Linear Technology LTC1265, LTC1265IS, LTC1265CS, LTC1265-5, LTC1265CS-5 Datasheet

LTC1265/LTC1265-3.3/LTC1265-5

1.2A, High Efficiency

Step-Down DC/DC Converter

FEATURES

■High Efficiency: Up to 95%

■Current Mode Operation for Excellent Line and Load Transient Response

■Internal 0.3Ω Power Switch (VIN = 10V)

■Short-Circuit Protection

■Low Dropout Operation: 100% Duty Cycle

■Low-Battery Detector

■Low 160µA Standby Current at Light Loads

■Active-High Micropower Shutdown: IQ < 15µA

■Peak Inductor Current Independent of Inductor Value

■Available in 14-pin SO Package

APPLICATIOUS

■5V to 3.3V Conversion

■Distributed Power Systems

■Step-Down Converters

■Inverting Converters

■Memory Backup Supply

■Portable Instruments

■Battery-Powered Equipment

■Cellular Telephones

DESCRIPTIOU

The LTC®1265 is a monolithic step-down current mode DC/DC converter featuring Burst Mode TM operation at low output current. The LTC1265 incorporates a 0.3Ω switch (VIN =10V) allowing up to 1.2A of output current.

Under no load condition, the converter draws only 160µA. In shutdown it typically draws a mere 5µA making this converter ideal for current sensitive applications. In dropout the internal P-channel MOSFET switch is turned on continuously maximizing the life of the battery source. The LTC1265 incorporates automatic power saving Burst Mode operation to reduce gate charge losses when the load currents drop below the level required for continuous operation.

The inductor current is user-programmable via an external current sense resistor. Operation up to 700kHz permits the use of small surface mount inductors and capacitors.

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

Burst Mode is a trademark of Linear Technology Corporation.

TYPICAL APPLICATIOU

VIN

5.4V TO

+

CIN†††

12V

68µF

0.1µF

PWR V

IN

V

IN

L1*

RSENSE**

20V

33µH

0.1Ω

VOUT

SHDN

SW

5V

1k

LTC1265-5

D1†

+

1A

COUT††

ITH

PGND

220µF

3900pF

130pF

CT

SENSE+

10V

SENSE–

1000pF

* COILTRONICS CTX33-4

SGND

**

IRC LRC2010-01-R100-J

† MBRS130LT3

†† AVX TPSE227K010

†††

AVX TPSE686K020

LTC1265-FO1

Figure 1. High Efficiency Step-Down Converter

	100
	95
(%)	90
EFFICIENCY	85
	80
	75
	70
	0.01

LTC1265-5 Efficiency

VIN = 6V

VIN = 9V

VIN = 12V

L = 33µH

VOUT = 5V

RSENSE = 0.1Ω

CT = 130pF

0.10	1.00
LOAD CURRENT (A)
	LTC1265 TA01

1

ELECTRICAL CHARACTERISTICS

LTC1265/LTC1265-3.3/LTC1265-5

ABSOLUTE WMAXIWMUWM RATINGSU

(Voltages Refer to GND Pin) (Note 1)

Input Supply Voltage (Pins 1, 2, 13) ..........	– 0.3V to 13V
DC Switch Current (Pin 14) ....................................	1.2A
Peak Switch Current (Pin 14) .................................	1.6A
Switch Voltage (Pin 14) ..................................	VIN – 13.0
Operating Temperature Range	0° to 70°C
LTC1265C ...............................................
LTC1265I ........................................	– 40°C to 85°C
Junction Temperature (Note 2) .............................	125°C
Storage Temperature Range ....................	– 65° to 150°C
Lead Temperature (Soldering, 10 sec)..................	300°C

PACKAGE/ORDER IUNFORWMATIOUN

				TOP VIEW			ORDER
							PART NUMBER
	PWR VIN	1			14	SW
							LTC1265CS
	VIN
		2			13	PWR VIN
	LBOUT	3			12	PGND	LTC1265CS-5
	LBIN	4			11	SGND	LTC1265CS-3.3
	CT
		5			10	SHDN	LTC1265IS
	ITH	6			9	N/C (VFB*)
	SENSE–					SENSE+
		7			8
				S PACKAGE
		14-LEAD PLASTIC SO
	*ADJUSTABLE OUTPUT VERSION
	TJMAX = 125°C, θJA = 110°C/W

Consult factory for Military grade parts.

