LINEAR TECHNOLOGY LT1618 Technical data

FEATURES

LT1618

Constant-Current/

Constant-Voltage 1.4MHz

Step-Up DC/DC Converter

U

DESCRIPTIO

■

Accurate Input/Output Current Control: ±5% Over
Temperature

■

Accurate Output Voltage Control: ±1%

■

Wide VIN Range: 1.6V to 18V

■

1.4MHz Switching Frequency

■

High Output Voltage: Up to 35V

■

Low V

■

Available in (3mm × 3mm × 0.8mm) 10-Pin DFN and

Switch: 200mV at 1A

CESAT

10-Pin MSOP Packages

U

APPLICATIO S

■

LED Backlight Drivers

■

USB Powered Boost/SEPIC Converters

■

Input Current Limited Boost/SEPIC Converters

■

Battery Chargers

U

TYPICAL APPLICATIO

The LT®1618 step-up DC/DC converter combines a tradi­tional voltage feedback loop and a unique current feedback
loop to operate as a constant-current, constant-voltage
source. This fixed frequency, current mode switcher oper­ates from a wide input voltage range of 1.6V to 18V, and
the high switching frequency of 1.4MHz permits the use of
tiny, low profile inductors and capacitors. The current
sense voltage is set at 50mV and can be adjusted using the
I

pin.

ADJ

Available in the 10-Pin (3mm × 3mm) Exposed Pad DFN
and 10-pin MSOP packages, the LT1618 provides a com­plete solution for constant-current applications.

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

(with Selectable 100mA/500mA Input Current Limit)

V

IN

5V

C1

4.7µF

3.3V

OFF

ON

0V

3.3V

100mA

500mA

0V

C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN EMK316BJ475
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CR43-100

USB to 12V Boost Converter

L1

3

8

9

13k

ISN

ISP

V

IN

SHDN

I

ADJ

10µH

2

LT1618

GND

SW

FB

V

C

5104

0.1Ω

20k

D1

7

R1
909k

1

R2
107k

2k

10nF

V

OUT

12V

C2

4.7µF

1618 TA01a

90

85

80

75

EFFICIENCY (%)

70

65

60

0

Efficiency Curve

20 60

40

LOAD CURRENT (mA)

100

120

160

1618 TA01b

sn1618 1618fas

80

140

1

LT1618

WW

W

ABSOLUTE AXI U RATI GS

U

(Note 1)

VIN, SHDN Voltage ................................................... 18V

SW Voltage .............................................................. 36V

ISP, ISN Voltage ...................................................... 36V

I

Voltage ............................................................... 6V

ADJ

FB Voltage .............................................................. 1.5V

Voltage .............................................................. 1.5V

V

C

UUW

PACKAGE/ORDER I FOR ATIO

TOP VIEW

10

FB

1

ISN

2

11

3

ISP

4

I

ADJ

5

GND

10-LEAD (3mm × 3mm) PLASTIC DFN

T

JMAX

DD PACKAGE

= 125°C, θJA = 43°C/W, θJC = 3°C/W

EXPOSED PAD (PIN 11) IS GND AND

MUST BE SOLDERED TO PCB

V

C

SHDN

9

V

8

IN

7

SW

6

SW

ORDER PART

NUMBER

LT1618EDD

DD PART

MARKING

LAFQ

Junction Temperature...........................................125°C

Operating Temperature Range (Note 2) .. – 40°C to 85°C
Storage Temperature Range

MSOP ............................................... – 65°C to 150°C

DFN ................................................. – 65°C to 125°C

Lead Temperature (Soldering, 10 sec) (MSOP) .... 300°C

ORDER PART

TOP VIEW

1

FB

2

ISN

3

ISP

4

I

ADJ

GND

5

MS PACKAGE

10-LEAD PLASTIC MSOP

T

= 125°C, θJA = 160°C/W

JMAX

10

V

C

9

SHDN
V

8

IN

SW

7

NC

6

NUMBER

LT1618EMS

MS PART

MARKING

LTNH

Consult LTC marketing for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 1.6V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Input Voltage 1.6 18 V

