Linear Technology LT1076CT7, LT1076CK, LT1076IT, LT1076IR, LT1076 Datasheet

FEATURES

■5A Onboard Switch (LT1074)

■100kHz Switching Frequency

■Greatly Improved Dynamic Behavior

■Available in Low Cost 5 and 7-Lead Packages

■Only 8.5mA Quiescent Current

■Programmable Current Limit

■Operates Up to 60V Input

■Micropower Shutdown Mode

APPLICATIOUS

■Buck Converter with Output Voltage Range of 2.5V to 50V

■Tapped-Inductor Buck Converter with 10A Output at 5V

■Positive-to-Negative Converter

■Negative Boost Converter

■Multiple Output Buck Converter

DESCRIPTIOU

The LT®1074 is a 5A (LT1076 is rated at 2A) monolithic bipolar switching regulator which requires only a few external parts for normal operation. The power switch, all oscillator and control circuitry, and all current limit components, are included on the chip. The topology is a classic

Step-Down Switching

Regulator

positive “buck” configuration but several design innovations allow this device to be used as a positive-to-negative converter, a negative boost converter, and as a flyback converter. The switch output is specified to swing 40V below ground, allowing the LT1074 to drive a tappedinductor in the buck mode with output currents up to 10A.

The LT1074 uses a true analog multiplier in the feedback loop. This makes the device respond nearly instantaneously to input voltage fluctuations and makes loop gain independent of input voltage. As a result, dynamic behavior of the regulator is significantly improved over previous designs.

On-chip pulse by pulse current limiting makes the LT1074 nearly bust-proof for output overloads or shorts. The input voltage range as a buck converter is 8V to 60V, but a selfboot feature allows input voltages as low as 5V in the inverting and boost configurations.

The LT1074 is available in low cost TO-220 or TO-3 packages with frequency pre-set at 100kHz and current limit at 6.5A (LT1076 = 2.6A). A 7-pin TO-220 package is also available which allows current limit to be adjusted down to zero. In addition, full micropower shutdown can be programmed. See Application Note 44 for design details.

A fixed 5V output, 2A version is also available. See LT1076-5.

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

TYPICAL APPLICATIOU

Basic Positive Buck Converter

L1**

50 H (LT1074)

VIN

VSW

100 H (LT1076)

5V

* USE MBR340 FOR LT1076

10V TO 40V

5A

** COILTRONICS #50-2-52 (LT1074)

LT1074

MBR745*

R1

#100-1-52 (LT1076)

2.8k

FB

1%

PULSE ENGINEERING, INC.

#PE-92114 (LT1074)

GND

VC

R3

R2

#PE-92102 (LT1076)

HURRICANE #HL-AK147QQ (LT1074)

2.21k

2.7k

#HL-AG210LL (LT1076)

+

1%

+

C3

†

C2

C1

†

RIPPLE CURRENT RATING ≥ IOUT/2

200 F

0.01 F

500 F

25V

LT1074•TA01

			Buck Converter Efficiency
							LT1074
	100
(%)	90				VOUT = 12V, VIN = 20V
EFFICIENCY	80
					VOUT = 5V, V IN = 15V
	70
					L =	50 H TYPE 52				CORE
	60
						DIODE = MBR735
	50
													5
	0		1		2		3		4						6

OUTPUT LOAD CURRENT (A)

LT1074•TPC27

1

LT1074/LT1076

ABSOLUTE

WAXIWUW

RATI

UGS

(Note 1)