The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 0V, unless otherwise specified.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

IFB

Feedback Current into Pin 9

LTC1265

0.2

1

µA

VFB

Feedback Voltage

LTC1265C

●

1.22

1.25

1.28

V

VIN = 9V, LTC1265I

●

1.20

1.25

1.30

V

VOUT

Regulator Output Voltage

LTC1265-3.3: ILOAD = 800mA

●

3.22

3.3

3.40

V

LTC1265-5: ILOAD = 800mA

●

4.9

5

5.2

V

VOUT

Output Voltage Line Regulation

VIN = 6.5V to 10V, ILOAD = 800mA

–40

0

40

mV

Output Voltage Load Regulation

LTC1265-3.3: 10mA < ILOAD < 800mA

40

65

mV

LTC1265-5: 10mA < ILOAD < 800mA

60

100

mV

Burst Mode Operation Output Ripple

ILOAD = 0mA

50

mVP-P

IQ

Input DC Supply Current (Note 3)

Active Mode: 3.5V < VIN < 10V

1.8

2.4

mA

Sleep Mode: 3.5V < VIN < 10V

160

230

µA

Sleep Mode: 5V < VIN < 10V (LTC1265-5)

160

230

µA

Shutdown: VSHDN = VIN, 3.5V < VIN < 10V

5

15

µA

VLBTRIP

Low-Battery Trip Point

1.15

1.25

1.35

V

ILBIN

Current into Pin 4

0.5

µA

ILBOUT

Current Sunk by Pin 3

VLBOUT = 0.4V, VLBIN = 0V

0.5

1.0

1.5

mA

VLBOUT = 5V, VLBIN = 10V

1.0

µA

V

8

– V

7

Current Sense Threshold Voltage

LTC1265: V

–

= 5V, V

9

= V /4 + 25mV (Forced)

25

mV

SENSE

OUT

V

–

= 5V, V

9

= V /4 – 25mV (Forced)

130

150

180

mV

SENSE

OUT

LTC1265-3.3: VSENSE– = VOUT + 100mV (Forced)

25

mV

VSENSE– = VOUT – 100mV (Forced)

130

150

180

mV

LTC1265-5: VSENSE– = VOUT + 100mV (Forced)

25

mV

V

–

= V

– 100mV (Forced)

130

150

180

mV

SENSE

OUT

RON

ON Resistance of Switch

LTC1265C

●

0.3

0.60

Ω

LTC1265I

0.3

0.70

Ω

I5

CT Pin Discharge Current

VOUT in Regulation, VSENSE– = VOUT

40

60

100

µA

VOUT = 0V

2

10

µA

tOFF

Switch Off Time (Note 4)

CT = 390pF, ILOAD = 800mA (LTC1265C)

●

4

5

6

µs

CT = 390pF, ILOAD = 800mA (LTC1265I)

●

3.5

5

7

µs

2

LTC1265/LTC1265-3.3/LTC1265-5

ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 0V, unless otherwise specified.

SYMBOL	PARAMETER	CONDITIONS		MIN	TYP	MAX	UNITS
VIH	Shutdown Pin High	Min Voltage at Pin 10 for Device to be in Shutdown		1.2			V
VIL	Shutdown Pin Low	Max Voltage at Pin 10 for Device to be Active				0.6	V
I10	Shutdown Pin Input Current	VSHDN = 8V				0.5	A

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

Note 2: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formulas:

LTC1265CS, LTC1265CS-3.3, LTC1265CS-5: TJ = TA + (PD • 110°C/W)

Note 3: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency.

Note 4: In applications where RSENSE is placed at ground potential, the off time increases by approximately 40%.

