability 12V < VCC < 25V, TJ = 25°C1%
Temperature Stability T
Amplitude Pin 4 1.7 V
Clock Ramp Reset Current V

Error Amplifier Section

Feedback Pin Input Voltage V
Input Bias Current VFB = 2.5V ● –2 µA
Open-Loop Voltage Gain 2 < VO < 4V ● 65 90 dB
Unity-Gain Bandwidth TJ = 25°C 1 2 MHz
Power Supply Rejection Ratio 12V < VCC < 25V ● 60 dB
Output Sink Current V
Output Source Current V

MIN

OSC

PIN 1

PIN 2

PIN 2

< 20mA ● –6 –25 mV

REF

< TJ < T

MAX

(Pin 4) = 2V, TJ = 25°C 7.9 8.2 8.5 mA

= 2.5V ● 2.42 2.50 2.58 V

= 2.7V, V
= 2.3V, V

= 1.1V ● 26 mA

PIN 1

= 5V ● – 0.5 –0.75 mA

PIN 1

–0.05 %/°C

2

LT1246/LT1247

LECTRICAL C CHARA TERIST

E

PARAMETER CONDITIONS MIN TYP MAX UNITS
Error Amplifier Section

Output Voltage High Level V
Output Voltage Low Level V

Current Sense Section

Gain ● 2.85 3.00 3.15 V/V
Maximum Current Sense Input Threshold V
Power Supply Rejection Ratio 70 dB
Input Bias Current ● –1 –10 µA
Delay to Output 30 ns
Blanking Time 60 ns
Blanking Override Voltage 1.5 V

Output Section

Output Low Level I

Output High Level I

Rise Time CL = 1nF, TJ = 25°C3070ns
Fall Time CL = 1nF, TJ = 25°C2060ns
Output Clamp Voltage IO = 1mA ● 18 19 V

Undervoltage Lockout

Start-Up Threshold LT1246 ● 15 16 17 V

Minimum Operating Voltage LT1246 ● 9.0 10 11 V

Hysteresis LT1246 ● 5.5 6.0 V

PWM

Maximum Duty Cycle TJ = 25°C 94 100 %
Minimum Duty Cycle TJ = 25°C0%

Total Device

Start-Up Current ● 170 250 µA
Operating Current ● 13 20 mA

PIN 2

PIN 2

PIN 3

= 20mA ● 0.25 0.4 V

OUT

= 200mA ● 0.75 2.2 V

I

OUT

= 20mA ● 12.0 V

OUT

= 200mA ● 11.75 V

I

OUT

LT1247

LT1247

LT1247

ICS

(Notes 1, 2)

= 2.3V, RL = 15k to GND ● 5 5.6 V
= 2.7V, RL = 15k to Pin 8 ● 0.2 1.1 V

< 1.1V ● 0.90 1.00 1.10 V

● 7.8 8.4 9.0 V

● 7.0 7.6 8.2 V

● 0.4 0.8 V

The ● denotes those specifications which apply over the full operating
temperature range.

Note 1: Unless otherwise specified, V

= 15V, RT = 10k, CT = 3.3nF.

CC

Note 2: Low duty cycle pulse techniques are used during test to maintain
junction temperature close to ambient.

3

LT1246/LT1247

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

VCC = 15V

UW

Y

PICA

17

16

15

(V)

CC

V

11

10

9

200

150

100

LPER

F

O

R

AT

CCHARA TERIST

E

C

ICS

LT1246 Undervoltage Lockout LT1247 Undervoltage Lockout Start-Up Current

200
180
160
140
120
100

80
60

START-UP CURRENT (µA)

40
20

0

–25 25 75 125

–50

START-UP THRESHOLD

MINIMUM OPERATING VOLTAGE

–50

0 50 100

–25 25 75 125

TEMPERATURE (°C)

LT1246 • TPC01

(V)

CC

V

11

10

9

8

7

6

–50

START-UP THRESHOLD

MINIMUM OPERATING VOLTAGE

–25 25 75 125

0 50 100

TEMPERATURE (°C)

1247 TPC02

Start-Up Current Supply Current Oscillator Frequency

15

START-UP

THRESHOLD

LT1246LT1247

14

13

(mA)

CC

I

12

0 50 100

TEMPERATURE (°C)

LT1246 • TPC03

50

START-UP CURRENT (µA)

0

0

Oscillator Sink Current Reference Short-Circuit Current Reference Voltage

8.7

8.6

8.5

8.4

8.3

8.2

8.1

8.0

7.9

OSCILLATOR SINK CURRENT (mA)

7.8

7.7
–50

4

TJ = 25°C

410146 16

2 8 12 18

VCC (V)

V

= 2V

PIN 4

0 50 100

–25 25 75 125

TEMPERATURE (°C)

1246/7 TPC04

LT1246 • TPC07

11

10

–25 25 75 125

–50

140

120

100

80

60

40

REFERENCE SHORT-CIRCUIT CURRENT (mA)

20

–50

0 50 100

TEMPERATURE (°C)

0 50 100

–25 25 75 125

TEMPERATURE (°C)

VCC = 15V

= 10k

R

T

= 3300pF

C

T

LT1246 • TPC05

LT1246 • TPC08



5.05

5.04

5.03

5.02

5.01

5.00

4.99

4.98

REFERENCE VOLTAGE (V)

4.97

4.96

4.95
–50

IO = 1mA

0 50 100

–25 25 75 125

TEMPERATURE (°C)

LT1246 • TPC09