Input Voltage	45V	ILIM Pin Voltage (Forced) ............................................	5.5V
LT1074/ LT1076 ..................................................		Maximum Operating Ambient Temperature Range
LT1074HV/LT1076HV .........................................	64V	Commercial .................................................	0°C to 70°C
Switch Voltage with Respect to Input Voltage		Industrial ................................................	–40°C to 85°C
LT1074/ LT1076 ..................................................	64V	Military .................................................	–55°C to 125°C
LT1074HV/LT1076HV .........................................	75V	Maximum Operating Junction Temperature Range
Switch Voltage with Respect to Ground Pin (VSW Negative)		Commercial ...............................................	0°C to 125°C
LT1074/LT1076 (Note 7) .....................................	35V	Industrial ..............................................	–40°C to 125°C
LT1074HV/LT1076HV (Note 7) ...........................	45V	Military .................................................	– 55°C to 150°C
Feedback Pin Voltage.....................................	–2V, +10V	Maximum Storage Temperature ...............	–65°C to 150°C
Shutdown Pin Voltage (Not to Exceed VIN) ..............	40V	Lead Temperature (Soldering, 10 sec) ......................	300°C

PACKAGE/ORDER IUFORWATIOU
	FRONT VIEW		ORDER PART		BOTTOM VIEW				ORDER PART
	5	VIN	NUMBER	VC		VIN			NUMBER
TAB IS	4	VSW	LT1076CQ		1	2			LT1074CK
GND	3	GND
	2	VC	LT1076IQ			3	CASE		LT1074HVCK
							IS GND
	1	FB/SENSE			4				LT1074MK
	Q PACKAGE
				FB		VSW			LT1074HVMK
	5-LEAD PLASTIC DD
	LT1076: θJC = 4°C, θJA = 30°C/W				K PACKAGE				LT1076CK
					4-LEAD TO-3 METAL CAN				LT1076HVCK
			LT1076CR		LT1074: θJC = 2.5°C, θJA = 35°C/W				LT1076MK
	FRONT VIEW				LT1076: θJC = 4°C, θJA = 35°C/W				LT1076HVMK
	7	SHDN	LT1076IR
TAB IS	6	VC	LT1076HVCR
	5	FB/SENSE
GND	4	GND	LT1076HVIR			FRONT VIEW			LT1074CT
	3	ILIM
	2	VSW				5		VIN	LT1074HVCT
	1	VIN
	R PACKAGE			TAB IS		4		VSW	LT1074IT
	7-LEAD PLASTIC DD					3		GND	LT1074HVIT
				GND
						2		VC
	LT1076: θJC = 4°C, θJA = 30°C/W								LT1076CT
						1		FB
									LT1076HVCT
			LT1074CT7			T PACKAGE
									LT1076IT
	FRONT VIEW				5-LEAD PLASTIC TO-220
			LT1074HVCT7		LEADS ARE FORMED STANDARD FOR				LT1076HVIT
	7	SHDN			STRAIGHT LEADS, ORDER FLOW 06
	6	VC	LT1074IT7
TAB IS	5	FB	LT1074HVIT7		LT1074: θJC = 2.5°C, θJA = 50°C/W
	4	GND
GND	3	ILIM	LT1076CT7		LT1076: θJC = 4°C, θJA = 50°C/W
	2	VSW
	1	VIN	LT1076HVCT7
	T7 PACKAGE
	7-LEAD PLASTIC TO-220
LT1074: θJC = 2.5°C, θJA = 50°C/W
	LT1076: θJC = 4°C, θJA = 50°C/W
*Assumes package is soldered to 0.5 IN2 of 1 oz. copper over internal ground plane or over back side plane.

2

LT1074/LT1076

ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Tj = 25°C, VIN = 25V, unless otherwise noted.