TYPICAL PERFORWMANUCE CHARACTERISTICS

EFFICIENCY (%)

Efficiency vs Load Current

100
95
					VIN = 5V
90
					VIN =					9V
85
					VIN = 12V
80
											LTC1265-3.3
											VOUT = 3.3V
75											RSENSE = 0.1Ω
											CT = 130pF
											COIL = CTX33-4
70
					0.10						1.00
0.01

LOAD CURRENT (A)

1265 G01

Efficiency vs Input Voltage	Efficiency vs Input Voltage
(VOUT = 5V)	(VOUT = 3.3V)

	100											100	LTC1265-3.3
	98											98
													RSENSE = 0.1Ω
	96			ILOAD = 250mA								96	CT = 130pF
	94											94	COIL = CTX33-4
(%)											(%)
	92	ILOAD = 800mA										92		ILOAD = 250mA
EFFICIENCY											EFFICIENCY
	90											90
	88											88
	86	LTC1265-5										86	ILOAD = 800mA
	84											84
		RSENSE = 0.1Ω
	82	CT = 130pF										82
		COIL = CTX33-4
	80	5	6	7		9		11	12	13		80	5	6	7		9		11	12	13
	4				8		10					4				8		10
				INPUT VOLTAGE (V)											INPUT VOLTAGE (V)
									1265 G02											LTC1265 G03

NORMALIZED FREQUENCY

Operating Frequency vs (VIN – VOUT)

1.2
1.0	0°C
	25°C
0.8	70°C

0.6

0.4

0.2

0

0 1 2 3 4 5 6 7 8 9 10 (VIN – VOUT) VOLTAGE (V)

1265 G04

		Switch Resistance
	0.9
	0.8
	0.7
	0.6
)							TJ	= 125	°C
(Ω	0.5
(ON)	0.4						TJ = 70°C
RDS
	0.3						TJ = 25°C
	0.2
							TJ = 0°C
	0.1
	0

3 4 5 6 7 8 9 10 11 12 13 INPUT VOLTAGE (V)

1265 G05

LEAKAGE CURRENT (nA)

Switch Leakage Current

300

VIN = 12V

270

240

210

180

150

120

90

60

30

0

0

20

40

60

80

100

TEMPERATURE (°C)

1265 G06

3

LTC1265/LTC1265-3.3/LTC1265-5

TYPICAL PERFORWMANUCE CHARACTERISTICS

SUPPLY CURRENT (mA)

DC Supply Current

2.1 DOES NOT INCLUDE

GATE CHARGE

1.8

1.5						ACTIVE MODE
1.2
0.9
0.6
0.3
						SLEEP	MODE
0
			4					10			12
0		2		6		8						14

INPUT VOLTAGE (V)

1265 G07

SUPPLY CURRENT ( A)

Supply Current in Shutdown

8 SHUTDOWN = 3V

7 TA = 25C

6

5

4

3

2

1

0

3

4

5

6

7

8

9

10

11

12

13

INPUT VOLTAGE (V)

1265 G08

Gate Charge Losses

	5.5
	5.0
(mA)	4.5				VIN = 12V
	4.0
CURRENT	3.5				VIN = 9V
	3.0
	2.5
SWITCHING
	2.0				VIN = 6V
	1.5
	1.0
	0.5
	0	200	400	600	800	1000
	0

FREQUENCY (kHz)

1265 G09

PIUN FUUNCTIOUNS

PWR VIN (Pins 1, 13): Supply for the Power MOSFET and its Driver. Must decouple this pin properly to ground. Must always tie Pins 1 and 13 together.

VIN (Pin 2): Main Supply for All the Control Circuitry in the LTC1265.

LBOUT (Pin 3): Open-Drain Output of the Low-Battery Comparator. This pin will sink current when Pin 4 (LBIN)

goes below 1.25V. During shutdown, this pin is high impedance.

LBIN (Pin 4): The (–) Input of the Low-Battery Comparator. The (+) input is connected to a reference voltage of 1.25V.

