Linear Technology LT1245, LT1242, LT1241, LT1244, LT1243 Datasheet

LT1241 Series

High Speed Current Mode

Pulse Width Modulators

FEATURES

■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

APPLICATIOUS

■Off-Line Converters

■DC/DC Converters

,LTC and LT are registered trademarks of Linear Technology Corporation.

DESCRIPTIOU

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 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 250μA. Cross-conduction current spikes in the output stage have been eliminated, making 500kHz operation practical. Several new features have been incorporated. 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 VOUT of 18V in the on state. The output and the reference output are actively

pulled low during undervoltage lockout.

BLOCK DIAGRAW

UV

REFERENCE ENABLE

LOCKOUT

5V REF

REFERENCE PULL-DOWN

8

VREF

MAIN BIAS

OUTPUT

7

VCC

PULL-DOWN

RT/CT	4		OSCILLATOR		T
COMP	1		5.6V	1V	S	6	OUTPUT
					R
			1mA
FB	2	–	2R	–	BLANKING	5	GND
		2.5V				18V
		+	R	+		+
ISENSE	3				1.5V	–
							1241 BD01

1

LT1241 Series

ABSOLUTE WAXIWUW RATIUGS

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		0 ° C to 100°C
LT124XC .............................................
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.

PACKAGE/ORDER IUFORWATIOU

							ORDER PART
			TOP VIEW				NUMBER
	COMP	1			8	VREF	LT124XCJ8
	FB					VCC
		2			7
							LT124XCN8
	ISENSE					OUTPUT
		3			6
							LT124XCS8
	RT/CT					GND
		4			5
							LT124XIN8
	J8 PACKAGE			N8 PACKAGE
							LT124XIS8
	8-LEAD CERDIP			8-LEAD PDIP
			S8 PACKAGE				LT124XMJ8
		8-LEAD PLASTIC SO
							S8 PART MARKING
	TJMAX = 125°C, θJA = 100°C/W (J8)
	TJMAX = 100°C, θJA = 130°C/W (N8)						124X
	TJMAX = 100°C, θJA = 150°C/W (S8)
							124XI

ELECTRICAL CHARACTERISTICS (Notes 1, 2)

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	●		3	20	mV
Load Regulation	1mA < IVREF < 20mA	●		– 6	– 25	mV
Temperature Stability				0.1		mV/°C
Total Output Variation	Line, Load, Temp	●	4.87		5.13	V
Output Noise Voltage	10Hz < F < 10kHz, TJ = 25°C			50		μV
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 = 13.0k, CT = 500pF, TJ = 25°C		228	248	268	kHz
Voltage Stability	12V < VCC < 25V, TJ = 25°C				1	%
Temperature Stability	TMIN < TJ < TMAX			– 0.05		%/°C
Amplitude	TJ = 25°C (Pin 4)			1.7		V
Clock Ramp Reset Current	VOSC (Pin 4) = 2V, TJ = 25°C		7.9	8.2	8.5	mA
Error Amplifier Section
Feedback Pin Input Voltage	VPIN1 = 2.5V	●	2.42	2.50	2.58	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		0.7	1.3	2	MHz
Power Supply Rejection Ratio	12V < VCC < 25V	●	60			dB
Output Sink Current	VPIN2 = 2.7V, VPIN1 = 1.1V	●	2	6		mA
Output Source Current	VPIN2 = 2.3V, VPIN1 = 5V	●	– 0.5	– 0.75		mA

2

LT1241 Series

ELECTRICAL CHARACTERISTICS (Notes 1, 2)