PARAMETER	CONDITIONS			MIN	TYP	MAX	UNITS
Switch “On” Voltage (Note 2)	LT1074 ISW = 1A, Tj ≥ 0°C					1.85	V
		ISW = 1A, Tj < 0°C				2.1	V
		ISW = 5A, Tj ≥ 0°C				2.3	V
		ISW = 5A, Tj < 0°C				2.5	V
	LT1076	ISW = 0.5A	●			1.2	V
		ISW = 2A	●			1.7	V
Switch “Off” Leakage	LT1074	VIN ≤ 25V, VSW = 0			5	300	µA
		VIN = VMAX, VSW = 0 (Note 8)			10	500	µA
	LT1076	VIN = 25V, VSW = 0				150	µA
		VIN = VMAX, VSW = 0 (Note 8)				250	µA
Supply Current (Note 3)	VFB = 2.5V, VIN ≤ 40V		●		8.5	11	mA
	40V < VIN < 60V		●		9	12	mA
	VSHUT = 0.1V (Device Shutdown) (Note 9)		●		140	300	µA
Minimum Supply Voltage	Normal Mode		●		7.3	8	V
	Startup Mode (Note 4)		●		3.5	4.8	V
Switch Current Limit (Note 5)	LT1074	ILIM Open	●	5.5	6.5	8.5	A
		RLIM = 10k (Note 6)			4.5		A
		RLIM = 7k (Note 6)			3		A
	LT1076	ILIM Open	●	2	2.6	3.2	A
		RLIM = 10k (Note 6)			1.8		A
		RLIM = 7k (Note 6)			1.2		A
Maximum Duty Cycle			●	85	90		%
Switching Frequency	Tj ≤ 125°C			90	100	110	kHz
			●	85		120	kHz
	Tj > 125°C		●	85		125	kHz
	VFB = 0V through 2kΩ (Note 5)				20		kHz
Switching Frequency Line Regulation	8V ≤ VIN ≤ VMAX (Note 8)		●		0.03	0.1	%/V
Error Amplifier Voltage Gain (Note 7)	1V ≤ VC ≤ 4V				2000		V/V
Error Amplifier Transconductance				3700	5000	8000	µmho
Error Amplifier Source and Sink Current	Source (VFB = 2V)			100	140	225	µA
	Sink (VFB = 2.5V)			0.7	1	1.6	mA
Feedback Pin Bias Current	VFB = VREF		●		0.5	2	µA
Reference Voltage	VC = 2V		●	2.155	2.21	2.265	V
Reference Voltage Tolerance	VREF (Nominal) = 2.21V				±0.5	±1.5	%
	All Conditions of Input Voltage, Output		●		±1	±2.5	%
	Voltage, Temperature and Load Current
Reference Voltage Line Regulation	8V ≤ VIN ≤ VMAX (Note 8)		●		0.005	0.02	%/V
VC Voltage at 0% Duty Cycle					1.5		V
	Over Temperature		●		– 4		mV/°C
Multiplier Reference Voltage					24		V
Shutdown Pin Current	VSH = 5V		●	5	10	20	µA
	VSH ≤ VTHRESHOLD ( 2.5V)		●			50	µA
Shutdown Thresholds	Switch Duty Cycle = 0		●	2.2	2.45	2.7	V
	Fully Shut Down		●	0.1	0.3	0.5	V
Thermal Resistance Junction to Case	LT1074					2.5	°C/W
	LT1076					4.0	°C/W

3

LT1074/LT1076

ELECTRICAL CHARACTERISTICS

Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.

Note 2: To calculate maximum switch “on” voltage at currents between low and high conditions, a linear interpolation may be used.

Note 3: A feedback pin voltage (VFB) of 2.5V forces the VC pin to its low clamp level and the switch duty cycle to zero. This approximates the zero load condition where duty cycle approaches zero.

Note 4: Total voltage from VIN pin to ground pin must be ≥ 8V after startup for proper regulation.

Note 5: Switch frequency is internally scaled down when the feedback pin voltage is less than 1.3V to avoid extremely short switch on times. During testing, VFB is adjusted to give a minimum switch on time of 1 s.

Note 6: I	LIM	≈ RLIM – 1k	(LT1074), I	LIM	≈ RLIM – 1k	(LT1076).
		2k			5.5k

Note 7: Switch to input voltage limitation must also be observed.

Note 8: VMAX = 40V for the LT1074/76 and 60V for the LT1074HV/76HV. Note 9: Does not include switch leakage.