Quiescent Current V

Reference Voltage Measured at FB Pin 1.250 1.263 1.276 V

Reference Voltage Line Regulation 1.6V < VIN < 18V 0.01 0.03 %/V

FB Pin Bias Current VFB = 1.263V, VIN = 1.8V ● ±2 ±12 nA

Error Amplifier Voltage Gain 180 V/V

Error Amplifier Transconductance ∆IC = ± 5µA 160 µmho

Error Amplifier Sink Current VFB = 1.35V, VC = 1V 15 µA

Error Amplifier Source Current VFB = 1.10V, VC = 1V 30 µA

Current Sense Voltage (ISP, ISN) VFB = 0V, V

ISP, ISN Pin Bias Currents (Note 3) V

(ISP, ISN) Common Mode Minimum Voltage 1.8 V

Switching Frequency VFB = 1V 1.25 1.4 1.6 MHz

Maximum Switch Duty Cycle ● 88 92 %

Switch Current Limit (Note 4) 1.5 2.1 2.8 A

The ● denotes specifications which apply over the full operating

= 1.6V, unless otherwise noted.

SHDN

= 1.6V, Not Switching 1.8 2.7 mA

SHDN

= 0V 0.1 1 µA

V

SHDN

● 1.243 1.263 1.283 V

= 0V ● 47.5 50 52.5 mV

IADJ

= 1.85V, V

ISP

= 0V 550 kHz

V

FB

= 1.80V, V

ISN

= 0V 50 80 µA

IADJ

sn1618 1618fas

2

LT1618

TEMPERATURE (°C)

–50

PEAK CURRENT (A)

125

1618 G03

0

75

2.5

2.0

1.5

1.0

0.5

0

–25 25 50

100

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T

The ● denotes specifications which apply over the full operating

= 25°C. VIN = 1.6V, V

A

= 1.6V, unless otherwise noted.

SHDN

PARAMETER CONDITIONS MIN TYP MAX UNITS

Switch V

CESAT

ISW = 1A (Note 4) 200 260 mV

Switch Leakage Current Switch Off, VSW = 5V 0.01 5 µA

SHDN Pin Current V

= 1.6V 5 20 µA

SHDN

Shutdown Threshold (SHDN Pin) 0.3 V

Start-Up Threshold (SHDN Pin) 1V

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

Note 2: The LT1618 is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating

temperature range are assured by design, characterization, and correlation
with statistical process controls.

Note 3: Bias currents flow into the ISP and ISN pins.
Note 4: Switch current limit and switch V

for the DD package is

CESAT

guaranteed by design and/or correlation to static test.

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Switch Saturation Voltage
(V

500

CE, SAT

)

FB Pin Voltage and Bias Current

1.270

Switch Current Limit

4

400

TJ = 125°C

300

200

SATURATION VOLTAGE (mV)

100

0

TJ = 25°C

TJ = –50°C

0

0.5

1.0

SWITCH CURRENT (A)

Current Sense Voltage
(I

Pin = 0V)

ADJ

52

51

50

49

CURRENT SENSE VOLTAGE (mV)

1.5

1618 G01

1.265

1.260

FEEDBACK VOLTAGE (V)

1.255

1.250

2.0

CURRENT SENSE VOLTAGE (mV)

VOLTAGE

–25 25 50

–50

0

TEMPERATURE (°C)

Current Sense Voltage
(V

60

50

40

30

20

10

ISP, ISN

)

CURRENT

75

100

FB PIN BIAS CURRENT (nA)

2

0

–2

–4

125

1618 G02

Quiescent Current

2.5

2.0

1.5

1.0

QUIESCENT CURRENT (mA)

0.5

VIN = 18V

VIN = 1.6V

48

–25 25 50

–50

0

TEMPERATURE (°C)

75

100

1618 G04

125

0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
I

PIN VOLTAGE (V)

ADJ

1618 G05

0

–25 25 50

–50

0

TEMPERATURE (°C)

75

100

1618 G06

sn1618 1618fas

125

3

LT1618

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Switching Frequency Frequency Foldback SHDN Pin Current

1.8

1.7

1.6

1.5

1.4

1.3

1.2

SWITCHING FREQUENCY (MHz)

1.1

1.0
–25 25 50

–50

VIN = 18V

0

TEMPERATURE (°C)

VIN = 1.6V

75

100

125

1618 G07

1.6
TJ = 25°C

1.4

1.2

1.0

0.8

0.6

0.4

SWITCHING FREQUENCY (MHz)