PARAMETER	CONDITIONS		MIN	TYP	MAX	UNITS
Error Amplifier Section
Output Voltage High Level	VPIN2 = 2.3V, RL = 15k to GND	●	5	5.6		V
Output Voltage Low Level	VPIN2 = 2.7V, RL = 15k to Pin 8	●		0.2	1.1	V
Current Sense Section
Gain		●	2.85	3.00	3.15	V/V
Maximum Current Sense Input Threshold	VPIN3 < 1.1V	●	0.90	1.00	1.10	V
Power Supply Rejection Ratio		●		70		dB
Input Bias Current		●		– 1	10	μA
Delay to Output		●		50	100	ns
Blanking Time				100		ns
Blanking Override Voltage				1.5		V
Output Section
Output Low Level	IOUT = 20mA	●		0.25	0.4	V
	IOUT = 200mA	●		0.75	2.2	V
Output High Level	IOUT = 20mA	●	12.0			V
	IOUT = 200mA	●	11.75			V
Rise Time	CL = 1nF, TJ = 25°C			50	80	ns
Fall Time	CL = 1.0nF, TJ = 25°C			30	60	ns
Output Clamp Voltage	IO = 1mA	●		18	19.5	V
Undervoltage Lockout
Start-Up Threshold			9.0	9.6	10.2	V
LT1241		●
LT1242/LT1244		●	15	16	17	V
LT1243/LT1245		●	7.8	8.4	9.0	V
Minimum Operating Voltage
LT1241/LT1243/LT1245		●	7.0	7.6	8.2	V
LT1242/LT1244		●	9.0	10	11	V
Hysteresis
LT1241			1.6	2.0		V
LT1242/LT1244			5.5	6.0		V
LT1243/LT1245			0.4	0.8		V
PWM
Maximum Duty Cycle	TJ = 25°C		46	48		%
LT1241/LT1244/LT1245
LT1242/LT1243	TJ = 25°C		94	96		%
Minimum Duty Cycle		●		0		%
Total Device
Start-Up Current		●		170	250	μA
Operating Current		●		7	10	mA

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

Note 1: Unless otherwise specified, VCC = 15V, RT = 10k, CT = 3.3nF.

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

3

LT1241 Series

TYPICAL PERFORWAUCE CHARACTERISTICS

																	Undervoltage Lockout –
Undervoltage Lockout – LT1241																	LT1242, LT1244
11																17
10																16					START-UP THRESHOLD
					START-UP THRESHOLD
9																15
(V)																(V)
CC																CC
V																V
8																11
				MINIMUM OPERATING VOLTAGE
7																10				MINIMUM OPERATING VOLTAGE
6																9
		–25			0	25		50		75		100		125				–25			0	25		50		75		100		125
–50																–50
						TEMPERATURE (°C)																TEMPERATURE (°C)
												LT1241 • TPC01																LT1241 • TPC02

VCC (V)

Undervoltage Lockout –

LT1243, LT1245

11

10

9

START-UP THRESHOLD

8 MINIMUM OPERATING VOLTAGE

7

6 –50 –25 0 25 50 75 100 125

TEMPERATURE (°C)

LT1241 • TPC03

Start-Up Current

Start-Up Current

Supply Current

	200											200									10
		START-UP										180										VCC = 15V
		THRESHOLD										160									9	RT = 10k
(μA)	150										(μA)											CT = 3300pF
												140
CURRENT		LT1243/5			LT1241			LT1242/4			CURRENT	120								CC	8
																				(mA)
	100											100
START-UP											START-UP	80								I	7
	50											60
												40									6
								TJ = 25°C				20
	0	2	4	6	8	10	12	14	16	18		0	–25	0	25	50	75	100	125		5	–25	0	25	50	75	100	125
	0											–50									–50
					VCC (V)									TEMPERATURE (°C)									TEMPERATURE (°C)

SUPPLY CURRENT (mA)

LT1241 • TPC04

Supply Current vs

Oscillator Frequency

10

9
8		LT1242, LT1243
7			LT1241, LT1244, LT1245

6

5

4

3

2VCC = 15V RT = 10k

1 CL = 15pF

0	100k	1M
10k

OSCILLATOR FREQUENCY (Hz)