BLOCK DIAGRAW

INPUT SUPPLY						LT1074
10µA			320 µA
0.3V
+		6V				500Ω
µ-POWER			6V TO ALL
		REGULATOR
SHUTDOWN			CIRCUITRY
		AND BIAS
–
				CURRENT
				LIMIT		0.04
2.35V	CURRENT			COMP	+
	LIMIT				C2
+						250 Ω
	SHUTDOWN				–
–
SHUTDOWN*		ILIM*	4.5V	10k

			FREQ SHIFT		R
			100kHz
				S	R/S	Q	G1
			OSCILLATOR		LATCH
			SYNC		R
			3V(p-p)
			VIN
				+			400 Ω	15 Ω
			Z
	+			C1
			ANALOG
	A1
	ERROR	X	MULTIPLIER	–
	AMP		XY	PULSE WIDTH
2.21V			Z	COMPARATOR
	–							SWITCH
			Y
								OUTPUT
								(VSW)
	FB	VC	24V (EQUIVALENT)			LT1076

0.1Ω

*AVAILABLE ON PACKAGES WITH PIN COUNTS GREATER THAN 5.

100Ω

SWITCH

OUTPUT (VSW )

LT1074 • BD01

4

LT1074/LT1076

BLOCK DIAGRAW DESCRIPTIOU

A switch cycle in the LT1074 is initiated by the oscillator setting the R/S latch. The pulse that sets the latch also locks out the switch via gate G1. The effective width of this pulse is approximately 700ns, which sets the maximum switch duty cycle to approximately 93% at 100kHz switching frequency. The switch is turned off by comparator C1, which resets the latch. C1 has a sawtooth waveform as one input and the output of an analog multiplier as the other input. The multiplier output is the product of an internal reference voltage, and the output of the error amplifier, A1, divided by the regulator input voltage. In standard buck regulators, this means that the output voltage of A1 required to keep a constant regulated output is independent of regulator input voltage. This greatly improves line transient response, and makes loop gain independent of input voltage. The error amplifier is a transconductance type with a GM at null of approximately 5000 mho. Slew current going positive is 140 A, while negative slew current is about 1.1mA. This asymmetry helps prevent overshoot on start-up. Overall loop frequency compensation is accomplished with a series RC network from VC to ground.

Switch current is continuously monitored by C2, which resets the R/S latch to turn the switch off if an overcurrent condition occurs. The time required for detection and switch turn off is approximately 600ns. So minimum switch “on” time in current limit is 600ns. Under dead shorted output conditions, switch duty cycle may have to be as low as 2% to maintain control of output current. This would require switch on time of 200ns at 100kHz switching frequency, so frequency is reduced at very low output

voltages by feeding the FB signal into the oscillator and creating a linear frequency downshift when the FB signal drops below 1.3V. Current trip level is set by the voltage on

the ILIM pin which is driven by an internal 320 A current source. When this pin is left open, it self-clamps at about

4.5V and sets current limit at 6.5A for the LT1074 and 2.6A for the LT1076. In the 7-pin package an external resistor

can be connected from the ILIM pin to ground to set a lower current limit. A capacitor in parallel with this resistor will

soft-start the current limit. A slight offset in C2 guarantees that when the ILIM pin is pulled to within 200mV of ground, C2 output will stay high and force switch duty cycle to zero.

The “Shutdown” pin is used to force switch duty cycle to zero by pulling the ILIM pin low, or to completely shut down the regulator. Threshold for the former is approximately 2.35V, and for complete shutdown, approximately 0.3V. Total supply current in shutdown is about 150 A. A 10 A pull-up current forces the shutdown pin high when left open. A capacitor can be used to generate delayed startup. A resistor divider will program “undervoltage lockout” if the divider voltage is set at 2.35V when the input is at the desired trip point.

The switch used in the LT1074 is a Darlington NPN (single NPN for LT1076) driven by a saturated PNP. Special patented circuitry is used to drive the PNP on and off very quickly even from the saturation state. This particular switch arrangement has no “isolation tubs” connected to the switch output, which can therefore swing to 40V below ground.

5