CT (Pin 5): External capacitor CT from Pin 5 to ground sets the switch off time. The operating frequency is dependent on the input voltage and CT.

ITH (Pin 6): Feedback Amplifier Decoupling Point. The current comparator threshold is proportional to Pin 6 voltage.

SENSE – (Pin 7): Connect to the (–) input of the current comparator. For LTC1265-3.3 and LTC1265-5, it also connects to an internal resistive divider which sets the output voltage.

SENSE+ (Pin 8): The (+) Pin to the Current Comparator. A built-in offset between Pins 7 and 8 in conjunction with RSENSE sets the current trip threshold.

N/C,VFB (Pin 9): For the LTC1265 adjustable version, this pin serves as the feedback pin from an external resistive divider used to set the output voltage. On the LTC1265-3.3 and LTC1265-5 versions, this pin is not used.

SHDN (Pin 10): Pulling this pin HIGH keeps the internal switch off and puts the LTC1265 in micropower shutdown. Do not float this pin.

SGND (Pin 11): Small-Signal Ground. Must be routed separately from other grounds to the (–) terminal of COUT.

PGND (Pin 12): Switch Driver Ground. Connects to the

(–) terminal of CIN. Anode of the Schottky diode must be connected close to this pin.

SW (Pin 14): Drain of the P-Channel MOSFET Switch. Cathode of the Schottky diode must be connected close to this pin.

4

LTC1265/LTC1265-3.3/LTC1265-5

FUNCTIONAL DIAGRAW		(Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)
U	U

SLEEP

+ S –

1, 13

PWR VIN

14 SW

12 PGND

		R
		Q
		S
	VTH2
	VTH1	–
		T
		+	2	VIN
5	CT	OFF-TIME	SENSE–
		CONTROL	VFB

SENSE+

8

– V +

–

25mV TO 150mV

C

	+			VOS
			13k	–
	ITH	6		G

+

3	LB0UT
	+	REFERENCE
	A3

–

11	4	LBIN
	SGND

SENSE–

7

9	VFB
	ADJUSTABLE
	VERSION

5pF

100k

10

SHDN

1265 FD

OPERATIOUN (Refer to Functional Diagram)

The LTC1265 uses a constant off-time architecture to switch its internal P-channel power MOSFET. The off time is set by an external timing capacitor at CT (Pin 5). The operating frequency is then determined by the off time and the difference between VIN and VOUT.

The output voltage is set by an internal resistive divider (LTC1265-3.3 and LTC1265-5) connected to SENSE– (Pin 7) or an external divider returned to VFB (Pin 9 for LTC1265). A voltage comparator V, and a gain block G, compare the divided output voltage with a reference voltage of 1.25V.

To optimize efficiency, the LTC1265 automatically switches between continuous and Burst Mode operation. The voltage comparator is the primary control element when the device is in Burst Mode operation, while the gain block controls the output voltage in continuous mode.

When the load is heavy, the LTC1265 is in continuous operation. During the switch ON time, current comparator C monitors the voltage between Pins 7 and 8 connected across an external shunt in series with the inductor. When

the voltage across the shunt reaches the comparator’s threshold value, its output signal will change state, setting the flip flop and turning the internal P-channel MOSFET off. The timing capacitor connected to Pin 5 is now allowed to discharge at a rate determined by the off-time controller.

When the voltage on the timing capacitor has discharged past VTH1, comparator T trips, sets the flip flop and causes the switch to turn on. Also, the timing capacitor is recharged. The inductor current will again ramp up until the current comparator C trips. The cycle then repeats.

When the load current increases, the output voltage decreases slightly. This causes the output of the gain stage (Pin 6) to increase the current comparator threshold, thus tracking the load current.

When the load is relatively light, the LTC1265 automatically goes into Burst Mode operation. The current loop is interrupted when the output voltage exceeds the desired regulated value. The hysteretic voltage comparator V trips when VOUT is above the desired output voltage, shutting off the switch and causing the capacitor to discharge. This

5