							LT1241 • TPC05										LT1241 • TPC06
	Oscillator Frequency										Oscillator Sink Current
	60									8.7
	58					VCC = 5V				8.6	VPIN4 = 2V
						RT = 10k			(mA)
	56					CT = 3300pF				8.5
FREQUENCY (kHz)	54								OSCILLATOR SINK CURRENT	8.4
	52									8.3
	50									8.2
	48									8.1
	46									8.0
	44									7.9
	42									7.8
	40	–25	0	25	50	75	100	125		7.7		–25	0	25	50	75	100	125
	–50									–50

TEMPERATURE (°C)

TEMPERATURE (°C)

LT1241 • TPC18

LT1241 • TPC07

LT1241 • TPC08

4

LT1241 Series

TYPICAL PERFORWAUCE CHARACTERISTICS

		Reference Voltage
	5.05
	5.04		IO =	1mA
(V)	5.03
	5.02
VOLTAGE
	5.01
REFERENCE	5.00
	4.99
	4.98
	4.97
	4.96
	4.95

–50 –25 0 25 50 75 100 125 TEMPERATURE (°C)

LT1241 • TPC10

Reference Short-Circuit Current

Feedback Pin Input Voltage

-CIRCUIT CURRENT (mA)	140									2.55
	120								INPUT VOLTAGE (V)	2.54
										2.53
	100									2.52
										2.51
	80									2.50
REFERENCE SHORT	60								FEEDBACK PIN	2.49
										2.48
	40									2.47
										2.46
	20									2.45
		–25	0	25	50	75	100	125			–25	0	25	50	75	100	125
	–50									–50

TEMPERATURE (°C)

TEMPERATURE (°C)

LT1241 • TPC09

LT1241 • TPC11

Error Amplifier Open-Loop Gain
and Phase	Current Sense Clamp Voltage

	100							225		1.05
OPEN-LOOP VOLTAGE GAIN (dB)					VCC = 15V				SENSE CLAMP VOLTAGE (V)	1.04
	80		GAIN		VO = 2.0V - 4.0V			180		1.03
					RL = 100k
	60				TA = 25°C			135		1.02
										1.01
					PHASE
	40							90		1.00
										0.99
	20							45		0.98
								(DEG)PHASE
										0.97
VOL	0							0	CURRENT
										0.96
A
	–20	100	1k	10k	100k	1M		–45		0.95	–25	0	25	50	75	100	125
	10						10M			–50
			FREQUENCY (Hz)									TEMPERATURE (°C)
						LT1241 • TPC16										LT1241 • TPC12

High Level Output	Low Level Output
Saturation Voltage	Saturation Voltage

	4.0				1.0
(V)	3.5			(V)
	2.5
VOLTAGE				VOLTAGE
	3.0	TJ = –55°C
							TJ = 125°C
SATURATION				SATURATION
	2.0	TJ = 25°C			0.5
	1.5	TJ = 125°C
OUTPUT	1.0			OUTPUT		°	TJ = –55°C
						TJ = 25 C
	0.5
	0	100	200		0	100	200
	0				0
		OUTPUT SOURCE CURRENT (mA)				OUTPUT SINK CURRENT (mA)

Current Sense Input Threshold

	1.2
(V)
THRESHOLD	1.0
	0.8
		TJ = –55°C
INPUT
	0.6	TJ =125°C
SENSE
	0.4
CURRENT	0.2	TJ = 25°C
	0

0 1 2 3 4 5 6 ERROR AMP OUTPUT VOLTAGE (V)

LT1241 • TPC17

Low Level Output Saturation

Voltage During Undervoltage

Lockout

	4.0
(V)	3.5
VOLTAGE	3.0
	2.5	TJ = –55°C
SATURATION
	2.0
	1.5	TJ = 25°C
OUTPUT	1.0	TJ = 125°C
	0.5
	0	5	10
	0
		OUTPUT SINK CURRENT (mA)

LT1241 • TPC13

LT1241 • TPC14

LT1241 • TPC15

5