0.2

0

0 0.2

0.4 0.6

FEEDBACK PIN VOLTAGE (V)

0.8

1.0

1618 G08

1.2

50

45

40

35

30

25

20

15

SHDN PIN CURRENT (µA)

10

5

0

0

UUU

PIN FUNCTIONS

(MS/DD)

TJ = –50°C

TJ = 25°C

TJ = 125°C

5

SHUTDOWN PIN VOLTAGE (V)

10

15

20

1618 G09

FB (Pin 1/Pin 1): Feedback Pin. Set the output voltage by
selecting values for R1 and R2 (see Figure 1):

RR

⎛

12

⎜
⎝

V

OUT

.

1 263

⎞

–

1=

⎟

V

⎠

ISN (Pin 2/Pin 2): Current Sense (–) Pin. The inverting
input to the current sense amplifier.

ISP (Pin 3/Pin 3): Current Sense (+) Pin. The noninverting
input to the current sense amplifier.

I

(Pin 4/Pin 4): Current Sense Adjust Pin. A DC voltage

ADJ

applied to this pin will reduce the current sense voltage. If
this adjustment is not needed, tie this pin to ground.

GND (Pin 5/Pin 5): Ground Pin. Tie this pin directly to local
ground plane.

NC (Pin 6/NA): No Connection for MS Package.

SW (NA/Pin 6): Switch Pin for DD Package. Connect this

pin to Pin 7.

SW (Pin 7/Pin 7): Switch Pin. This is the collector of the
internal NPN power switch. Minimize the metal trace area
connected to this pin to minimize EMI.

VIN (Pin 8/Pin 8): Input Supply Pin. Bypass this pin with
a capacitor to ground as close to the device as possible.

SHDN (Pin 9/Pin 9): Shutdown Pin. Tie this pin higher
than 1V to turn on the LT1618; tie below 0.3V to turn it off.

VC (Pin 10/Pin 10): Compensation Pin for Error Amplifier.
Connect a series RC from this pin to ground. Typical values
are 2kΩ and 10nF.

Exposed Pad (NA/Pin 11): The Exposed Pad on the DD
package is GND and must be soldered to the PCB GND for
optimum thermal performance.

4

sn1618 1618fas

BLOCK DIAGRA

LT1618

W

V

IN

C1 C2

SHDN

V

8

DRIVER

L1

IN

SW

79

Q1

+

0.02Ω

×5

–

1.4MHz

OSCILLATOR

S

Q

R

A3

+

Σ

+

+

–

5 10

GND

V

C

R

C

C

D1

ISP

3

+

A1

×25

–

A2

–

1.263V

+

C

ISN

2

–

I

ADJ

4

FB

1

R

SENSE

V

OUT

R1

R2

Figure 1. LT1618 Block Diagram

U

OPERATIO

The LT1618 uses a constant frequency, current mode
control scheme to provide excellent line and load regula­tion. Operation can be best understood by referring to the
Block Diagram in Figure 1. At the start of each oscillator
cycle, the SR latch is set, turning on power switch Q1. The
signal at the noninverting input of PWM comparator A3 is
a scaled-down version of the switch current (summed
together with a portion of the oscillator ramp). When this
signal reaches the level set by the output of error amplifier
A2, comparator A3 resets the latch and turns off the power
switch. In this manner, A2 sets the correct peak current
level to keep the output in regulation. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. A2 has two
inverting inputs, one from the voltage feedback loop, and
one from the current feedback loop. Whichever inverting

input is higher takes precedence, forcing the converter
into either a constant-current or a constant-voltage mode.
The LT1618 is designed to transition cleanly between the
two modes of operation. Current sense amplifier A1 senses
the voltage between the ISP and ISN pins and provides a
25× level-shifted version to error amplifier A2. When the
voltage between ISP and ISN reaches 50mV, the output of
A1 provides 1.263V to one of the noninverting inputs of A2
and the converter is in constant-current mode. If the
current sense voltage exceeds 50mV, the output of A1 will
increase causing the output of A2 to decrease, thus
reducing the amount of current delivered to the output. In
this manner the current sense voltage is regulated to
50mV. Similarly, if the FB pin increases above 1.263V, the
output of A2 will decrease to reduce the peak current level
and regulate the output (constant-voltage mode).

sn1618 1618fas

5